ODG Linter Js By Dragons Gamers

✴️ Code Quality for typescript ✴️!

Standard Code

Introduction
- Installation
File Name Convention
Semi Rule
Quotes Rule
Indent Rule
Line Break Rule
EOL last Rule
Max Line Len Rule
Camel Case Rule
Strict
Padded Block Rule
Lines Between Class Members
No Multi Assign Rule
Explicit Member Accessibility Rule
Default Param Last Rule
Space Before Function Paren
Exception Handled
Array Space
Key Word Space
Space Format
UTF-8 Only
No Space in Parentheses
No Multiple Space
Useless String Concat
No Self Assign
Force Return Type
Array Bracket Line
Unused Vars
Comma Spacing
Comma Dangle
Arrow Spacing
Prefer Arrow Function
Prefer Destructuring
Arrow Function Body
Arrow Function Parentheses
Arrow Function No Break Line
No Empty Block
No Long Syntax
Useless Parens
Useless Boolean
Useless Alias
Return New line
Comment Multi Line Prefer
No throw Literal
No Unreachable
No Multiline String
No Unsafe Assign
Disallow Script Url
Disallow Undefined
Function Name
Function Name Match
No Use Future Reserved Words
No Generator Without Yield
Inverted Assertion Arguments
Max Union Size
No Redundant Optional
Prefer Type Guard
No Production Debug
Unused Named Groups
Get And Setters
Function Style
No Else Return
No Console Spaces
No Hex Escape
Prefer Array Flat Map
Prefer String Slice
Prefer Modern DOM
Prefer Prefix Number
Numeric Separators Style
Prefer Default Parame
No Avoid Reverse
Prefer Code Point
Bool Param Default
Class Name Convention
Comma Or Logical Or Case
No Constructor Bind
No Thenable
No Unreadable Iife
Prefer Native Cast
Prefer Logical Operator Over Ternary
Prefer Event Target
Prefer Object From Entries
Prefer Array From Map
Prefer Array Flat
This Pattern
Use Dot
Use this
Use IsNan
Dot Object Format
No Trailing Space
Type Format
Max Statements Per Line
No Constant Condition
No Debugger
Not Duplicate Case
Regex Block
No Overwrite Exception
No Extra Semi
No Function Overwrite
No Delete Var
No Lone Blocks
No Proto
No Declare in Block
No Nonoctal Decimal Escape
No Import Absolute Path
No Webpack Loader Syntax
No Magic Number
Security Negation
Regex Space
Array No Space
Valid TypeOf
Strict equality
No Label
Full Decimal Number
No Global Overwrite
Not Used New
No New Function
No Redeclare
No Self Compare
Loop Valid
Useless Scape
No Yoda
No Undefined declare
No New require
No New Object
No New Symbol
Var Size
Max Depth
Max Params
Max Statements
Operator Assignment
Require Yield
Prefer Rest Params
Symbol Description
No Await Return
Max Class Per File
No Constructor Return
Prefer Exponentiation Operator
Prefer Object Spread
Accessor Pairs
Default Case Last
Prefer Literals
Useless Condition
No Multiple Empty Line
No Misused New
No Semicolon Before spacing
Disallow Type
Disallow Empty Function
Disallow Duplicate Imports
Disallow Unnecessary Type
Disallow Caller
Disallow Underscore
Disallow Param Reassign
Prefer Const
Array Type
Disallow Await sync function
Method Signature Style
No Unnecessary Type Assertion
No Unsafe Call
No Var
Operator Break-Line
Generator Function Stars
No Unsafe Optional Chaining
Array Callback
Space Types
Curly
Quote Props
Brace Style
Comma Style
Object Break Line
Object Curly Newline
No Negative Condition
No Duplicated Branches
No Identical Functions
No Inverted Boolean Check
No Nested Switch
No Nested Template Literals
No Redundant Boolean
Prefer Immediate Return
Prefer Object Literal
Prefer Single Boolean Return
No Shadow
Parentheses New Line
No Func Call Spacing
Array Element New Line
Wrap Iife
Disallow Template Tag Space
No Space Spread
Inline IF
New Instance Use Parentheses
Logical Assignment Operators
No With
Promise Rules
- No New Statics
- No Return Wrap
- Param Name
- Always Return
- No Nesting
- No Return Finally
- Valid Params
- No Async Promise Executor
- No Misused Promises
Import
- New Line After Import
- Import Deprecated
- No Mutable Export
- No Amd
- Prefer Default Export
- Max Imports
- No CommanJs Export
- No Useless Path Import
- No Extraneous Dependencies
- Import Order
- No Anonymous Default Export
- Prefer Node Protocol
- Prefer export from
- No Empty Import
- Export End File
- Import First
- No Named Default
Documentation
- Space Comment
- Capitalized Comments
- Comment Align
- Disallow Space Align
- Validate Param
- Validate Syntax
- Validate Tag Name
- Validate Types
- Validate Values
- Empty Tags
- Validate Block
- Disallow Extra Asterisk
- Disallow Default Value
Regex Rules
- No Optional With Assertion
- No Control Character
- No Dupe Condition
- No Empty Alternative
- No Empty Capturing Group
- No Empty Character Class
- No Empty Group
- No Empty Look Rounds Assertion
- No Escape Backspace
- No Invalid Regexp
- No Lazy Ends
- No Misleading Unicode Character
- No Optional Assertion
- No Potentially Useless Back Reference
- No Useless Assertions
- No Useless Back Reference
- No Useless Dollar Replacements
- Strict Regex
- Confusing Quantifier
- Control Character Escape
- Negation
- No Dupe Characters Character Class
- No Invisible Character
- No Legacy Features
- No Non Standard Flag
- No Obscure Range
- No Trivially Nested Quantifier
- No Unused Capturing Group
- No Useless Character Class
- No Useless Flag
- No Useless Lazy
- No Useless Quantifier
- No Useless Range
- No Useless Two Num Quantifier
- No Zero Quantifier
- Optimal LookAround Quantifier
- Optimal Quantifier Concatenation
- Prefer Quantifier
- Prefer Range
- Sort Alternatives
- Hexadecimal Escape
- Match Any
- No Useless Escape
- No Useless Non Capturing Group
- Prefer Character Class
- Prefer D
- Prefer Plus
- Prefer Question Quantifier
- Prefer Star Quantifier
- Prefer Unicode Code Point Escapes
- Prefer W
- Sort Character Class Elements
- Sort Flags
- Prefer Named Capture Group
- Prefer Regexp Exec
- Existing Groups
- No Misleading Capturing Group
- No Extra Lookaround Assertions
Security
- Eval Disabled
- Detect Unsafe Regex
- Buffer No Assert
- No Exec Child Process
- No Mustache Scape
- No Csrf Before Method Override
- No Literal Fs Filename
- No Pseudo Random Bytes
- Prevent Secret Token
- Prevent Literal Code
- No Import Dynamic
- Security SSL
- Security Encrypt Access
- Certificate Transparency
- No Log confidential information
- Content Length
- Anti Trojan Source No Bidi
- Cookie Httponly
- Protect XSS Render
- Force Integrity
- DNS prefetching
- No Prototype Builtins
- File Permissions
- File Upload
- Frame Ancestors
- Hashing Insecurity
- Hidden Files
- No Dynamic Delete
- Cors
- Csrf
- Aws Security
- Security Cookie
- Insecure Cookie
- No Clear Text Protocols
- Insecure Jwt Token
- No Hardcoded Credentials
- No Hardcoded ip
- No Os Command From Path
- No Unsafe Unzip
- Sockets
- Helmet Security
- Unverified Certificate
- Sql Queries
Catch Error Name
Consistent Destructured
Consistent Function Scope
Exception With Message
Escape Case
New For Builtin
No Abusive Eslint Disable
Prefer for-of
No Array Push Push
No Await Chased
No Document Cookie
No Empty File
No Instance Of Array
No Invalid Remove Event Listener
No Lonely If
No Nested Ternary
No New Buffer
No This Assignment
No Useless Fallback In Spread
No Useless Length Check
No Useless Spread
No Useless Switch Case
No Useless Undefined
Prefer Add Event Listener
Prefer Array Find
Prefer Array Index Of
Prefer Array Some
Prefer Dom Node Append
Prefer Include
Prefer Keyboard Event Key
Prefer Negative Index
Prefer Optional Catch Binding
Prefer Prototype Methods
Prefer Query Selector
Prefer Regexp Test
Prefer Spread
Prefer String Replace All
Prefer String Starts Ends With
Prefer String Trim Start End
Prefer Switch
Prefer Ternary
Multiline Ternary Multiple
Prevent Abbreviations
Consistent Relative URL
Require Array Join Separator
Require Number To Fixed Digits Argument
Template String Indent
Adjacent Overload Signatures
Ban Ts Comment
Ban Tslint Comment
Class Literal Property Style
Consistent Generic Constructors
Consistent Indexed Object Style
Consistent Type Assertions
Consistent Type Definitions
Consistent Type Exports
Consistent Type Imports
Member Delimiter Style
Member Ordering
No Confusing Non Null Assertion
No Extra Non Null Assertion
No For In Array
No Inferrable Type
No This Alias
Non Nullable Type Assertion Style
Parameter Properties
Prefer Enum Initializers
Prefer Function Type
Prefer Namespace Keyword
Prefer Nullish Coalescing
Prefer Optional Chain
Prefer Readonly
Prefer Reduce Type Parameter
Promise Function Async
Require Array Sort Compare
Sort Type Constituents
Space Before Blocks
Performance
- No Alert
- No Loop Func
Errors
- Construtor Super Invalid
- Getter Return
- No Class Assign
- No Compare Neg Zero
- Useless Loop
- No Loss Of Precision
- No Unsafe Argument
- No Dupe Class Members
- No Dupe Keys
- Import Default Not Fount
- Import Namespace
- No Self Import
- Export Name
- Import Namespace
- No Import Resolve
- Import Not Found
- JS Type
- No Invalid This
- No This Before Super
- No Obj Calls
- No Empty Pattern
- No Useless Computed Key
- Useless Call Code
- Useless Catch Code
- Useless Expression Code
- Useless Return Code
- Useless Construct Code
- No Use Before Define
- No Base To String
- Not IN Primitive type
- Useless String Operation
- Super Invocation
- No Useless Intersection
- Stateful Regex
Possible Errors
- For Direction
- No Extra Bind
- No Template Curly In String
- No Dupe Else If
- No Unused Private Class Members
- No Constant Binary Expression
- No Unneeded Ternary
- No Unsafe Negation
- Text Encoding Identifier Case
- Arguments Order
- Assertions In Tests
- Test Multiple Possible Assert
- No Disable Timeout
- No Empty Static Block
- No Fallthrough
- No Octal
- Octal Scape
- No Global Assign
- No Case Declarations
- No Confusing Void Expression
- No Duplicate Enum Values
- No Floating Promises
- No Invalid Void Type
- No Non Null Asserted Nullish Coalescing
- No Non Null Asserted Optional Chain
- No Unsafe Declaration Merging
- No Useless Empty Export
- Restrict Template Expressions
- Return Await Try Catch
- Switch Exhaustiveness Check
- Index Of Compare To Positive Number
- No Invariant Returns
- Inconsistent Function Call
- Duplicate Conditions
- Redundant Type Aliases
- No Element Overwrite
- No Empty Collection
- No Extra Arguments
- No Identical Expressions
- No Ignored Return
- No Use Of Empty Return Value
- No Collection Size Mischeck
- No Gratuitous Expressions
- No Unused Collection
- No Inconsistent Docblock
YAML / JSON

Introduction

Installation

Add dependence to package.json

npm install eslint @odg/eslint-config
# or
yarn add -D eslint @odg/eslint-config

Add extends in your .eslintrc file

{
    "extends": [ "@odg" ]
}

Add script in your package.json file

{
    "scripts": {
        "lint": "eslint --ext .js,.jsx,.ts,.tsx,.json,.jsonc,.json5,.yml,.yaml,.xml,.txt,.svg,.properties,.gradle,.java,.cpp,.c,.cs,.html,.css,.groovy,.gitignore,.npmignore,.toml,.env,.example,.sample,.ini,.php,.bat,.powershell,.ps1,.sh,.bash,.eslintrc",
    }
}

Test: npm run lint or yarn lint

File Name Convention

File Name Convention

https://github.com/selaux/eslint-plugin-filenames

👍 Examples of correct code

// File name Foo.ts
export default class Foo {
}

👎 Examples of incorrect code

// File name FooClass.ts
export default class Foo {
}

Semi Rule

Requires semicolons at the end of statements

https://eslint.org/docs/rules/semi#semi https://eslint.org/docs/rules/semi-style

👍 Examples of correct code

var name = "ODG";

object.method = function() {
    // ...
};

class Foo {
    bar = 1;
}

foo();
[1, 2, 3].forEach(bar);

for (
    var i = 0;
    i < 10;
    ++i
) {
    foo();
}

class C {
    static {
        foo();
    }
}

👎 Examples of incorrect code

var name = "ODG"

object.method = function() {
    // ...
}

class Foo {
    bar = 1
}

foo()
;[1, 2, 3].forEach(bar)

for (
    var i = 0
    ; i < 10
    ; ++i
) {
    foo()
}

class C {
    static {
        foo()
        ;bar()
    }
}

Quotes Rule

Requires the use of double quotes wherever possible

Enforces the use of double quotes for all JSX attribute values that don’t contain a double quote.

https://eslint.org/docs/rules/quotes https://eslint.org/docs/rules/jsx-quotes

👍 Examples of correct code

var double = "double";
var backtick = `back
tick`;  // backticks are allowed due to newline
var backtick = tag`backtick`;

👎 Examples of incorrect code

var single = 'single';
var unescaped = 'a string containing "double" quotes';
var backtick = `back\ntick`; // you can use \n in single or double quoted strings

Indent Rule

Requires indent with 4 spaces Tabs Disallow

https://eslint.org/docs/rules/indent#indent https://sonarsource.github.io/rspec/#/rspec/S3973/javascript https://eslint.org/docs/latest/rules/no-tabs https://eslint.org/docs/latest/rules/no-mixed-spaces-and-tabs

👍 Examples of correct code

if (a) {
    b=c;
    function foo(d) {
        e=f;
    }
}

👎 Examples of incorrect code

if (a) {
  b=c;
  function foo(d) {
    e=f;
  }
}

Line Break Rule

Enforces the usage of Unix line endings: \n for LF.

https://eslint.org/docs/rules/linebreak-style#linebreak-style

👍 Examples of correct code

var a = 'a'; // \n

👎 Examples of incorrect code

var a = 'a'; // \r\n

EOL last Rule

Force empty end line

https://eslint.org/docs/rules/eol-last#eol-last

👍 Examples of correct code

function doSmth() {
  var foo = 2;
} // \n

👎 Examples of incorrect code

function doSmth() {
  var foo = 2;
}

Max Line Len Rule

Max line len is 120

https://eslint.org/docs/rules/max-len#max-len

👍 Examples of correct code

var foo = {
    "bar": "This is a bar.",
    "baz": { "qux": "This is a qux" },
    "difficult": "to read",
};

👎 Examples of incorrect code

var foo = { "bar": "This is a bar.", "baz": { "qux": "This is a qux" }, "difficult": "to read" };

Camel Case Rule

Force use camel case variable

https://eslint.org/docs/rules/camelcase#camelcase

👍 Examples of correct code

import { no_camelcased as camelCased } from "external-module";

var myFavoriteColor   = "#112C85";
var _myFavoriteColor  = "#112C85";
var myFavoriteColor_  = "#112C85";
var MY_FAVORITE_COLOR = "#112C85";

👎 Examples of incorrect code

import { no_camelcased } from "external-module"

var my_favorite_color = "#112C85";

function do_something() {
    // ...
}

Strict

A strict mode directive is a "use strict" literal at the beginning of a script or function body. It enables strict mode semantics.

https://eslint.org/docs/latest/rules/strict

👍 Examples of correct code

"use strict";

function foo() {
}

👎 Examples of incorrect code

function foo() {
}

Padded Block Rule

force empty line in blocks

https://eslint.org/docs/rules/padded-blocks#padded-blocks

{
  "classes": "always",
  "blocks": "never",
  "switches": "never",
}

👍 Examples of correct code

class ClassName {

  variable = 1;

}

switch (a) {
    case 0: foo();
}

if (a) {
    a = b;
}

👎 Examples of incorrect code

class ClassName {
  variable = 1;

}

class ClassName {

  variable = 1;
}

switch (a) {

    case 0: foo();

}

if (a) {

    a = b;

}

Lines Between Class Members

Enforces consistent spacing before function parenthesis.

https://eslint.org/docs/rules/lines-between-class-members#lines-between-class-members

👍 Examples of correct code

class MyClass {
  x;

  foo() {
    //...
  }

  bar() {
    //...
  }
}

👎 Examples of incorrect code

class MyClass {
  x;
  foo() {
    //...
  }
  bar() {
    //...
  }
}

No Multi Assign Rule

Chaining the assignment of variables can lead to unexpected results and be difficult to read. Disabled.

https://eslint.org/docs/rules/no-multi-assign#no-multi-assign

👍 Examples of correct code

var a = 5;
var b = 5;
var c = 5;

const foo = "baz";
const bar = "baz";

let a = c;
let b = c;

class Foo {
    a = 10;
    b = 10;
}

a = "quux";
b = "quux";

👎 Examples of incorrect code

var a = b = c = 5;

const foo = bar = "baz";

let a =
    b =
    c;

class Foo {
    a = b = 10;
}

a = b = "quux";

Explicit Member Accessibility Rule

Force specific public/private or protected visibility

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/explicit-member-accessibility.md

{
    "anonymous": "never",
    "named": "never",
    "asyncArrow": "always"
}

👍 Examples of correct code

class ClassName {
    public a = 1;
    protected c = 2;
    private b = 3;
}

👎 Examples of incorrect code

class ClassName {
    a = 1;
    c = 2;
    b = 3;
}

Default Param Last Rule

Enforces default parameters to be last.

https://eslint.org/docs/rules/default-param-last#default-param-last

👍 Examples of correct code

function f(a = 0) {}
function f(a: number, b = 0) {}
function f(a: number, b?: number) {}
function f(a: number, b?: number, c = 0) {}
function f(a: number, b = 0, c?: number) {}
class Foo {
  constructor(public a, private b = 0) {}
}
class Foo {
  constructor(public a, private b?: number) {}
}

👎 Examples of incorrect code

function f(a = 0, b: number) {}
function f(a: number, b = 0, c: number) {}
function f(a: number, b?: number, c: number) {}
class Foo {
  constructor(public a = 10, private b: number) {}
}
class Foo {
  constructor(public a?: number, private b: number) {}
}

Space Before Function Paren

Enforces default parameters to be last.

https://eslint.org/docs/latest/rules/space-before-function-paren https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/space-before-function-paren.md

👍 Examples of correct code

foo(function() {

})

function foo() {

}

(async () => {})()

👎 Examples of incorrect code

foo(function () {

})

function foo () {

}

(async() => {})()

Exception Handled

Enforces callback error handling.

https://eslint.org/docs/rules/handle-callback-err https://github.com/weiran-zsd/eslint-plugin-node/blob/HEAD/docs/rules/handle-callback-err.md

👍 Examples of correct code

function loadData (err, data) {
    if (err) {
        console.log(err.stack);
    }
    doSomething();
}

function loadData (exception, data) {
    if (exception) {
        console.log(exception);
    }
    doSomething();
}

function generateError (err) {
    if (err) {
        throw new Exception(err.message);
    }
}

👎 Examples of incorrect code

function loadData (err, data) {
    doSomething();
}
function loadData (exception, data) {
    doSomething();
}

Class Name

This rule requires constructor names to begin with a capital letter.

https://eslint.org/docs/rules/new-cap

👍 Examples of correct code

var friend = new Person();

👎 Examples of incorrect code

var friend = new person();
var friend = Person();

Array Space

requires one or more spaces or newlines inside array brackets, and disallow space inside of computed properties.

https://eslint.org/docs/rules/array-bracket-spacing#array-bracket-spacing

https://eslint.org/docs/rules/computed-property-spacing#computed-property-spacing

👍 Examples of correct code

var arr = [ 'foo', 'bar' ];
var [ x, y ] = z;

var c = arr[0];

👎 Examples of incorrect code

var arr = ['foo', 'bar'];
var [x,y] = z;

var c = arr[ 0 ];

var c = object[ "foo" ];
var c = object["foo" ];
var c = object[ "foo"];

Key Word Space

Enforces consistent spacing before and after keywords.

https://eslint.org/docs/rules/keyword-spacing#keyword-spacing https://eslint.org/docs/rules/yield-star-spacing

👍 Examples of correct code

if (foo) {
    //...
} else if (bar) {
    //...
} else {
    //...
}

try {

} catch(e) {
    // code ...
}

function *generator() {
  yield *other();
}

👎 Examples of incorrect code

if(foo){
    //...
}else if(bar){
    //...
}else{
    //...
}

try{

}catch(e){
    // code ...
}

function*generator() {
  yield*other();
}

function* generator() {
  yield* other();
}

function * generator() {
  yield * other();
}

Space Format

This rule enforces consistency regarding the spaces after

https://eslint.org/docs/rules/space-unary-ops#space-unary-ops

👍 Examples of correct code

a++;
++a;

--a;
a--;

async function foo() {
    await bar;
}

if (!foo) {

}

const value = +"3";

👎 Examples of incorrect code

a ++;
++ a;

-- a;
a --;

async function foo() {
    await(bar);
}

if (! foo) {

}

const value = + "3";

UTF-8 Only

Disallow the Unicode Byte Order Mark (BOM).

https://eslint.org/docs/rules/unicode-bom#unicode-bom

No Space in Parentheses

Disallows or enforce spaces inside of parentheses.

https://eslint.org/docs/rules/space-in-parens#space-in-parens

👍 Examples of correct code

foo();

foo('bar');

foo(/* bar */);

var foo = (1 + 2) * 3;
(function () { return 'bar'; }());

👎 Examples of incorrect code

foo( );

foo( 'bar');
foo('bar' );
foo( 'bar' );

foo( /* bar */ );

var foo = ( 1 + 2 ) * 3;
( function () { return 'bar'; }() );

No Multiple Space

Disallows multiple consecutive spaces.

https://eslint.org/docs/rules/no-multi-spaces#no-multi-spaces

👍 Examples of correct code

var a = 1;

if(foo === "bar") {}

a << b

var arr = [ 1, 2 ];
var a = [];
var baz = [];

a ? b : c

👎 Examples of incorrect code

var a =  1;

if(foo   === "bar") {}

a <<  b

var arr  = [1,  2];
var c    = [];
var baz =  [];

a ?  b  : c

Useless String Concat

Disallows useless string concat.

https://eslint.org/docs/rules/no-useless-concat#no-useless-concat

👍 Examples of correct code

var c = a + b;
var c = '1' + a;
var a = 1 + '1';
var c = 1 - 2;
// when the string concatenation is multiline
var c = "foo" +
    "bar";

👎 Examples of incorrect code

var a = `some` + `string`;

// these are the same as "10"
var a = '1' + '0';
var a = '1' + `0`;
var a = `1` + '0';
var a = `1` + `0`;

No Self Assign

Disallows assignments where both sides are exactly the same.

https://eslint.org/docs/rules/no-self-assign#no-self-assign

👍 Examples of correct code

foo = bar;
[a, b] = [b, a];

// This pattern is warned by the `no-use-before-define` rule.
let foo = foo;

// The default values have an effect.
[foo = 1] = [foo];

// non-self-assignments with properties.
obj.a = obj.b;
obj.a.b = obj.c.b;
obj.a.b = obj.a.c;
obj[a] = obj["a"];

👎 Examples of incorrect code

foo = foo;

[a, b] = [a, b];

[a, ...b] = [x, ...b];

({a, b} = {a, x});

foo &&= foo;
foo ||= foo;
foo ??= foo;

Force Return Type

Force fill return type in typescript

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/explicit-function-return-type.md

👍 Examples of correct code

function test(): void {
  return;
}

// A return value of type number
var fn = function (): number {
  return 1;
};

// A return value of type string
var arrowFn = (): string => 'test';

class Test {
  // No return value should be expected (void)
  method(): void {
    return;
  }
}

👎 Examples of incorrect code

function test() {
  return;
}

// Should indicate that a number is returned
var fn = function () {
  return 1;
};

// Should indicate that a string is returned
var arrowFn = () => 'test';

class Test {
  // Should indicate that no value is returned (void)
  method() {
    return;
  }
}

Array Bracket Line

Requires consistent usage of linebreaks for each pair of brackets. It reports an error if one bracket in the pair has a linebreak inside it and the other bracket does not.

https://eslint.org/docs/rules/array-bracket-newline#consistent

👍 Examples of correct code

var a = [];
var c = [ 1 ];
var d = [
    1
];
var f = [
    function foo() {
        dosomething();
    }
];

👎 Examples of incorrect code

var a = [1
];
var b = [
    1];
var c = [function foo() {
    dosomething();
}
]
var d = [
    function foo() {
        dosomething();
    }]

Unused Vars

Variables that are declared and not used anywhere in the code are most likely an error due to incomplete refactoring. Such variables take up space in the code and can lead to confusion by readers.

https://eslint.org/docs/rules/no-unused-vars#no-unused-vars

👍 Examples of correct code

var x = 10;
alert(x);

// foo is considered used here
myFunc(function foo() {
    // ...
}.bind(this));

(function(foo) {
    return foo;
})();

var myFunc;
myFunc = setTimeout(function() {
    // myFunc is considered used
    myFunc();
}, 50);

// Only the second argument from the destructured array is used.
function getY([, y]) {
    return y;
}

👎 Examples of incorrect code

// It checks variables you have defined as global
some_unused_var = 42;

var x;

// Write-only variables are not considered as used.
var y = 10;
y = 5;

// A read for a modification of itself is not considered as used.
var z = 0;
z = z + 1;

// By default, unused arguments cause warnings.
(function(foo) {
    return 5;
})();

// Unused recursive functions also cause warnings.
function fact(n) {
    if (n < 2) return 1;
    return n * fact(n - 1);
}

// When a function definition destructures an array, unused entries from the array also cause warnings.
function getY([x, y]) {
    return y;
}

Comma Spacing

Putting default parameter at last allows function calls to omit optional tail arguments.

https://eslint.org/docs/rules/comma-spacing#options

👍 Examples of correct code

var foo = 1, bar = 2
    , baz = 3;
var arr = [1, 2];
var arr = [1,, 3]
var obj = {"foo": "bar", "baz": "qur"};
foo(a, b);
new Foo(a, b);
function foo(a, b){}
a, b

👎 Examples of incorrect code

var foo = 1 ,bar = 2;
var arr = [1 , 2];
var obj = {"foo": "bar" ,"baz": "qur"};
foo(a ,b);
new Foo(a ,b);
function foo(a ,b){}
a ,b

Comma Dangle

This rule enforces consistent use of trailing commas in object and array literals.

https://eslint.org/docs/rules/comma-dangle#comma-dangle https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/comma-dangle.md

👍 Examples of correct code

var foo = {
    bar: "baz",
    qux: "quux",
    bar: "baz",
};
function baz(
    a,
    b,
    c,
) {
    // code ...
}

👎 Examples of incorrect code

var foo = {
    bar: "baz",
    qux: "quux",
    bar: "baz"
};
function baz(
    a,
    b,
    c
) {
    // code ...
}

Arrow Spacing

This rule normalize style of spacing before/after an arrow function’s arrow(=>).

https://eslint.org/docs/latest/rules/arrow-spacing

👍 Examples of correct code

() => {};
(a) => {};
() => {'\n'};

👎 Examples of incorrect code

()=> {};
() =>{};
(a)=> {};
(a) =>{};
a =>a;
a=> a;
()=> {'\n'};
() =>{'\n'};

Prefer Arrow Function

Requires using arrow functions for callbacks.

https://eslint.org/docs/rules/prefer-arrow-callback#prefer-arrow-callback

👍 Examples of correct code

foo(a => a);
foo(() => this.a);
foo(function*() { yield; });

👎 Examples of incorrect code

foo(function(a) { return a; });
foo(function() { return this.a; }.bind(this));

Prefer Destructuring

Require destructuring from arrays and/or objects

https://eslint.org/docs/latest/rules/prefer-destructuring https://sonarsource.github.io/rspec/#/rspec/S3514/javascript

👍 Examples of correct code

var [ foo ] = array;
var foo = array[someIndex];

var { foo } = object;

var foo = object.bar;

let foo;
({ foo } = object);

👎 Examples of incorrect code

// With `array` enabled
var foo = array[0];

// With `object` enabled
var foo = object.foo;
var foo = object['foo'];

Arrow Function Body

Enforces no braces where they can be omitted

https://eslint.org/docs/rules/arrow-body-style#arrow-body-style

👍 Examples of correct code

() => {};
(a) => {};
(a) => a;
(a) => {'\n'}
a.then((foo) => {});
a.then((foo) => { if (true) {} });

👎 Examples of incorrect code

a => {};
a => a;
a => {'\n'};
a.then(foo => {});
a.then(foo => a);
a(foo => { if (true) {} });

Arrow Function Parentheses

Enforces parentheses around arguments in all cases.

https://eslint.org/docs/rules/arrow-parens

👍 Examples of correct code

let foo = () => 0;
let foo = (retv, name) => {
    retv[name] = true;
    return retv;
};
let foo = () => ({
    bar: {
        foo: 1,
        bar: 2,
    }
});
let foo = () => { bar(); };
let foo = () => {};
let foo = () => { /* do nothing */ };
let foo = () => {
    // do nothing.
};
let foo = () => ({ bar: 0 });

👎 Examples of incorrect code

let foo = () => {
    return 0;
};
let foo = () => {
    return {
       bar: {
            foo: 1,
            bar: 2,
        }
    };
};

Arrow Function No Break Line

Enforces parentheses around arguments in all cases.

https://eslint.org/docs/rules/arrow-parens

👍 Examples of correct code

(foo) => bar;

(foo) => (bar);

(foo) => bar => baz;

(foo) => (
  bar()
);

// functions with block bodies allowed with this rule using any style
// to enforce a consistent location for this case, see the rule: `brace-style`
(foo) => {
  return bar();
}

(foo) =>
{
  return bar();
}

👎 Examples of incorrect code

(foo) =>
  bar;

(foo) =>
  (bar);

(foo) =>
  bar =>
    baz;

(foo) =>
(
  bar()
);

No Empty Block

Disallows empty block statements.

https://eslint.org/docs/rules/no-empty#no-empty

👍 Examples of correct code

if (!foo) {
    // code
}

while (foo) {
    // code
}

try {
    doSomething();
} catch (ex) {
    // continue regardless of error
}

try {
    doSomething();
} finally {
    /* continue regardless of error */
}

👎 Examples of incorrect code

if (foo) {
} else {
  // code
}

while (foo) {
}

switch(foo) {
}

try {
    doSomething();
} catch(ex) {

} finally {

}

No Long Syntax

Disallow Array constructors

https://eslint.org/docs/latest/rules/no-array-constructor

👍 Examples of correct code

const arr: Array<number> = [ 1, 2, 3 ];
const arr: Array<Foo> = [ x, y, z ];

Array(500);
new Array(someOtherArray.length);

👎 Examples of incorrect code

const arr = Array(0, 1, 2);
const arr = new Array(0, 1, 2);

Useless Parens

Disallows unnecessary parentheses.

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/no-extra-parens.md https://eslint.org/docs/rules/no-extra-parens#no-extra-parens

👍 Examples of correct code

a = (b * c);

(a * b) + c;

for (a in (b, c));

for (a in (b));

for (a of (b));

typeof (a);

(function(){} ? a() : b());

class A {
    [(x)] = 1;
}

class B {
    x = (y + z);
}

👎 Examples of incorrect code

a = (b * c);

(a * b) + c;

for (a in (b, c));

for (a in (b));

for (a of (b));

typeof (a);

(function(){} ? a() : b());

class A {
    [(x)] = 1;
}

class B {
    x = (y + z);
}

Useless Boolean

Disallow useless code

https://eslint.org/docs/rules/no-useless-constructor#options

👍 Examples of correct code

var foo = !!bar;
var foo = Boolean(bar);

function foo() {
    return !!bar;
}

var foo = bar ? !!baz : !!bat;

👎 Examples of incorrect code

var foo = !!!bar;

var foo = !!bar ? baz : bat;

var foo = Boolean(!!bar);

var foo = new Boolean(!!bar);

if (!!foo) {
    // ...
}

if (Boolean(foo)) {
    // ...
}

while (!!foo) {
    // ...
}

do {
    // ...
} while (Boolean(foo));

for ( ;!!foo; ) {
    // ...
}

Useless Alias

Disallows renaming import, export, and destructured assignments to the same name.

https://eslint.org/docs/rules/no-useless-rename

👍 Examples of correct code

import * as foo from "foo";
import { foo } from "bar";
import { foo as bar } from "baz";
import { "foo" as bar } from "baz";

export { foo };
export { foo as bar };
export { foo as bar } from "foo";

let { foo } = bar;
let { foo: bar } = baz;
let { [foo]: foo } = bar;

function foo({ bar }) {}
function foo({ bar: baz }) {}

({ foo }) => {}
({ foo: bar }) => {}

👎 Examples of incorrect code

import { foo as foo } from "bar";
import { "foo" as foo } from "bar";
export { foo as foo };
export { foo as "foo" };
export { foo as foo } from "bar";
export { "foo" as "foo" } from "bar";
let { foo: foo } = bar;
let { 'foo': foo } = bar;
function foo({ bar: bar }) {}
({ foo: foo }) => {}

Return New line

Force new line before return

https://eslint.org/docs/rules/newline-before-return#newline-before-return

👍 Examples of correct code

function foo(bar) {
  var baz = 'baz';

  if (bar()) {
    return true;
  }

  if (!bar) {
    bar = baz;

    return baz;
  }

  return bar;
}

👎 Examples of incorrect code

function foo(bar) {
  var baz = 'baz';
  if (bar()) {
    return true;
  }
  if (!bar) {
    bar = baz;
    return bar;
  }
  return bar;
}

Comment Multi Line Prefer

Prefer Multi-line comment formated

https://eslint.org/docs/rules/newline-before-return#newline-before-return

👍 Examples of correct code

/*
 * this line
 * calls foo()
 */
foo();

// single-line comment

👎 Examples of incorrect code

// this line
// calls foo()
foo();

/* this line
calls foo() */
foo();

/* this comment
 * is missing a newline after /*
 */

/*
 * this comment
 * is missing a newline at the end */

/*
* the star in this line should have a space before it
 */

/*
 * the star on the following line should have a space before it
*/

No throw Literal

Create custom class to Throw

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/no-throw-literal.md https://eslint.org/docs/rules/prefer-promise-reject-errors#prefer-promise-reject-errors

👍 Examples of correct code

class CustomError extends Error {
  // ...
};

const e = new CustomError("error");
throw e;

throw new CustomError("error");

function err() {
  return new CustomError();
}
throw err();

const foo = {
  bar: new CustomError();
}
throw foo.bar;

// promises

Promise.reject(new CustomError("something bad happened"));

Promise.reject(new TypeError("something bad happened"));

new Promise(function(resolve, reject) {
  reject(new CustomError("something bad happened"));
});

var foo = getUnknownValue();
Promise.reject(foo);

👎 Examples of incorrect code

throw new Error();

throw 'error';

throw 0;

throw undefined;

throw null;

const err = new Error();
throw 'an ' + err;

const err = new Error();
throw `${err}`;

const err = '';
throw err;

function err() {
  return '';
}
throw err();

const foo = {
  bar: '',
};
throw foo.bar;

// Promise

Promise.reject("something bad happened");

Promise.reject(5);

Promise.reject();

new Promise(function(resolve, reject) {
  reject("something bad happened");
});

new Promise(function(resolve, reject) {
  reject();
});

No Unreachable

No Unreachable code

https://eslint.org/docs/rules/no-unreachable https://sonarsource.github.io/rspec/#/rspec/S6079/javascript

👍 Examples of correct code

function foo() {
    function bar() {
        return 1;
    }

    return bar();

}

function bar() {
    var x;
    return x;
}

switch (foo) {
    case 1:
        break;
}

👎 Examples of incorrect code

function foo() {
    return true;
    console.log("done");
}

function bar() {
    throw new Error("Oops!");
    console.log("done");
}

while(value) {
    break;
    console.log("done");
}

throw new Error("Oops!");
console.log("done");

function baz() {
    if (Math.random() < 0.5) {
        return;
    } else {
        throw new Error();
    }
    console.log("done");
}

for (;;) {}
console.log("done");

No Multiline String

Prevent break line in string

https://eslint.org/docs/rules/no-multi-str#no-multi-str

👍 Examples of correct code

var x = "some very\nlong text";

var x = "some very " +
        "long text";

👎 Examples of incorrect code

var x = "some very \
long text";

No Unsafe Assign

Disallows assigning any to variables and properties.

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/no-unsafe-assignment.md

👍 Examples of correct code

const x = 1,
  y = 1;
const [x] = [1];
[x] = [1] as [number];

function foo(a = 1) {}
class Foo {
  constructor(private a = 1) {}
}
class Foo {
  private a = 1;
}

// generic position examples
const x: Set<string> = new Set<string>();
const x: Map<string, string> = new Map<string, string>();
const x: Set<string[]> = new Set<string[]>();
const x: Set<Set<Set<string>>> = new Set<Set<Set<string>>>();

👎 Examples of incorrect code

const x = 1 as any,
  y = 1 as any;
const [x] = 1 as any;
const [x] = [] as any[];
const [x] = [1 as any];
[x] = [1] as [any];

function foo(a = 1 as any) {}
class Foo {
  constructor(private a = 1 as any) {}
}
class Foo {
  private a = 1 as any;
}

// generic position examples
const x: Set<string> = new Set<any>();
const x: Map<string, string> = new Map<string, any>();
const x: Set<string[]> = new Set<any[]>();
const x: Set<Set<Set<string>>> = new Set<Set<Set<any>>>();

Disallow Script Url

Using javascript: URLs is considered by some as a form of eval.

https://eslint.org/docs/rules/no-script-url

👍 Examples of correct code

location.href = "#";

👎 Examples of incorrect code

location.href = "javascript:void(0)";

location.href = `javascript:void(0)`;

Disallow Undefined

Disallows the use of undeclared variables unless mentioned in /*global*/ comments.

https://eslint.org/docs/rules/no-undef

👍 Examples of correct code

/* global someFunction, a */

var foo = someFunction();
var bar = a + 1;

👎 Examples of incorrect code

var foo = someFunction();
var bar = a + 1;

Function Name

Requires function expressions to have a name, if the name isn't assigned automatically per the ECMAScript specification.

https://eslint.org/docs/rules/func-names https://sonarsource.github.io/rspec/#/rspec/S100/javascript

👍 Examples of correct code

/* global someFunction, a */

var foo = someFunction();
var bar = a + 1;

👎 Examples of incorrect code

Foo.prototype.bar = function() {};

(function() {
    // ...
}())

export default function() {}

Function Name Match

This rule requires function names to match the name of the variable or property to which they are assigned. The rule will ignore property assignments where the property name is a literal that is not a valid identifier in the ECMAScript version specified in your configuration (default ES5).

https://eslint.org/docs/latest/rules/func-name-matching

👍 Examples of correct code

var foo = function foo() {};
var foo = function() {};
var foo = () => {};
foo = function foo() {};

obj.foo = function foo() {};
obj['foo'] = function foo() {};
obj['foo//bar'] = function foo() {};
obj[foo] = function bar() {};

var obj = {foo: function foo() {}};
var obj = {[foo]: function bar() {}};
var obj = {'foo//bar': function foo() {}};
var obj = {foo: function() {}};

obj['x' + 2] = function bar(){};
var [ bar ] = [ function bar(){} ];
({[foo]: function bar() {}})

class C {
    foo = function foo() {};
    baz = function() {};
}

// private names are ignored
class D {
    #foo = function foo() {};
    #bar = function foo() {};
    baz() {
        this.#foo = function foo() {};
        this.#foo = function bar() {};
    }
}

module.exports = function foo(name) {};
module['exports'] = function foo(name) {};

👎 Examples of incorrect code

var foo = function bar() {};
foo = function bar() {};
obj.foo = function bar() {};
obj['foo'] = function bar() {};
var obj = {foo: function bar() {}};
({['foo']: function bar() {}});

class C {
    foo = function bar() {};
}

No Use Future Reserved Words

"future reserved words" should not be used as identifiers Special identifiers should not be bound or assigned

https://sonarsource.github.io/rspec/#/rspec/S2137/javascript

👍 Examples of correct code

var elements = document.getElementsByName("foo"); // Compliant
var someData = { package: true };

result = 17;
++result;
var obj = { set p(arg) { } };
var result;
try { } catch (args) { }
function x(arg) { }
function args() { }
var y = function fun() { };
var f = new Function("args", "return 17;");

👎 Examples of incorrect code

var package = document.getElementsByName("foo"); // Noncompliant
eval = 17; // Noncompliant
arguments++; // Noncompliant
++eval; // Noncompliant
var obj = { set p(arguments) { } }; // Noncompliant
var eval; // Noncompliant
try { } catch (arguments) { } // Noncompliant
function x(eval) { } // Noncompliant
function arguments() { } // Noncompliant
var y = function eval() { }; // Noncompliant
var f = new Function("arguments", "return 17;"); // Noncompliant

No Generator Without Yield

Generators should "yield" something

https://sonarsource.github.io/rspec/#/rspec/S3531/javascript

👍 Examples of correct code

function* myGen(a, b) {
  let answer = 0;
  while (answer < 42) {
    answer += a * b;
    yield answer;
  }
}

👎 Examples of incorrect code

function* myGen(a, b) {  // Noncompliant
  let answer = 0;
  answer += a * b;
}

Inverted Assertion Arguments

Assertion arguments should be passed in the correct order

https://sonarsource.github.io/rspec/#/rspec/S3415/javascript

👍 Examples of correct code

const assert = require('chai').assert;
const expect = require('chai').expect;
const should = require('chai').should();

it("inverts arguments", function() {
    assert.equal(aNumber, 42);
    expect(aNumber).to.equal(42);
    should.fail(aNumber, 42);
});

👎 Examples of incorrect code

const assert = require('chai').assert;
const expect = require('chai').expect;
const should = require('chai').should();

it("inverts arguments", function() {
    assert.equal(42, aNumber); // Noncompliant
    expect(42).to.equal(aNumber); // Noncompliant
    should.fail(42, aNumber);  // Noncompliant
});

Max Union Size

Union types should not have too many elements

https://sonarsource.github.io/rspec/#/rspec/S4622/javascript

👍 Examples of correct code

type MyUnionType = MyType1 | MyType2 | MyType3 | MyType4; // Compliant, "type" statements are ignored
let x: MyUnionType;

function foo(value: string, padding: MyUnionType) {
    // ...
}

👎 Examples of incorrect code

let x: MyType1 | MyType2 | MyType3 | MyType4; // Noncompliant

function foo(p1: string, p2: MyType1 | MyType2 | MyType3 | MyType4) { // Noncompliant
    // ...
}

No Redundant Optional

Optional property declarations should not use both '?' and 'undefined' syntax

https://sonarsource.github.io/rspec/#/rspec/S4782/javascript

👍 Examples of correct code

interface Person {
  name: string;
  address: string | undefined;
  pet?: Animal;
}

👎 Examples of incorrect code

interface Person {
  name: string;
  address? : string | undefined;   // Noncompliant, "?" should be removed
  pet?: Animal | undefined; // Noncompliant, "undefined" should be removed
}

Prefer Type Guard

Type guards should be used

https://sonarsource.github.io/rspec/#/rspec/S4322/javascript

👍 Examples of correct code

function isSomething(x: BaseType) : x is Something {
  return (<Something>x).foo !== undefined;
}

if (isSomething(v)) {
  v.foo();
}

👎 Examples of incorrect code

function isSomething(x: BaseType) : boolean { // Noncompliant
  return (<Something>x).foo !== undefined;
}

if (isSomething(v)) {
  (<Something>v).foo();
}

No Production Debug

Delivering code in production with debug features activated is security-sensitive

https://sonarsource.github.io/rspec/#/rspec/S4507/javascript

👍 Examples of correct code

const express = require('express');
const errorhandler = require('errorhandler');

let app = express();

if (process.env.NODE_ENV === 'development') {  // Compliant
  app.use(errorhandler());  // Compliant
}

👎 Examples of incorrect code

const express = require('express');
const errorhandler = require('errorhandler');

let app = express();
app.use(errorhandler()); // Sensitive

Unused Named Groups

Why use named groups only to never use any of them later on in the code?

This rule raises issues every time named groups are: defined but never called anywhere in the code through their name; defined but called elsewhere in the code by their number instead; referenced while not defined.

https://sonarsource.github.io/rspec/#/rspec/S5860/javascript

👍 Examples of correct code

const date = "01/02";

const datePattern = /(?<month>[0-9]{2})\/(?<year>[0-9]{2})/;
const dateMatched = date.match(datePattern);

if (dateMatched !== null) {
  checkValidity(dateMatched.groups.month, dateMatched.groups.year);
}

// ...

const score = "14:1";

const scorePattern = /(?<player1>[0-9]+):(?<player2>[0-9]+)/;
const scoreMatched = score.match(scorePattern);

if (scoreMatched !== null) {
  checkScore(scoreMatched.groups.player1);
  checkScore(scoreMatched.groups.player2);
}

👎 Examples of incorrect code

const date = "01/02";

const datePattern = /(?<month>[0-9]{2})\/(?<year>[0-9]{2})/;
const dateMatched = date.match(datePattern);

if (dateMatched !== null) {
  checkValidity(dateMatched[1], dateMatched[2]); // Noncompliant - numbers instead of names of groups are used
  checkValidity(dateMatched.groups.day); // Noncompliant - there is no group called "day"
}

// ...

const score = "14:1";

const scorePattern = /(?<player1>[0-9]+):(?<player2>[0-9]+)/; // Noncompliant - named groups are never used
const scoreMatched = score.match(scorePattern);

if (scoreMatched !== null) {
  checkScore(score);
}

Get And Setters

A getter and setter for the same property don’t necessarily have to be defined adjacent to each other.

https://eslint.org/docs/latest/rules/grouped-accessor-pairs

👍 Examples of correct code

var foo = {
    get a() {
        return this.val;
    },
    set a(value) {
        this.val = value;
    },
    b: 1
};

var bar = {
    set b(value) {
        this.val = value;
    },
    get b() {
        return this.val;
    },
    a: 1
}

class Foo {
    set a(value) {
        this.val = value;
    }
    get a() {
        return this.val;
    }
    b(){}
}

const Bar = class {
    static get a() {
        return this.val;
    }
    static set a(value) {
        this.val = value;
    }
}

👎 Examples of incorrect code

var foo = {
    get a() {
        return this.val;
    },
    b: 1,
    set a(value) {
        this.val = value;
    }
};

var bar = {
    set b(value) {
        this.val = value;
    },
    a: 1,
    get b() {
        return this.val;
    }
}

class Foo {
    set a(value) {
        this.val = value;
    }
    b(){}
    get a() {
        return this.val;
    }
}

const Bar = class {
    static get a() {
        return this.val;
    }
    b(){}
    static set a(value) {
        this.val = value;
    }
}

Function Style

Enforce the consistent use of either function declarations or expressions

https://eslint.org/docs/latest/rules/func-style

👍 Examples of correct code

function foo() {
    // ...
}

// Methods (functions assigned to objects) are not checked by this rule
SomeObject.foo = function() {
    // ...
};

👎 Examples of incorrect code

var foo = function() {
    // ...
};

var foo = () => {};

No Else Return

Disallow else blocks after return statements in if statements

https://eslint.org/docs/latest/rules/no-else-return

👍 Examples of correct code

function foo() {
    if (x) {
        return y;
    }

    return z;
}

function foo() {
    if (x) {
        return y;
    } else if (z) {
        var t = "foo";
    } else {
        return w;
    }
}

function foo() {
    if (x) {
        if (z) {
            return y;
        }
    } else {
        return z;
    }
}

function foo() {
    if (error) {
        return 'It failed';
    } else if (loading) {
        return "It's still loading";
    }
}

👎 Examples of incorrect code

function foo() {
    if (x) {
        return y;
    } else {
        return z;
    }
}

function foo() {
    if (x) {
        return y;
    } else if (z) {
        return w;
    } else {
        return t;
    }
}

function foo() {
    if (x) {
        return y;
    } else {
        var t = "foo";
    }

    return t;
}

function foo() {
    if (error) {
        return 'It failed';
    } else {
        if (loading) {
            return "It's still loading";
        }
    }
}

// Two warnings for nested occurrences
function foo() {
    if (x) {
        if (y) {
            return y;
        } else {
            return x;
        }
    } else {
        return z;
    }
}

No Console Spaces

The console.log() method and similar methods joins the parameters with a space, so adding a leading/trailing space to a parameter, results in two spaces being added.

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/no-console-spaces.md

👍 Examples of correct code

console.log('abc');
console.log('abc', 'def');

console.log('abc ');
console.log(' abc');

console.log('abc  ', 'def');
console.log('abc\t', 'def');
console.log('abc\n', 'def');

console.log(`
    abc
`);

👎 Examples of incorrect code

console.log('abc ', 'def');
console.log('abc', ' def');

console.log("abc ", " def");
console.log(`abc `, ` def`);

console.debug('abc ', 'def');
console.info('abc ', 'def');
console.warn('abc ', 'def');
console.error('abc ', 'def');

No Hex Escape

Enforce the use of Unicode escapes instead of hexadecimal escapes

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/no-hex-escape.md

👍 Examples of correct code

const foo = '\u001B';
const foo = `\u001B${bar}`;

👎 Examples of incorrect code

const foo = '\x1B';
const foo = `\x1B${bar}`;

Prefer Array Flat Map

Prefer .flatMap(…) over .map(…).flat()

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/prefer-array-flat-map.md

👍 Examples of correct code

const foo = bar.flatMap(element => unicorn(element));
const foo = bar.map(element => unicorn(element)).flat(2);
const foo = bar.map(element => unicorn(element)).foo().flat();
const foo = bar.flat().map(element => unicorn(element));

👎 Examples of incorrect code

const foo = bar.map(element => unicorn(element)).flat();
const foo = bar.map(element => unicorn(element)).flat(1);

Prefer String Slice

Prefer String#slice() over String#substr() and String#substring()

String#substr() and String#substring() are the two lesser known legacy ways to slice a string. It's better to use String#slice() as it's a more popular option with clearer behavior that has a consistent Array counterpart.

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/prefer-string-slice.md

👍 Examples of correct code

foo.slice(beginIndex, endIndex);

👎 Examples of incorrect code

foo.substr(start, length);
foo.substring(indexStart, indexEnd);

Prefer Modern DOM

Prefer .before() over .insertBefore(), .replaceWith() over .replaceChild(), prefer one of .before(), .after(), .append() or .prepend() over insertAdjacentText() and insertAdjacentElement()

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/prefer-modern-dom-apis.md

👍 Examples of correct code

foo.replaceWith(bar);
foo.replaceWith('bar');
foo.replaceWith(bar, 'baz'));

foo.before(bar)
foo.before('bar')
foo.before(bar, 'baz')

foo.prepend(bar)
foo.prepend('bar')
foo.prepend(bar, 'baz')

foo.append(bar)
foo.append('bar')
foo.append(bar, 'baz')

foo.after(bar)
foo.after('bar')
foo.after(bar, 'baz')

👎 Examples of incorrect code

foo.replaceChild(baz, bar);

foo.insertBefore(baz, bar);

foo.insertAdjacentText('position', bar);

foo.insertAdjacentElement('position', bar);

Prefer Prefix Number

Prefer Number static properties over global ones

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/prefer-number-properties.md

👍 Examples of correct code

const foo = Number.parseInt('10', 2);

const foo = Number.parseFloat('10.5');

const foo = Number.isNaN(10);

const foo = Number.isFinite(10);

if (Object.is(foo, Number.NaN)) {}

const isPositiveZero = value => value === 0 && 1 / value === Number.POSITIVE_INFINITY;

const isNegativeZero = value => value === 0 && 1 / value === Number.NEGATIVE_INFINITY;

👎 Examples of incorrect code

const foo = parseInt('10', 2);

const foo = parseFloat('10.5');

const foo = isNaN(10);

const foo = isFinite(10);

if (Object.is(foo, NaN)) {}

const isPositiveZero = value => value === 0 && 1 / value === Infinity;

const isNegativeZero = value => value === 0 && 1 / value === -Infinity;

const {parseInt} = Number;
const foo = parseInt('10', 2);

Numeric Separators Style

Enforce the style of numeric separators by correctly grouping digits

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/numeric-separators-style.md

👍 Examples of correct code

const foo = 1_234_444;
const foo = 1_234.567_89;
const foo = 0xAB_CD_EF;
const foo = 0b1000_1111;
const foo = 0o10_4421;
const foo = 1_294_287_712n;

👎 Examples of incorrect code

const foo = 1_23_4444;
const foo = 1_234.56789;
const foo = 0xAB_C_D_EF;
const foo = 0b10_00_1111;
const foo = 0o1_0_44_21;
const foo = 1_294_28771_2n;

Prefer Default Parame

Prefer default parameters over reassignment

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/prefer-default-parameters.md

👍 Examples of correct code

function abc(foo = 'bar') {}
function abc(foo) {
    const parameter = foo || bar();
}

👎 Examples of incorrect code

function abc(foo) {
    foo = foo || 'bar';
}
function abc(foo) {
    const bar = foo || 'bar';
}

No Avoid Reverse

Prefer Array#flat() over legacy techniques to flatten arrays

https://github.com/freaktechnik/eslint-plugin-array-func#avoid-reverse

👍 Examples of correct code

const string = array.reduceRight((p, c) => p + c, "");

const reverseString = array.reduce((p, c) => p + c, "");

👎 Examples of incorrect code

const string = array.reverse().reduce((p, c) => p + c, '');

const reverseString = array.reverse().reduceRight((p, c) => p + c, '');

Bool Param Default

Optional boolean parameters should have default value

https://sonarsource.github.io/rspec/#/rspec/S4798/javascript

👍 Examples of correct code

function countPositiveNumbers(arr: number[], countZero = false) {
  // ...
}

function toggleProperty(property: string, value: boolean) {
  setProperty(property, value);
}

function togglePropertyToCalculatedValue(property: string) {
  setProperty(property, calculateProperty());
}

👎 Examples of incorrect code

// Noncompliant, default value for 'countZero' should be defined
function countPositiveNumbers(arr: number[], countZero?: boolean) {
  // ...
}

function toggleProperty(property: string, value?: boolean) { // Noncompliant, a new function should be defined
  if (value !== undefined) {
    setProperty(property, value);
  } else {
    setProperty(property, calculateProperty());
  }
}

Class Name Convention

Class names should comply with a naming convention

https://sonarsource.github.io/rspec/#/rspec/S101/javascript

👍 Examples of correct code

// With default provided regular expression /^[A-Z][a-zA-Z0-9]*$/:
class MyClass { }

👎 Examples of incorrect code

class my_class { }

No Class prototype

Class methods should be used instead of "prototype" assignments

https://sonarsource.github.io/rspec/#/rspec/S3525/javascript

👍 Examples of correct code

class MyClass {
  constructor(initializerArgs = []) {
    this._values = [...initializerArgs];
  }

  doSomething() {
    //...
  }
}

👎 Examples of incorrect code

function MyNonClass(initializerArgs = []) {
  this._values = [...initializerArgs];
}

MyNonClass.prototype.doSomething = function () {  // Noncompliant
  // ...
}

Comma Or Logical Or Case

Comma and logical OR operators should not be used in switch cases

https://sonarsource.github.io/rspec/#/rspec/S3616/javascript

👍 Examples of correct code

switch (a) {
  case 1:
  case 2:
    doTheThing(a);
  case 3:
  case 4:
    doThatThing(a);
  case 5:
    doTheOtherThing(a);
  default:
    console.log('Neener, neener!');
}

👎 Examples of incorrect code

switch (a) {
  case 1,2:  // Noncompliant; only 2 is ever handled by this case
    doTheThing(a);
  case 3 || 4: // Noncompliant; only '3' is handled
    doThatThing(a);
  case 5:
    doTheOtherThing(a);
  default:
    console.log('Neener, neener!');  // this happens when a==1 or a == 4
}

No Constructor Bind

Prefer class properties to equivalent setup steps taken in a class' constructor method.

https://github.com/markalfred/eslint-plugin-no-constructor-bind

👍 Examples of correct code

class User {
  greet = () => 'hello'
}

👎 Examples of incorrect code

class User {
  constructor() {
    this.greet = this.greet.bind(this)
  }
  greet() { return 'Hello' }
}

Prefer Code Point

Prefer String#codePointAt(…) over String#charCodeAt(…) and String.fromCodePoint(…) over String.fromCharCode(…)

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/prefer-code-point.md

👍 Examples of correct code

const unicorn = '🦄'.codePointAt(0).toString(16);
const unicorn = String.fromCodePoint(0x1f984);

👎 Examples of incorrect code

const unicorn = '🦄'.charCodeAt(0).toString(16);
const unicorn = String.fromCharCode(0x1f984);

No Thenable

If an object is defined as "thenable", once it's accidentally used in an await expression, it may cause problems:

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/no-thenable.md

👍 Examples of correct code

export {then as success};
const foo = {
    success() {}
};
class Foo {
    success() {}
}
const foo = bar.then;

👎 Examples of incorrect code

export {then};
const foo = {
    then() {}
};
const foo = {
    get then() {}
};
foo.then = function () {}
class Foo {
    then() {}
}
class Foo {
    static then() {}
}

No Unreadable Iife

IIFE with parenthesized arrow function body is considered unreadable.

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/no-unreadable-iife.md

👍 Examples of correct code

const bar = getBar();
const foo = bar ? bar.baz : baz;

const getBaz = bar => (bar ? bar.baz : baz);
const foo = getBaz(getBar());

const foo = (bar => {
    return bar ? bar.baz : baz;
})(getBar());

👎 Examples of incorrect code

const foo = (bar => (bar ? bar.baz : baz))(getBar());
const foo = ((bar, baz) => ({bar, baz}))(bar, baz);

Prefer Native Cast

Prefer using String, Number, BigInt, Boolean, and Symbol directly

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/prefer-native-coercion-functions.md

👍 Examples of correct code

const toBoolean = Boolean;

if (Number(foo) === 1) {}

const hasTruthyValue = array.some(Boolean);

const toStringObject = value => new String(value);

const toObject = value => Object(value);

👎 Examples of incorrect code

const toBoolean = value => Boolean(value);
function toNumber(value) {
    return Number(value);
}

if (toNumber(foo) === 1) {}
const hasTruthyValue = array.some(element => element);

Prefer Logical Operator Over Ternary

Prefer using a logical operator over a ternary

Ideally, most reported cases have an equivalent Logical OR(||) expression. The rule intentionally provides suggestions instead of auto-fixes, because in many cases, the nullish coalescing operator (??) should be preferred.

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/prefer-logical-operator-over-ternary.md

👍 Examples of correct code

foo ?? bar;
foo || bar;
foo ? bar : baz;
foo.bar ?? foo.baz
foo?.bar ?? baz

👎 Examples of incorrect code

foo ? foo : bar;
foo.bar ? foo.bar : foo.baz
foo?.bar ? foo.bar : baz
!bar ? foo : bar;

Prefer Event Target

Prefer EventTarget over EventEmitter

While EventEmitter is only available in Node.js, EventTarget is also available in Deno and browsers.

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/prefer-event-target.md

👍 Examples of correct code

class Foo extends EventTarget {}

const target = new EventTarget();

👎 Examples of incorrect code

import {EventEmitter} from 'node:event';
class Foo extends EventEmitter {}

const emitter = new EventEmitter();

Prefer Object From Entries

Prefer using Object.fromEntries(…) to transform a list of key-value pairs into an object

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/prefer-object-from-entries.md

👍 Examples of correct code

const object = Object.fromEntries(pairs);

const object = new Map(pairs);

👎 Examples of incorrect code

const object = pairs.reduce(
    (object, [key, value]) => ({...object, [key]: value}),
    {}
);

const object = pairs.reduce(
    (object, [key, value]) => ({...object, [key]: value}),
    Object.create(null)
);

const object = pairs.reduce(
    (object, [key, value]) => Object.assign(object, {[key]: value}),
    {}
);

const object = _.fromPairs(pairs);

Prefer Array From Map

Prefer using the mapFn callback of Array.from over an immediate .map() call on the Array.from result.

https://github.com/freaktechnik/eslint-plugin-array-func#from-map

👍 Examples of correct code

Array.from(iterable).map((t) => t.id);

Array.from(iterable, (t) => t.id).map((id) => id[0]);

👎 Examples of incorrect code

Array.from(iterable, (t) => t.id);

Array.from(iterable, function(t) { this.format(t); }, this);

const arr = Array.from(iterable);
const mappedArray = arr.map((t) => t.id);

Prefer Array Flat

Use .flat() to flatten an array of arrays. This rule currently recognizes two patterns and can replace them with a .flat() call:

https://github.com/freaktechnik/eslint-plugin-array-func#prefer-flat https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/prefer-array-flat.md

👍 Examples of correct code

const concatFlat = array.flat();

const reverseFlat = array.reduce((p, n) => n.concat(p), []);

const otherReduce = array.reduce((p, n) => n + p, 0);

👎 Examples of incorrect code

const concatFlat = [].concat(...array);

const reduceFlat = array.reduce((p, n) => p.concat(n), []);

This Pattern

Enforces consistent naming when capturing the current execution context.
Disallows this keywords outside of classes or class-like objects.

https://eslint.org/docs/rules/consistent-this#consistent-this

👍 Examples of correct code

var bar = function() {};

const cat = {
  meow: function() {}
}

class C {
    #bar = function() {};
    baz = function() {};
}

quux ??= function() {};

(function bar() {
    // ...
}())

export default function foo() {}

👎 Examples of incorrect code

var that = 42;

var self = this;

that = 42;

self = this;

this.a = 0;
baz(() => this);

(function() {
    this.a = 0;
    baz(() => this);
})();

function foo() {
    this.a = 0;
    baz(() => this);
}

var foo = function() {
    this.a = 0;
    baz(() => this);
};

Use Dot

Enforces use dot notation

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/dot-notation.md https://eslint.org/docs/rules/dot-notation

👍 Examples of correct code

var x = foo.bar;

var x = foo[bar];

👎 Examples of incorrect code

var x = foo["bar"];

Use This

Enforces that this is used when only this type is returned.

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/prefer-return-this-type.md

👍 Examples of correct code

class Foo {
  f1(): this {
    return this;
  }
  f2() {
    return this;
  }
  f3 = (): this => {
    return this;
  };
  f4 = () => {
    return this;
  };
}

class Base {}
class Derived extends Base {
  f(): Base {
    return this;
  }
}

👎 Examples of incorrect code

class Foo {
  f1(): Foo {
    return this;
  }
  f2 = (): Foo => {
    return this;
  };
  f3(): Foo | undefined {
    return Math.random() > 0.5 ? this : undefined;
  }
}

Use Is-Nan

Require calls to isNaN() when checking for NaN

https://eslint.org/docs/latest/rules/use-isnan

👍 Examples of correct code

if (isNaN(foo)) {
    // ...
}

if (!isNaN(foo)) {
    // ...
}

👎 Examples of incorrect code

if (foo == NaN) {
    // ...
}

if (foo != NaN) {
    // ...
}

if (foo == Number.NaN) {
    // ...
}

if (foo != Number.NaN) {
    // ...
}

Dot Object Format

The dot in a member expression should be on the same line as the property portion.
Disallows whitespace before properties.

https://eslint.org/docs/rules/dot-location#dot-location https://eslint.org/docs/rules/no-whitespace-before-property

👍 Examples of correct code

foo.bar

foo[bar]

foo[ bar ]

foo.bar.baz

foo
  .bar().baz()

foo
  .bar()
  .baz()

👎 Examples of incorrect code

foo [bar]

foo. bar

foo .bar

foo. bar. baz

foo. bar()
  .baz()

foo
  .bar(). baz()

No Trailing Space

Not allow trailing whitespace

https://eslint.org/docs/rules/dot-location#dot-location https://eslint.org/docs/rules/no-whitespace-before-property

👍 Examples of correct code

var foo = 0;
var baz = 5;

class A {

  b = 1;

}

👎 Examples of incorrect code

var foo = 0;//•••••
var baz = 5;//••

class A {
//••
  b = 1;
//••
}
//••

Type Format

Require consistent spacing around type annotations.

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/type-annotation-spacing.md

👍 Examples of correct code

let foo: string = "bar";

function foo(): string {}

class Foo {
    name: string;
}

type Foo = () => {};

👎 Examples of incorrect code

let foo:string = "bar";
let foo :string = "bar";
let foo : string = "bar";

function foo():string {}
function foo() :string {}
function foo() : string {}

class Foo {
    name:string;
}

class Foo {
    name :string;
}

class Foo {
    name : string;
}

type Foo = ()=>{};
type Foo = () =>{};
type Foo = ()=> {};

Max Statements Per Line

This rule enforces a maximum number of statements allowed per line.

https://eslint.org/docs/rules/max-statements-per-line

👍 Examples of correct code

var bar, baz;
if (condition) bar = 1;
for (var i = 0; i < length; ++i);
switch (discriminant) { default: }
function foo() { }
var foo = function foo() { };
(function foo() { })();

👎 Examples of incorrect code

var bar; var baz;
if (condition) { bar = 1; }
for (var i = 0; i < length; ++i) { bar = 1; }
switch (discriminant) { default: break; }
function foo() { bar = 1; }
var foo = function foo() { bar = 1; };
(function foo() { bar = 1; })();

No Constant Condition

Disallows constant expressions in conditions. always use variables instead.

https://eslint.org/docs/rules/no-constant-condition#no-constant-condition

👍 Examples of correct code

if (x === 0) {
    doSomething();
}

for (let i = 0; i < 2; i++) {

}

while (typeof x === "undefined") {
    doSomething();
}

do {
    doSomething();
} while (x);

var result = x !== 0 ? a : b;

👎 Examples of incorrect code

if (false) {
    doSomethingUnfinished();
}

if (void x) {
    doSomethingUnfinished();
}

if (x &&= false) {
    doSomethingNever();
}

if (class {}) {
    doSomethingAlways();
}

if (new Boolean(x)) {
    doSomethingAlways();
}

if (Boolean(1)) {
    doSomethingAlways();
}

if (undefined) {
    doSomethingUnfinished();
}

if (x ||= true) {
    doSomethingAlways();
}

for (;-2;) {
    doSomethingForever();
}

while (typeof x) {
    doSomethingForever();
}

do {
    doSomethingForever();
} while (x = -1);

var result = 0 ? a : b;

No Debugger

Debugger not recommended use break point.

https://eslint.org/docs/rules/no-debugger#no-debugger

👍 Examples of correct code

function isTruthy(x) {
    return Boolean(x); // set a breakpoint at this line
}

👎 Examples of incorrect code

function isTruthy(x) {
    debugger;
    return Boolean(x);
}

Not Duplicate Case

No Duplicate case in switch

https://eslint.org/docs/rules/no-duplicate-case#no-duplicate-case

👍 Examples of correct code

switch (a) {
    case 1:
        break;
    case 2:
        break;
    case 3:
        break;
    default:
        break;
}

switch (a) {
    case one:
        break;
    case two:
        break;
    case three:
        break;
    default:
        break;
}

switch (a) {
    case "1":
        break;
    case "2":
        break;
    case "3":
        break;
    default:
        break;

👎 Examples of incorrect code

switch (a) {
    case 1:
        break;
    case 2:
        break;
    case 1:         // duplicate test expression
        break;
    default:
        break;
}

switch (a) {
    case one:
        break;
    case 2:
        break;
    case one:         // duplicate test expression
        break;
    default:
        break;
}

switch (a) {
    case "1":
        break;
    case "2":
        break;
    case "1":         // duplicate test expression
        break;
    default:
        break;
}

Regex Block

Disallows empty character classes in regular expressions.
No Invalid Regex

https://eslint.org/docs/rules/no-empty-character-class#no-empty-character-class

👍 Examples of correct code

/^abc/.test("abcdefg"); // true
"abcdefg".match(/^abc/); // ["abc"]

/^abc[a-z]/.test("abcdefg"); // true
"abcdefg".match(/^abc[a-z]/); // ["abcd"]

// valid regex
RegExp('.')
new RegExp
this.RegExp('[')

👎 Examples of incorrect code

/^abc[]/.test("abcdefg"); // false
"abcdefg".match(/^abc[]/); // null

// invalid regex
RegExp('[')
RegExp('.', 'z')
new RegExp('\\')

No Overwrite Exception

Disallows empty character classes in regular expressions.

https://eslint.org/docs/rules/no-ex-assign#no-ex-assign

👍 Examples of correct code

try {
    // code
} catch (e) {
    var foo = 10;
}

👎 Examples of incorrect code

try {
    // code
} catch (e) {
    e = 10;
}

No Extra Semi

Disallows unnecessary semicolons.

https://eslint.org/docs/rules/no-extra-semi#no-extra-semi

👍 Examples of correct code

var x = 5;

function foo() {
    // code
}

var bar = function() {
    // code
};

class C {
    field;

    method() {
        // code
    }

    static {
        // code
    }
}

👎 Examples of incorrect code

var x = 5;;

function foo() {
    // code
};

class C {
    field;;

    method() {
        // code
    };

    static {
        // code
    };
};

No Function Overwrite

Disallows reassigning function declarations.

https://eslint.org/docs/rules/no-func-assign#no-func-assign

👍 Examples of correct code

var foo = function () {}
foo = bar;

function foo(foo) { // `foo` is shadowed.
}

👎 Examples of incorrect code

function foo() {}
foo = bar;

function foo() {
    foo = bar;
}

var a = function hello() {
  hello = 123;
};

No Delete Var

Disallow deleting variables.

https://eslint.org/docs/latest/rules/no-delete-var

👍 Examples of correct code

// don't remove

👎 Examples of incorrect code

var x;
delete x;

No Lone Blocks

Disallow unnecessary nested blocks

https://eslint.org/docs/latest/rules/no-lone-blocks

👍 Examples of correct code

while (foo) {
    bar();
}

if (foo) {
    if (bar) {
        baz();
    }
}

function bar() {
    baz();
}

{
    let x = 1;
}

{
    const y = 1;
}

{
    class Foo {}
}

aLabel: {
}

class C {
    static {
        lbl: {
            if (something) {
                break lbl;
            }

            foo();
        }
    }
}

👎 Examples of incorrect code

if (foo) {
    bar();
    {
        baz();
    }
}

function bar() {
    {
        baz();
    }
}

{
    function foo() {}
}

{
    aLabel: {
    }
}

class C {
    static {
        {
            foo();
        }
    }
}

No Proto

Disallow the use of the proto property

https://eslint.org/docs/latest/rules/no-proto

👍 Examples of correct code

var a = Object.getPrototypeOf(obj);

Object.setPrototypeOf(obj, b);

var c = { __proto__: a };

👎 Examples of incorrect code

var a = obj.__proto__;

var a = obj["__proto__"];

obj.__proto__ = b;

obj["__proto__"] = b;

No Declare in Block

Disallows variable or function declarations in nested blocks.

https://eslint.org/docs/rules/no-func-assign#no-func-assign

👍 Examples of correct code

function doSomething() { }

function doSomethingElse() {
    function doAnotherThing() { }
}

class C {
    static {
        function doSomething() { }
    }
}

if (test) {
    asyncCall(id, function (err, data) { });
}

var fn;
if (test) {
    fn = function fnExpression() { };
}

if (foo) var a;

👎 Examples of incorrect code

if (test) {
    function doSomething() { }
}

function doSomethingElse() {
    if (test) {
        function doAnotherThing() { }
    }
}

if (foo) function f(){}

class C {
    static {
        if (test) {
            function doSomething() { }
        }
    }
}

No Nonoctal Decimal Escape

Disallow \8 and \9 escape sequences in string literals

https://eslint.org/docs/latest/rules/no-nonoctal-decimal-escape

👍 Examples of correct code

"8";

"9";

var foo = "w8less";

var bar = "December 19";

var baz = "Don't use \\8 and \\9 escapes.";

var quux = "\0\u0038";

👎 Examples of incorrect code

"\8";

"\9";

var foo = "w\8less";

var bar = "December 1\9";

var baz = "Don't use \8 and \9 escapes.";

var quux = "\0\8";

No Import Absolute Path

Node.js allows the import of modules using an absolute path such as /home/xyz/file.js. That is a bad practice as it ties the code using it to your computer, and therefore makes it unusable in packages distributed on npm for instance.

https://github.com/import-js/eslint-plugin-import/blob/HEAD/docs/rules/no-absolute-path.md

👍 Examples of correct code

import _ from 'lodash';
import foo from 'foo';
import foo from './foo';

var _ = require('lodash');
var foo = require('foo');
var foo = require('./foo');

👎 Examples of incorrect code

import f from '/foo';
import f from '/some/path';

var f = require('/foo');
var f = require('/some/path');

No Webpack Loader Syntax

Forbid Webpack loader syntax in imports.

https://github.com/import-js/eslint-plugin-import/blob/HEAD/docs/rules/no-webpack-loader-syntax.md

👍 Examples of correct code

import myModule from 'my-module';
import theme from './theme.css';

var myModule = require('my-module');
var theme = require('./theme.css');

👎 Examples of incorrect code

import myModule from 'my-loader!my-module';
import theme from 'style!css!./theme.css';

var myModule = require('my-loader!./my-module');
var theme = require('style!css!./theme.css');

No Magic Number

Disallow magic numbers

https://eslint.org/docs/latest/rules/no-magic-numbers

👍 Examples of correct code

var TAX = 0.25;

var dutyFreePrice = 100,
    finalPrice = dutyFreePrice + (dutyFreePrice * TAX);

👎 Examples of incorrect code

var dutyFreePrice = 100,
    finalPrice = dutyFreePrice + (dutyFreePrice * 0.25);

// or

var data = ['foo', 'bar', 'baz'];

var dataLast = data[2];

Security Negation

Disallows variable or function declarations in nested blocks.

https://eslint.org/docs/rules/no-negated-in-lhs#no-negated-in-lhs

👍 Examples of correct code

if(!(key in object)) {
    // key is not in object
}

👎 Examples of incorrect code

if(!key in object) {
    // operator precedence makes it equivalent to (!key) in object
    // and type conversion makes it equivalent to (key ? "false" : "true") in object
}

Regex Space

Disallows multiple spaces in regular expression literals.

https://eslint.org/docs/rules/no-regex-spaces#no-regex-spaces

👍 Examples of correct code

var re = /foo {3}bar/;
var re = new RegExp("foo {3}bar");

// better
var re = /foo {3}bar/;
var re = new RegExp("foo\s{3}bar");

👎 Examples of incorrect code

var re = /foo   bar/;
var re = new RegExp("foo   bar");

Array No Space

Disallows sparse arrays.

https://eslint.org/docs/rules/no-sparse-arrays#no-sparse-arrays

👍 Examples of correct code

var items = [];
var items = new Array(23);

// trailing comma (after the last element) is not a problem
var colors = [ "red", "blue", ];

👎 Examples of incorrect code

var items = [,];
var colors = [ "red",, "blue" ];

Valid Type-Of

Enforces comparing typeof expressions against valid strings.

https://eslint.org/docs/rules/valid-typeof#valid-typeof

👍 Examples of correct code

typeof foo === "string"
typeof bar == "undefined"
typeof foo === baz
typeof bar === typeof qux

👎 Examples of incorrect code

typeof foo === "strnig"
typeof foo == "undefimed"
typeof bar != "nunber"
typeof bar !== "fucntion"

Strict equality

Requires the use of === and !== instead of == and !=. It is considered good practice to use the type-safe equality operators === and !== instead of their regular counterparts == and !=.

https://eslint.org/docs/rules/eqeqeq#eqeqeq

👍 Examples of correct code

a === b
foo === true
bananas !== 1
value === undefined
typeof foo === 'undefined'
'hello' !== 'world'
0 === 0
true === true
foo === null

👎 Examples of incorrect code

a == b
foo == true
bananas != 1
value == undefined
typeof foo == 'undefined'
'hello' != 'world'
0 == 0
true == true
foo == null

No Label

Disallows unnecessary labels.
Labeled statements in JavaScript are used in conjunction with break and continue to control flow around multiple loops. For example:

https://eslint.org/docs/rules/no-extra-label#no-extra-label https://eslint.org/docs/rules/no-labels#no-labels

👍 Examples of correct code

while (a) {
    break;
}

for (let i = 0; i < 10; ++i) {
    break;
}

switch (a) {
    case 0:
        break;
}

👎 Examples of incorrect code

A: while (a) {
    break A;
}

B: for (let i = 0; i < 10; ++i) {
    break B;
}

C: switch (a) {
    case 0:
        break C;
}

Full Decimal Number

Disallows leading or trailing decimal points in numeric literals.

https://eslint.org/docs/rules/no-floating-decimal#no-floating-decimal

👍 Examples of correct code

var num = 0.5;
var num = 2.0;
var num = -0.7;

👎 Examples of incorrect code

var num = .5;
var num = 2.;
var num = -.7;

No Global Overwrite

Disallows reassignment of native objects.

https://eslint.org/docs/rules/no-floating-decimal#no-floating-decimal

👍 Examples of correct code

a = 1
var b = 1
b = 2

onload = function() {}

/*global a:writable*/
a = 1

👎 Examples of incorrect code

Object = null;
undefined = 1;
window = {};
length = 1;
top = 1;

/*global a:readonly*/
a = 1

Not Used New

Disallows new operators outside of assignments or comparisons.

https://eslint.org/docs/rules/no-new#no-new

👍 Examples of correct code

var thing = new Thing();

Thing();

👎 Examples of incorrect code

new Thing();

No New Function

Disallows new operators with the Function object. It's possible to create functions in JavaScript from strings at runtime using the Function constructor, such as:

https://eslint.org/docs/rules/no-new-func#no-new-func

👍 Examples of correct code

var x = function (a, b) {
    return a + b;
};

👎 Examples of incorrect code

var x = new Function("a", "b", "return a + b");
var x = Function("a", "b", "return a + b");
var x = Function.call(null, "a", "b", "return a + b");
var x = Function.apply(null, ["a", "b", "return a + b"]);
var x = Function.bind(null, "a", "b", "return a + b")();

// assuming that the result of Function.bind(...) will be eventually called.
var f = Function.bind(null, "a", "b", "return a + b");

No Redeclare

Disallows variable redeclarations.

https://eslint.org/docs/rules/no-redeclare#no-redeclare https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/no-redeclare.md

👍 Examples of correct code

var a = 3;
a = 10;

class C {
    foo() {
        var b = 3;
        b = 10;
    }

    static {
        var c = 3;
        c = 10;
    }
}

👎 Examples of incorrect code

var a = 3;
var a = 10;

class C {
    foo() {
        var b = 3;
        var b = 10;
    }

    static {
        var c = 3;
        var c = 10;
    }
}

No Self Compare

Disallows comparisons where both sides are exactly the same.

https://eslint.org/docs/rules/no-self-compare#no-self-compare

👍 Examples of correct code

var x = 10;
var y = 10;
if (x === y) {
    x = 20;
}

👎 Examples of incorrect code

var x = 10;
if (x === x) {
    x = 20;
}

Loop Valid

Disallows unmodified conditions of loops.

https://eslint.org/docs/rules/no-unmodified-loop-condition#no-unmodified-loop-condition

👍 Examples of correct code

while (node) {
    doSomething(node);
    node = node.parent;
}

for (var j = 0; j < items.length; ++j) {
    doSomething(items[j]);
}

// OK, the result of this binary expression is changed in this loop.
while (node !== root) {
    doSomething(node);
    node = node.parent;
}

// OK, the result of this ternary expression is changed in this loop.
while (node ? A : B) {
    doSomething(node);
    node = node.parent;
}

// A property might be a getter which has side effect...
// Or "doSomething" can modify "obj.foo".
while (obj.foo) {
    doSomething(obj);
}

// A function call can return various values.
while (check(obj)) {
    doSomething(obj);
}

👎 Examples of incorrect code

var node = something;

while (node) {
    doSomething(node);
}
node = other;

for (var j = 0; j < items.length; ++i) {
    doSomething(items[j]);
}

while (node !== root) {
    doSomething(node);
}

Useless Scape

Disallows unnecessary escape characters.

https://eslint.org/docs/rules/no-useless-escape#no-useless-escape

👍 Examples of correct code

"\"";
'\'';
"\x12";
"\u00a9";
"\371";
"xs\u2111";
`\``;
`\${${foo}}`;
`$\{${foo}}`;
/\\/g;
/\t/g;
/\w\$\*\^\./;
/[[]/;
/[\]]/;
/[a-z-]/;

👎 Examples of incorrect code

"\'";
'\"';
"\#";
"\e";
`\"`;
`\"${foo}\"`;
`\#{foo}`;
/\!/;
/\@/;
/[\[]/;
/[a-z\-]/;

No Yoda

Disallows "Yoda" conditions.

https://eslint.org/docs/rules/yoda#yoda

Yoda conditions are so named because the literal value of the condition comes first while the variable comes second. For example, the following is a Yoda condition:

👍 Examples of correct code

if (5 & value) {
    // ...
}

if (value === "red") {
    // ...
}

if (value === `red`) {
    // ...
}

if (`${value}` === `red`) {

}

👎 Examples of incorrect code

if ("red" === color) {
    // ...
}

if (`red` === color) {
    // ...
}

if (`red` === `${color}`) {
    // ...
}

if (true == flag) {
    // ...
}

if (5 > count) {
    // ...
}

if (-1 < str.indexOf(substr)) {
    // ...
}

if (0 <= x && x < 1) {
    // ...
}

No Undefined declare

Disallows initializing variables to undefined.

https://eslint.org/docs/rules/no-undef-init#no-undef-init

👍 Examples of correct code

var foo;
let bar;

const foo = undefined;

let { bar = undefined } = baz;

[quux = undefined] = quuux;

(foo = undefined) => {};

class Foo {
    bar = undefined;
}

👎 Examples of incorrect code

var foo = undefined;
let bar = undefined;

No New require

Disallows new operators with calls to require.

https://eslint.org/docs/rules/no-new-require#no-new-require

👍 Examples of correct code

var AppHeader = require('app-header');
var appHeader = new AppHeader();

👎 Examples of incorrect code

var appHeader = new require('app-header');

No New Object

Disallow Object constructors

https://eslint.org/docs/rules/no-new-object

👍 Examples of correct code

var myObject = new CustomObject();

var myObject = {};

var Object = function Object() {};
new Object();

👎 Examples of incorrect code

var myObject = new Object();

new Object();

No New Symbol

Disallow new operators with the Symbol object

https://eslint.org/docs/latest/rules/no-new-symbol

👍 Examples of correct code

var foo = Symbol('foo');

// Ignores shadowed Symbol.
function bar(Symbol) {
    const baz = new Symbol("baz");
}

👎 Examples of incorrect code

var foo = new Symbol('foo');

Var Size

Enforce minimum identifier lengths

https://eslint.org/docs/latest/rules/id-length

👍 Examples of correct code

var num = 5;
function _f() { return 42; }
function _func() { return 42; }
obj.el = document.body;
var foo = function (evt) { /* do stuff */ };
try {
    dangerousStuff();
} catch (error) {
    // ignore as many do
}
var myObj = { apple: 1 };
(num) => { num * num };
function foo(num = 0) { }
class MyClass { }
class Foo { method() {} }
class Foo { #method() {} }
class Foo { field = 1 }
class Foo { #field = 1 }
function foo(...args) { }
function foo([longName]) { }
var { prop } = {};
var { prop: [longName] } = {};
var [longName] = arr;
function foo({ prop }) { }
function foo({ a: prop }) { }
var { prop } = {};
var { a: prop } = {};
({ prop: obj.longName } = {});
var data = { "x": 1 };  // excused because of quotes
data["y"] = 3;  // excused because of calculated property access

👎 Examples of incorrect code

var x = 5;
obj.e = document.body;
var foo = function (e) { };
try {
    dangerousStuff();
} catch (e) {
    // ignore as many do
}
var myObj = { a: 1 };
(a) => { a * a };
class x { }
class Foo { x() {} }
class Foo { #x() {} }
class Foo { x = 1 }
class Foo { #x = 1 }
function foo(...x) { }
function foo([x]) { }
var [x] = arr;
var { prop: [x]} = {};
function foo({x}) { }
var { x } = {};
var { prop: a} = {};
({ prop: obj.x } = {});

Max Depth

Enforce a maximum depth that blocks can be nested

https://eslint.org/docs/latest/rules/max-depth

👍 Examples of correct code

function foo() {
    for (;;) { // Nested 1 deep
        while (true) { // Nested 2 deep
            if (true) { // Nested 3 deep
            }
        }
    }
}

👎 Examples of incorrect code

function foo() {
    for (;;) { // Nested 1 deep
        while (true) { // Nested 2 deep
            if (true) { // Nested 3 deep
                if (true) { // Nested 4 deep
                    if (true) { // Nested 5 deep
                    }
                }
            }
        }
    }
}

Max Params

Enforce a maximum number of parameters in function definitions

https://eslint.org/docs/latest/rules/max-params https://www.npmjs.com/package/eslint-plugin-better-max-params

👍 Examples of correct code

function foo (bar, baz, qux) {
    doSomething();
}

let foo = (bar, baz, qux) => {
    doSomething();
};

👎 Examples of incorrect code

function foo (bar, baz, qux, qxx) {
    doSomething();
}

let foo = (bar, baz, qux, qxx) => {
    doSomething();
};

Max Statements

Enforce a maximum number of parameters in function definitions

https://eslint.org/docs/latest/rules/max-statements

👍 Examples of correct code

function foo() {
  var foo1 = 1;
  var foo2 = 2;
  var foo3 = 3;
  var foo4 = 4;
  var foo5 = 5;
  var foo6 = 6;
  var foo7 = 7;
  var foo8 = 8;
  var foo9 = 9;
  var foo10 = 10;
  return function () {

    // The number of statements in the inner function does not count toward the
    // statement maximum.

    return 42;
  };
}

let foo = () => {
  var foo1 = 1;
  var foo2 = 2;
  var foo3 = 3;
  var foo4 = 4;
  var foo5 = 5;
  var foo6 = 6;
  var foo7 = 7;
  var foo8 = 8;
  var foo9 = 9;
  var foo10 = 10;
  return function () {

    // The number of statements in the inner function does not count toward the
    // statement maximum.

    return 42;
  };
}

👎 Examples of incorrect code

function foo() {
  var foo1 = 1;
  var foo2 = 2;
  var foo3 = 3;
  var foo4 = 4;
  var foo5 = 5;
  var foo6 = 6;
  var foo7 = 7;
  var foo8 = 8;
  var foo9 = 9;
  var foo10 = 10;

  var foo11 = 11; // Too many.
}

let foo = () => {
  var foo1 = 1;
  var foo2 = 2;
  var foo3 = 3;
  var foo4 = 4;
  var foo5 = 5;
  var foo6 = 6;
  var foo7 = 7;
  var foo8 = 8;
  var foo9 = 9;
  var foo10 = 10;

  var foo11 = 11; // Too many.
};

Operator Assignment

Require or disallow assignment operator shorthand where possible

https://eslint.org/docs/latest/rules/operator-assignment

👍 Examples of correct code

x = y;
x += y;
x = y * z;
x = (x * y) * z;
x[0] /= y;
x[foo()] = x[foo()] % 2;
x = y + x; // `+` is not always commutative (e.g. x = "abc")

👎 Examples of incorrect code

x = x + y;
x = y * x;
x[0] = x[0] / y;
x.y = x.y << z;

Require Yield

Require generator functions to contain yield

https://eslint.org/docs/latest/rules/require-yield

👍 Examples of correct code

function* foo() {
  yield 5;
  return 10;
}

function foo() {
  return 10;
}

// This rule does not warn on empty generator functions.
function* foo() { }

👎 Examples of incorrect code

function* foo() {
  return 10;
}

Prefer Rest Params

Require rest parameters instead of arguments

https://eslint.org/docs/latest/rules/prefer-rest-params

👍 Examples of correct code

function foo(...args) {
    console.log(args);
}

function foo(action, ...args) {
    action.apply(null, args); // or `action(...args)`, related to the `prefer-spread` rule.
}

// Note: the implicit arguments can be overwritten.
function foo(arguments) {
    console.log(arguments); // This is the first argument.
}
function foo() {
    var arguments = 0;
    console.log(arguments); // This is a local variable.
}

👎 Examples of incorrect code

function foo() {
    console.log(arguments);
}

function foo(action) {
    var args = Array.prototype.slice.call(arguments, 1);
    action.apply(null, args);
}

function foo(action) {
    var args = [].slice.call(arguments, 1);
    action.apply(null, args);
}

Symbol Description

Require symbol descriptions

https://eslint.org/docs/latest/rules/symbol-description

👍 Examples of correct code

var foo = Symbol("some description");

// or

var someString = "some description";
var bar = Symbol(someString);

👎 Examples of incorrect code

var foo = Symbol();

No Await Return

Disallow unnecessary return await

https://eslint.org/docs/latest/rules/no-return-await

👍 Examples of correct code

async function foo() {
    return bar();
}

async function foo() {
    await bar();
    return;
}

// This is essentially the same as `return await bar();`, but the rule checks only `await` in `return` statements
async function foo() {
    const x = await bar();
    return x;
}

// In this example the `await` is necessary to be able to catch errors thrown from `bar()`
async function foo() {
    try {
        return await bar();
    } catch (error) {}
}

👎 Examples of incorrect code

async function foo() {
    return await bar();
}

Max Class Per File

Enforce a maximum 1 classes per file

https://eslint.org/docs/latest/rules/max-classes-per-file

👍 Examples of correct code

class Foo {}

👎 Examples of incorrect code

class Foo {}
class Bar {}

No Constructor Return

Disallow returning value from constructor

https://eslint.org/docs/latest/rules/no-constructor-return

👍 Examples of correct code

class C {
    constructor(c) {
        this.c = c;
    }
}

class D {
    constructor(f) {
        if (!f) {
            return;  // Flow control.
        }

        f();
    }
}

👎 Examples of incorrect code

class A {
    constructor(a) {
        this.a = a;
        return a;
    }
}

class B {
    constructor(f) {
        if (!f) {
            return 'falsy';
        }
    }
}

Prefer Exponentiation Operator

Prefira o operador de exponencial

https://eslint.org/docs/latest/rules/prefer-exponentiation-operator

👍 Examples of correct code

const foo = 2 ** 8;

const bar = a ** b;

let baz = (a + b) ** (c + d);

let quux = (-1) ** n;

👎 Examples of incorrect code

const foo = Math.pow(2, 8);

const bar = Math.pow(a, b);

let baz = Math.pow(a + b, c + d);

let quux = Math.pow(-1, n);

Prefer Object Spread

Disallow using Object.assign with an object literal as the first argument and prefer the use of object spread instead.

https://eslint.org/docs/latest/rules/prefer-object-spread

👍 Examples of correct code

({ ...foo });

({ ...baz, foo: 'bar' });

// Any Object.assign call without an object literal as the first argument
Object.assign(foo, { bar: baz });

Object.assign(foo, bar);

Object.assign(foo, { bar, baz });

Object.assign(foo, { ...baz });

👎 Examples of incorrect code

Object.assign({}, foo);

Object.assign({}, {foo: 'bar'});

Object.assign({ foo: 'bar'}, baz);

Object.assign({}, baz, { foo: 'bar' });

Object.assign({}, { ...baz });

// Object.assign with a single argument that is an object literal
Object.assign({});

Object.assign({ foo: bar });

Accessor Pairs

Enforce getter and setter pairs in objects and classes

https://eslint.org/docs/latest/rules/accessor-pairs

👍 Examples of correct code

var o = {
    set a(value) {
        this.val = value;
    },
    get a() {
        return this.val;
    }
};

var myObject = { d: 1 };
Object.defineProperty(myObject, 'c', {
    set: function(value) {
        this.val = value;
    },
    get: function() {
        return this.val;
    }
});

👎 Examples of incorrect code

var o = {
    set a(value) {
        this.val = value;
    }
};

var myObject = { d: 1 };
Object.defineProperty(myObject, 'c', {
    set: function(value) {
        this.val = value;
    }
});

Default Case Last

Enforce default clauses in switch statements to be last

https://eslint.org/docs/latest/rules/default-case-last

👍 Examples of correct code

👎 Examples of incorrect code

switch (foo) {
    default:
        bar();
        break;
    case "a":
        baz();
        break;
}

switch (foo) {
    case 1:
        bar();
        break;
    default:
        baz();
        break;
    case 2:
        quux();
        break;
}

switch (foo) {
    case "x":
        bar();
        break;
    default:
    case "y":
        baz();
        break;
}

switch (foo) {
    default:
        break;
    case -1:
        bar();
        break;
}

switch (foo) {
  default:
    doSomethingIfNotZero();
  case 0:
    doSomethingAnyway();
}

Prefer Literals

Suggests using template literals instead of string concatenation.

https://eslint.org/docs/rules/prefer-template#prefer-template

👍 Examples of correct code

var str = "Hello World!";
var str = `Hello, ${name}!`;
var str = `Time: ${12 * 60 * 60 * 1000}`;

// This is reported by `no-useless-concat`.
var str = "Hello, " +
  "World!";

👎 Examples of incorrect code

var str = "Hello, " + name + "!";
var str = "Time: " + (12 * 60 * 60 * 1000);

Useless Condition

Prevents conditionals where the type is always truthy or always falsy.

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/no-unnecessary-condition.md

👍 Examples of correct code

function head<T>(items: T[]) {
  // Necessary, since items.length might be 0
  if (items.length) {
    return items[0].toUpperCase();
  }
}

function foo(arg: string) {
  // Necessary, since foo might be ''.
  if (arg) {
  }
}

function bar(arg?: string | null) {
  // Necessary, since arg might be nullish
  return arg?.length;
}

[0, 1, 2, 3].filter(t => t); // number can be truthy or falsy

👎 Examples of incorrect code

function head<T>(items: T[]) {
  // items can never be nullable, so this is unnecessary
  if (items) {
    return items[0].toUpperCase();
  }
}

function foo(arg: 'bar' | 'baz') {
  // arg is never nullable or empty string, so this is unnecessary
  if (arg) {
  }
}

function bar<T>(arg: string) {
  // arg can never be nullish, so ?. is unnecessary
  return arg?.length;
}

// Checks array predicate return types, where possible
[
  [1, 2],
  [3, 4],
].filter(t => t); // number[] is always truthy

No multiple empty line

we should avoid using lots of white spaces, this takes up the screen and tries to supply a bad organization with spaces

https://eslint.org/docs/rules/no-multiple-empty-lines

👍 Examples of correct code

function foo<T>(items: T[]) {

}

function bar(arg: string) {

}

👎 Examples of incorrect code

function foo<T>(items: T[]) {
// \r\n
// \r\n
}
// \n
// \n
function bar(arg: string) {
// \n
// \n
}

No Misused New

Block misused new instance

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/no-misused-new.md

👍 Examples of correct code

class C {
  constructor() {}
}
interface I {
  new (): C;
}

👎 Examples of incorrect code

class C {
  new(): C;
}

interface I {
  new (): I;
  constructor(): void;
}

No Semicolon Before spacing

Disallows space before semicolon.

https://eslint.org/docs/rules/semi-spacing

👍 Examples of correct code

var foo;
var foo; var bar;
throw new Error("error");
while (a) { break; }
for (i = 0; i < 10; i++) {}

👎 Examples of incorrect code

var foo ;
var foo;var bar;
throw new Error("error") ;
while (a) { break ; }
for (i = 0 ; i < 10 ; i++) {}
for (i = 0;i < 10;i++) {}

Disallow Type

Disallow specific types from being used. Disallow any use.

https://typescript-eslint.io/rules/ban-types https://typescript-eslint.io/rules/no-explicit-any

👍 Examples of correct code

// use lower-case primitives for consistency
const str: string = 'foo';
const bool: boolean = true;
const num: number = 1;
const symb: symbol = Symbol('foo');

// use a proper function type
const func: () => number = () => 1;

// use safer object types
const lowerObj: object = {};

const capitalObj1: number = 1;
const capitalObj2: { a: string } = { a: 'string' };

const curly1: number = 1;
const curly2: Record<'a', string> = { a: 'string' };

interface ObjInterface {
    a: number
}
const obj: ObjInterface = { a: 1 };

👎 Examples of incorrect code

// use lower-case primitives for consistency
const str: String = 'foo';
const bool: Boolean = true;
const num: Number = 1;
const symb: Symbol = Symbol('foo');

// use a proper function type
const func: Function = () => 1;

// use safer object types
const capitalObj1: Object = 1;
const capitalObj2: Object = { a: 'string' };

const curly1: {} = 1;
const curly2: {} = { a: 'string' };

const obj: any = { a: 1 };

Disallow Empty Function

Disallow empty functions

https://typescript-eslint.io/rules/no-empty-function

👍 Examples of correct code

function foo() {
    // do nothing.
}

var foo = function() {
    // any clear comments.
};

var foo = () => {
    bar();
};

function* foo() {
    // do nothing.
}

var foo = function*() {
    // do nothing.
};

var obj = {
    foo: function() {
        // do nothing.
    },

    foo: function*() {
        // do nothing.
    },

    foo() {
        // do nothing.
    },

    *foo() {
        // do nothing.
    },

    get foo() {
        // do nothing.
    },

    set foo(value) {
        // do nothing.
    }
};

class A {
    constructor() {
        // do nothing.
    }

    foo() {
        // do nothing.
    }

    *foo() {
        // do nothing.
    }

    get foo() {
        // do nothing.
    }

    set foo(value) {
        // do nothing.
    }

    static foo() {
        // do nothing.
    }

    static *foo() {
        // do nothing.
    }

    static get foo() {
        // do nothing.
    }

    static set foo(value) {
        // do nothing.
    }
}

👎 Examples of incorrect code

function foo() {}

var foo = function() {};

var foo = () => {};

function* foo() {}

var foo = function*() {};

var obj = {
    foo: function() {},

    foo: function*() {},

    foo() {},

    *foo() {},

    get foo() {},

    set foo(value) {}
};

class A {
    constructor() {}

    foo() {}

    *foo() {}

    get foo() {}

    set foo(value) {}

    static foo() {}

    static *foo() {}

    static get foo() {}

    static set foo(value) {}
}

Disallow Duplicate Imports

Disallow duplicate imports.

https://typescript-eslint.io/rules/no-duplicate-imports https://github.com/import-js/eslint-plugin-import/blob/HEAD/docs/rules/no-duplicates.md

👍 Examples of correct code

import { merge, find } from "module";

import something from "another-module";
// or
import * as something from "another-module";

👎 Examples of incorrect code

import { merge } from "module";
import something from "another-module";
import { find } from "module";

Disallow Unnecessary Type

Disallows unnecessary constraints on generic types.

https://typescript-eslint.io/rules/no-unnecessary-type-constraint

👍 Examples of correct code

interface Foo<T> {}

type Bar<T> = {};

class Baz<T> {
    qux<U> { }
}

const Quux = <T>() => {};

function Quuz<T>() {}

👎 Examples of incorrect code

interface FooAny<T extends any> {}
interface FooUnknown<T extends unknown> {}

type BarAny<T extends any> = {};
type BarUnknown<T extends unknown> = {};

class BazAny<T extends any> {
  quxUnknown<U extends unknown>() {}
}

class BazUnknown<T extends unknown> {
  quxUnknown<U extends unknown>() {}
}

const QuuxAny = <T extends any>() => {};
const QuuxUnknown = <T extends unknown>() => {};

function QuuzAny<T extends any>() {}
function QuuzUnknown<T extends unknown>() {}

Disallow Caller

The use of arguments.caller and arguments.callee make several code optimizations impossible. They have been deprecated in future versions of JavaScript and their use is forbidden in ECMAScript 5 while in strict mode.

https://eslint.org/docs/rules/no-caller

👍 Examples of correct code

function foo(n) {
    if (n <= 0) {
        return;
    }

    foo(n - 1);
}

[1,2,3,4,5].map(function factorial(n) {
    return !(n > 1) ? 1 : factorial(n - 1) * n;
});

👎 Examples of incorrect code

function foo(n) {
    if (n <= 0) {
        return;
    }

    arguments.callee(n - 1);
}

[1,2,3,4,5].map(function(n) {
    return !(n > 1) ? 1 : arguments.callee(n - 1) * n;
});

Disallow Underscore

This rule disallows dangling underscores in identifiers.

https://eslint.org/docs/rules/no-underscore-dangle

👍 Examples of correct code

var _ = require('underscore');
var obj = _.contains(items, item);
obj.__proto__ = {};
var file = __filename;
function foo(_bar) {};
const foo = { onClick(_bar) {} };
const foo = (_bar) => {};

👎 Examples of incorrect code

var foo_;
var __proto__ = {};
foo._bar();
function _foo(_bar) {};

Disallow Param Reassign

This rule aims to prevent unintended behavior caused by modification or reassignment of function parameters.

https://eslint.org/docs/rules/no-param-reassign

👍 Examples of correct code

function foo(bar) {
    var baz = bar;
}

👎 Examples of incorrect code

function foo(bar) {
    bar = 13;
}

function foo(bar) {
    bar++;
}

function foo(bar) {
    for (bar in baz) {}
}

function foo(bar) {
    for (bar of baz) {}
}

Prefer Const

If a variable is never reassigned, using the const declaration is better.

https://eslint.org/docs/rules/prefer-const https://typescript-eslint.io/rules/prefer-as-const

👍 Examples of correct code

const a = 0;

// it's never initialized.
let a;
console.log(a);

// it's reassigned after initialized.
let a;
a = 0;
a = 1;
console.log(a);

// it's initialized in a different block from the declaration.
let a;
if (true) {
    a = 0;
}
console.log(a);

// it's initialized in a different scope.
let a;
class C {
    #x;
    static {
        a = obj => obj.#x;
    }
}

// it's initialized at a place that we cannot write a variable declaration.
let a;
if (true) a = 0;
console.log(a);

// `i` gets a new binding each iteration
for (const i in [1, 2, 3]) {
  console.log(i);
}

// `a` gets a new binding each iteration
for (const a of [1, 2, 3]) {
  console.log(a);
}

let foo = 'bar';
let foo = 'bar' as const;
let foo: 'bar' = 'bar' as const;
let bar = 'bar' as string;
let foo = <string>'bar';
let foo = { bar: 'baz' };

👎 Examples of incorrect code

let a = 3;
console.log(a);

let a;
a = 0;
console.log(a);

class C {
    static {
        let a;
        a = 0;
        console.log(a);
    }
}

// `i` is redefined (not reassigned) on each loop step.
for (let i in [1, 2, 3]) {
    console.log(i);
}

// `a` is redefined (not reassigned) on each loop step.
for (let a of [1, 2, 3]) {
    console.log(a);
}

let bar: 2 = 2;
let foo = <'bar'>'bar';
let foo = { bar: 'baz' as 'baz' };

Array Type

Requires using either T[] instead of Array<T>.

https://typescript-eslint.io/rules/array-type

👍 Examples of correct code

const x: string[] = ['a', 'b'];
const y: readonly string[] = ['a', 'b'];

const a: Array<string | number> = ['a', 'b'];

👎 Examples of incorrect code

const x: Array<string> = ['a', 'b'];
const y: ReadonlyArray<string> = ['a', 'b'];

Disallow Await sync function

Disallows awaiting a value that is not a Thenable.

https://typescript-eslint.io/rules/await-thenable

👍 Examples of correct code

await Promise.resolve('value');

const createValue = async () => 'value';
await createValue();

👎 Examples of incorrect code

await 'value';

const createValue = () => 'value';
await createValue();

Method Signature Style

Enforces using a particular method signature syntax.

https://typescript-eslint.io/rules/method-signature-style

👍 Examples of correct code

interface T1 {
  func(arg: string): number;
}
type T2 = {
  func(arg: boolean): void;
};
interface T3 {
  func(arg: number): void;
  func(arg: string): void;
  func(arg: boolean): void;
}

👎 Examples of incorrect code

interface T1 {
  func: (arg: string) => number;
}
type T2 = {
  func: (arg: boolean) => void;
};
// this is equivalent to the overload
interface T3 {
  func: ((arg: number) => void) &
    ((arg: string) => void) &
    ((arg: boolean) => void);
}

No Unnecessary Type Assertion

Warns if a type assertion does not change the type of an expression.

https://typescript-eslint.io/rules/no-unnecessary-type-assertion

👍 Examples of correct code

const foo = <number>3;

const foo = 3 as number;
const foo: number = 3;

const foo = 'foo' as const;

👎 Examples of incorrect code

const foo = 3;
const bar = foo!;

const foo = <3>3;

type Foo = 3;
const foo = <Foo>3;

type Foo = 3;
const foo = 3 as Foo;

No Unsafe Call

This rule disallows calling any variable that is typed as any.

https://typescript-eslint.io/rules/no-unsafe-call

👍 Examples of correct code

declare const typedVar: () => void;
declare const typedNested: { prop: { a: () => void } };

typedVar();
typedNested.prop.a();

(() => {})();

new Map();

String.raw`foo`;

👎 Examples of incorrect code

declare const anyVar: any;
declare const nestedAny: { prop: any };

anyVar();
anyVar.a.b();

nestedAny.prop();
nestedAny.prop['a']();

new anyVar();
new nestedAny.prop();

anyVar`foo`;
nestedAny.prop`foo`;

No Var

Requires let or const instead of var.

https://eslint.org/docs/rules/no-var

👍 Examples of correct code

let x = "y";
const CONFIG = {};

// init var is better
let y;
if (CONFIG.y) {
    y = CONFIG.y;
}
console.log(y);

👎 Examples of incorrect code

var x = "y";
var CONFIG = {};

if (CONFIG.y) {
    var y = CONFIG.y;
}
console.log(y);

Operator Break-Line

The operator-linebreak rule is aimed at enforcing a particular operator line break style

https://eslint.org/docs/rules/operator-linebreak

👍 Examples of correct code

foo = 1 + 2;

foo = 1
    + 2
    + 3;

foo = 5;

if (
  someCondition
  || otherCondition
) {
  // code ...
}

answer = everything
  ? 42
  : foo;

class Foo {
    a  = 1;
    [b]  = 2;
    d = 4;
}

👎 Examples of incorrect code

foo = 1 +
      2;

foo =
    5;

foo
    = 5;

if (
    someCondition ||
    otherCondition
) {
}

answer = everything ?
  42 :
  foo;

class Foo {
    a =
        1;
    [b] =
        2;
    [c
    ] =
        3;
}

Generator Function Stars

Enforces spacing before * in generator functions.

https://eslint.org/docs/rules/generator-star-spacing

👍 Examples of correct code

function *generator() {}

var anonymous = function *() {};

var shorthand = { *generator() {} };

👎 Examples of incorrect code

function* generator() {}

var anonymous = function* () {};

var shorthand = { * generator() {} };

No Unsafe Optional Chaining

Enforces spacing before * in generator functions.

https://eslint.org/docs/rules/no-unsafe-optional-chaining

👍 Examples of correct code

(obj?.foo)?.();

obj?.foo();

(obj?.foo ?? bar)();

obj?.foo.bar;

obj.foo?.bar;

foo?.()?.bar;

(obj?.foo ?? bar)`template`;

new (obj?.foo ?? bar)();

var baz = {...obj?.foo};

const { bar } = obj?.foo || baz;

async function foo () {
  const { bar } = await obj?.foo || baz;
   (await obj?.foo)?.();
   (await obj?.foo)?.bar;
}

👎 Examples of incorrect code

(obj?.foo)();

(obj?.foo).bar;

(foo?.()).bar;

(foo?.()).bar();

(obj?.foo ?? obj?.bar)();

(foo || obj?.foo)();

(obj?.foo && foo)();

(foo ? obj?.foo : bar)();

(foo, obj?.bar).baz;

(obj?.foo)`template`;

new (obj?.foo)();

[...obj?.foo];

bar(...obj?.foo);

1 in obj?.foo;

bar instanceof obj?.foo;

for (bar of obj?.foo);

const { bar } = obj?.foo;

[{ bar } = obj?.foo] = [];

with (obj?.foo);

class A extends obj?.foo {}

var a = class A extends obj?.foo {};

async function foo () {
    const { bar } = await obj?.foo;
   (await obj?.foo)();
   (await obj?.foo).bar;
}

Array Callback

Array has several methods for filtering, mapping, and folding. If we forget to write return statement in a callback of those, it's probably a mistake.

https://eslint.org/docs/rules/array-callback-return https://sonarsource.github.io/rspec/#/rspec/S3796/javascript

👍 Examples of correct code

var indexMap = myArray.reduce(function(memo, item, index) {
    memo[item] = index;
    return memo;
}, {});

var foo = Array.from(nodes, function(node) {
    if (node.tagName === "DIV") {
        return true;
    }
    return false;
});

var bar = foo.map(node => node.getAttribute("id"));

👎 Examples of incorrect code

var indexMap = myArray.reduce(function(memo, item, index) {
    memo[item] = index;
}, {});

var foo = Array.from(nodes, function(node) {
    if (node.tagName === "DIV") {
        return true;
    }
});

var bar = foo.filter(function(x) {
    if (x) {
        return true;
    } else {
        return;
    }
});

Space Types

Requires spacing around infix operators.

https://eslint.org/docs/rules/space-infix-ops https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/space-infix-ops.md https://eslint.org/docs/rules/key-spacing

👍 Examples of correct code

a + b

a ? b : c

const a = {b:1};

var {a = 0} = bar;

function foo(a = 0) { }

function foo(a = 0): string | number { }

var obj = { "foo": 42 };

👎 Examples of incorrect code

a+b

a+ b

a +b

a?b:c

const a={b:1};

var {a=0}=bar;

function foo(a=0) { }

function foo(): string|number { }

var obj = { "foo" : 42 };
var obj = { "foo" :42 };

Curly

Requires following curly brace conventions.

https://eslint.org/docs/rules/curly

👍 Examples of correct code

if (foo) {
    bar();
    baz();
} else {
    buz();
}

if (foo) {
    bar();
} else if (faa) {
    bor();
} else {
    other();
    things();
}

if (true)
    foo();
else
    baz();

if (foo)
    foo++;

👎 Examples of incorrect code

if (foo) {
    bar();
    baz();
} else
    buz();

if (foo)
    bar();
else if (faa)
    bor();
else {
    other();
    things();
}

if (true)
    foo();
else {
    baz();
}

if (foo) {
    foo++;
}

Quote Props

Requires quotes around all object literal property names if any name strictly requires quotes, otherwise disallows quotes around object property names.

https://eslint.org/docs/rules/quote-props

👍 Examples of correct code

var object1 = {
    "foo": "bar",
    "baz": 42,
    "qux-lorem": true
};

var object2 = {
    foo: 'bar',
    baz: 42
};

👎 Examples of incorrect code

var object1 = {
    foo: "bar",
    "baz": 42,
    "qux-lorem": true
};

var object2 = {
    'foo': 'bar',
    'baz': 42
};

Brace Style

Enforces consistent brace style for blocks.

https://eslint.org/docs/rules/brace-style

👍 Examples of correct code

function foo() {
  return true;
}

if (foo) {
  bar();
}

if (foo) {
  bar();
} else {
  baz();
}

try {
  somethingRisky();
} catch(e) {
  handleError();
}

class C {
    static {
        foo();
    }
}

// when there are no braces, there are no problems
if (foo) bar();
else if (baz) boom();

👎 Examples of incorrect code

function foo()
{
  return true;
}

if (foo)
{
  bar();
}

try
{
  somethingRisky();
} catch(e)
{
  handleError();
}

if (foo) {
  bar();
}
else {
  baz();
}

class C
{
    static
    {
        foo();
    }
}

Comma Style

This rule enforce consistent comma style in array literals, object literals, and variable declarations.

https://eslint.org/docs/rules/comma-style

👍 Examples of correct code

var foo = 1, bar = 2;

var foo = 1,
    bar = 2;

var foo = ["apples",
           "oranges"];

function bar() {
    return {
        "a": 1,
        "b:": 2
    };
}

👎 Examples of incorrect code

var foo = 1,
    bar = 2;

var foo = ["apples",
           "oranges"];

function bar() {
    return {
        "a": 1,
        "b:": 2
    };
}

Object Break Line

Enforces placing object properties on separate lines.

https://eslint.org/docs/rules/object-property-newline

👍 Examples of correct code

const obj1 = { foo: "foo", bar: "bar", baz: "baz" };

const obj2 = {
    foo: "foo",
    bar: "bar",
    baz: "baz",
};

const user = process.argv[2];
const obj3 = {
    user,
    [ process.argv[3] ? "foo" : "bar" ]: 0,
    baz: [
        1,
        2,
        4,
        8,
    ]
};

👎 Examples of incorrect code

const obj1 = {
    foo: "foo", bar: "bar", baz: "baz"
};

const obj2 = {
    foo: "foo", bar: "bar",
    baz: "baz"
};

const obj3 = {
    [process.argv[3] ? "foo" : "bar"]: 0, baz: [
        1,
        2,
        4,
        8
    ]
};

Object Curly Newline

A number of style guides require or disallow line breaks inside of object braces and other tokens.

https://eslint.org/docs/latest/rules/object-curly-newline

👍 Examples of correct code

let a = {};
let b = { foo: 1 };
let c = { foo: 1, bar: 2 };
let d = {
    foo: 1,
    bar: 2,
};
let e = {
    foo: function() {
        dosomething();
    }
};

let { f } = obj;
let { g, h } = obj;
let {
    i,
    j
} = obj;
let { k = () => dosomething() } = obj;

👎 Examples of incorrect code

let b = {
    foo: 1
};
let c = { foo: 1,
    bar: 2 };
let d = {foo: 1
    , bar: 2};
let e = { foo: function() {
    dosomething();
} };

let {i,
    j} = obj;

No Negative Condition

Negated conditions are more difficult to understand. Code can be made more readable by inverting the condition instead.

https://eslint.org/docs/latest/rules/no-negated-condition

👍 Examples of correct code

if (!a) {
    doSomething();
}

if (!a) {
    doSomething();
} else if (b) {
    doSomething();
}

if (a != b) {
    doSomething();
}

a ? b : c

👎 Examples of incorrect code

if (!a) {
    doSomething();
} else {
    doSomethingElse();
}

if (a != b) {
    doSomething();
} else {
    doSomethingElse();
}

if (a !== b) {
    doSomething();
} else {
    doSomethingElse();
}

!a ? c : b

No Duplicated Branches

Having two cases in a switch statement or two branches in an if chain with the same implementation is at best duplicate code, and at worst a coding error. If the same logic is truly needed for both instances, then in an if chain they should be combined, or for a switch, one should fall through to the other.

https://github.com/SonarSource/eslint-plugin-sonarjs/blob/HEAD/docs/rules/no-duplicated-branches.md

👍 Examples of correct code

switch (i) {
  case 1:
  case 3:
    doFirstThing();
    doSomething();
    break;
  case 2:
    doSomethingDifferent();
    break;
  default:
    doTheRest();
}

if ((a >= 0 && a < 10) || (a >= 20 && a < 50)) {
  doFirstThing();
  doTheThing();
} else if (a >= 10 && a < 20) {
  doTheOtherThing();
} else {
  doTheRest();
}

// Or

switch (i) {
  case 1:
    doFirstThing();
    doSomething();
    break;
  case 2:
    doSomethingDifferent();
    break;
  case 3:
    doFirstThing();
    doThirdThing();
    break;
  default:
    doTheRest();
}

if (a >= 0 && a < 10) {
  doFirstThing();
  doTheThing();
} else if (a >= 10 && a < 20) {
  doTheOtherThing();
} else if (a >= 20 && a < 50) {
  doFirstThing();
  doTheThirdThing();
} else {
  doTheRest();
}

👎 Examples of incorrect code

switch (i) {
  case 1:
    doFirstThing();
    doSomething();
    break;
  case 2:
    doSomethingDifferent();
    break;
  case 3: // Noncompliant; duplicates case 1's implementation
    doFirstThing();
    doSomething();
    break;
  default:
    doTheRest();
}

if (a >= 0 && a < 10) {
  doFirstThing();
  doTheThing();
} else if (a >= 10 && a < 20) {
  doTheOtherThing();
} else if (a >= 20 && a < 50) {
  // Noncompliant; duplicates first condition
  doFirstThing();
  doTheThing();
} else {
  doTheRest();
}

No Identical Functions

When two functions have the same implementation, either it was a mistake - something else was intended - or the duplication was intentional, but may be confusing to maintainers. In the latter case, the code should be refactored.

https://github.com/SonarSource/eslint-plugin-sonarjs/blob/HEAD/docs/rules/no-identical-functions.md

👍 Examples of correct code

function calculateCode() {
  doTheThing();
  doOtherThing();
  return code;
}

function getName() {
  return calculateCode();
}

👎 Examples of incorrect code

function calculateCode() {
  doTheThing();
  doOtherThing();
  return code;
}

function getName() { // Noncompliant
  doTheThing();
  doOtherThing();
  return code;
}

No Inverted Boolean Check

It is needlessly complex to invert the result of a boolean comparison. The opposite comparison should be made instead.

https://github.com/SonarSource/eslint-plugin-sonarjs/blob/HEAD/docs/rules/no-inverted-boolean-check.md

👍 Examples of correct code

if (a !== 2) { ... }

👎 Examples of incorrect code

if (!(a === 2)) { ... }  // Noncompliant

No Nested Switch

Nested switch structures are difficult to understand because you can easily confuse the cases of an inner switch as belonging to an outer statement. Therefore nested switch statements should be avoided.

https://github.com/SonarSource/eslint-plugin-sonarjs/blob/HEAD/docs/rules/no-nested-switch.md

👍 Examples of correct code

function foo(n, m) {
  switch (n) {
    case 0:
      return bar(m);
    case 1:
      // ...
    default:
      // ...
  }
}

function bar(m) {
  switch(m) {
    // ...
  }
}

👎 Examples of incorrect code

function foo(n, m) {
  switch (n) {
    case 0:
      switch (m) {  // Noncompliant; nested switch
        // ...
      }
    case 1:
      // ...
    default:
      // ...
  }
}

No Nested Template Literals

Template literals (previously named "template strings") are an elegant way to build a string without using the + operator to make strings concatenation more readable.

https://github.com/SonarSource/eslint-plugin-sonarjs/blob/HEAD/docs/rules/no-nested-template-literals.md

👍 Examples of correct code

let color = "red";
let count = 3;
let apples = color ? `${count} ${color}` : count;
let message = `I have ${apples} apples`;

👎 Examples of incorrect code

let color = "red";
let count = 3;
let message = `I have ${color ? `${count} ${color}` : count} apples`;

No Redundant Boolean

Redundant Boolean literals should be removed from expressions to improve readability.

https://github.com/SonarSource/eslint-plugin-sonarjs/blob/HEAD/docs/rules/no-redundant-boolean.md

👍 Examples of correct code

if (booleanMethod()) { /* ... */ }
if (!booleanMethod()) { /* ... */ }
if (booleanMethod()) { /* ... */ }
doSomething(true);
doSomething(booleanMethod());

👎 Examples of incorrect code

if (booleanMethod() == true) { /* ... */ }
if (booleanMethod() == false) { /* ... */ }
if (booleanMethod() || false) { /* ... */ }
doSomething(!false);
doSomething(booleanMethod() == true);

Prefer Immediate Return

Declaring a variable only to immediately return or throw it is a bad practice.

https://github.com/SonarSource/eslint-plugin-sonarjs/blob/HEAD/docs/rules/prefer-immediate-return.md

👍 Examples of correct code

function ms(hours, minutes, seconds) {
  return ((hours * 60 + minutes) * 60 + seconds) * 1000;
}

👎 Examples of incorrect code

function ms(hours, minutes, seconds) {
  const duration = ((hours * 60 + minutes) * 60 + seconds) * 1000;

  return duration;
}

Prefer Object Literal

Object literal syntax, which initializes an object's properties inside the object declaration is cleaner and clearer than the alternative: creating an empty object, and then giving it properties one by one.

https://github.com/SonarSource/eslint-plugin-sonarjs/blob/HEAD/docs/rules/prefer-object-literal.md

👍 Examples of correct code

var person = {
  firstName: "John",
  middleInitial: "Q",
  lastName: "Public",
};

👎 Examples of incorrect code

var person = {}; // Noncompliant
person.firstName = "John";
person.middleInitial = "Q";
person.lastName = "Public";

Prefer Single Boolean Return

Return of boolean literal statements wrapped into if-then-else flow should be simplified.

https://github.com/SonarSource/eslint-plugin-sonarjs/blob/HEAD/docs/rules/prefer-single-boolean-return.md

👍 Examples of correct code

return expression;

👎 Examples of incorrect code

if (expression) {
  return true;
} else {
  return false;
}

// or

if (expression) {
  return true;
}
return false;

No Shadow

Enforces placing object properties on separate lines.

Then any code used within the same scope would not get the global undefined, but rather the local version with a very different meaning.

https://eslint.org/docs/rules/no-shadow https://eslint.org/docs/rules/no-shadow-restricted-names https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/no-shadow.md https://sonarsource.github.io/rspec/#/rspec/S1527/javascript

👍 Examples of correct code

var a = 3;
function b() {
    var c = 10;
}

var b = function () {
    var c = 10;
}

function b(a) {
    var c = 10;
}
b(a);

function f(a, b){}

var elements = document.getElementsByName("foo"); // Compliant
var someData = { package: true };                 // Compliant, as it is not used as an identifier here

👎 Examples of incorrect code

var a = 3;
function b() {
    var a = 10;
}

var b = function () {
    var a = 10;
}

function b(a) {
    a = 10;
}
b(a);

if (true) {
    let a = 5;
}

function NaN(){}

!function(Infinity){};

var undefined = 5;

try {} catch(eval){}

var package = document.getElementsByName("foo"); // Noncompliant

Parentheses New Line

This rule enforces consistent line breaks inside parentheses of function parameters or arguments.

https://eslint.org/docs/latest/rules/function-paren-newline https://eslint.org/docs/latest/rules/function-call-argument-newline

👍 Examples of correct code

function foo(
  bar,
  baz
) {}

var foo = function(bar, baz) {};

var foo = (
  bar
) => {};

foo(
  function() {
    return baz;
  }
);

foo("one", "two", "three");
// or
foo(
    "one",
    "two",
    "three"
);

bar("one", "two", {
    one: 1,
    two: 2
});
// or
bar(
    "one",
    "two",
    { one: 1, two: 2 }
);
// or
bar(
    "one",
    "two",
    {
        one: 1,
        two: 2
    }
);

baz("one", "two", (x) => {
    console.log(x);
});
// or
baz(
    "one",
    "two",
    (x) => {
        console.log(x);
    }
);

👎 Examples of incorrect code

function foo(bar,
  baz
) {}

var foo = function(bar,
baz) {};

var foo = (
  bar) => {};

foo(
  function() {
    return baz;
  });

foo("one", "two",
    "three");
//or
foo("one",
    "two", "three");

bar("one", "two",
    { one: 1, two: 2}
);

baz("one", "two",
    (x) => { console.log(x); }
);

No Func Call Spacing

Disallows space between the function name and the opening parenthesis.

https://eslint.org/docs/latest/rules/func-call-spacing

👍 Examples of correct code

fn();

👎 Examples of incorrect code

fn ();

fn
();

Array Element New Line

This rule enforces line breaks between array elements.

https://eslint.org/docs/latest/rules/array-element-newline

👍 Examples of correct code

var a = [];
var b = [1];
var c = [1, 2];
var d = [1, 2, 3];
var e = [
    1,
    2
];
var f = [
    1,
    2,
    3
];
var g = [
    function foo() {
        dosomething();
    }, function bar() {
        dosomething();
    }, function baz() {
        dosomething();
    }
];
var h = [
    function foo() {
        dosomething();
    },
    function bar() {
        dosomething();
    },
    function baz() {
        dosomething();
    }
];

👎 Examples of incorrect code

var a = [
    1, 2,
    3
];
var b = [
    function foo() {
        dosomething();
    }, function bar() {
        dosomething();
    },
    function baz() {
        dosomething();
    }
];

Wrap Iife

Enforces always wrapping the function expression

https://eslint.org/docs/latest/rules/wrap-iife

👍 Examples of correct code

var x = (function () { return { y: 1 }; })(); // wrapped function expression

👎 Examples of incorrect code

var x = function () { return { y: 1 };}(); // unwrapped
var x = (function () { return { y: 1 };}()); // wrapped call expression

Disallow Template Tag Space

Disallows spaces between a tag function and its template literal.

https://eslint.org/docs/latest/rules/template-tag-spacing

👍 Examples of correct code

func`Hello world`;

👎 Examples of incorrect code

func `Hello world`;

No Space Spread

Enforce spacing between rest and spread operators and their expressions

https://eslint.org/docs/latest/rules/rest-spread-spacing

👍 Examples of correct code

fn(...args)
[...arr, 4, 5, 6]
let [a, b, ...arr] = [1, 2, 3, 4, 5];
function fn(...args) { console.log(args); }
let { x, y, ...z } = { x: 1, y: 2, a: 3, b: 4 };
let n = { x, y, ...z };

👎 Examples of incorrect code

fn(... args)
[... arr, 4, 5, 6]
let [a, b, ... arr] = [1, 2, 3, 4, 5];
function fn(... args) { console.log(args); }
let { x, y, ... z } = { x: 1, y: 2, a: 3, b: 4 };
let n = { x, y, ... z };

Inline IF

Disallows a newline before a single-line statement.

https://eslint.org/docs/latest/rules/nonblock-statement-body-position

👍 Examples of correct code

if (foo) bar();
else baz();

while (foo) bar();

for (let i = 1; i < foo; i++) bar();

do bar(); while (foo)

if (foo) { // block statements are always allowed with this rule
  bar();
} else {
  baz();
}

👎 Examples of incorrect code

if (foo)
  bar();
else
  baz();

while (foo)
  bar();

for (let i = 1; i < foo; i++)
  bar();

do
  bar();
while (foo)

New Instance Use Parentheses

This rule can enforce or disallow parentheses when invoking a constructor with no arguments using the new keyword.

https://eslint.org/docs/latest/rules/new-parens

👍 Examples of correct code

var person = new Person();
var person = new Person("name");

👎 Examples of incorrect code

var person = new Person;
var person = new (Person);

Logical Assignment Operators

The shorthand can be used if the assignment target and the left expression of a logical expression are the same. For example a = a || b can be shortened to a ||= b.

https://eslint.org/docs/latest/rules/logical-assignment-operators

👍 Examples of correct code

a = b
a += b
a ||= b
a = b || c
a || (b = c)

if (a) a = b

👎 Examples of incorrect code

a = a || b
a = a && b
a = a ?? b
a || (a = b)
a && (a = b)
a ?? (a = b)

No With

The with statement is potentially problematic because it adds members of an object to the current scope, making it impossible to tell what a variable inside the block actually refers to.

https://eslint.org/docs/rules/no-with

👍 Examples of correct code

const r = ({x, y}) => Math.sqrt(x * x + y * y);

👎 Examples of incorrect code

with (point) {
    r = Math.sqrt(x * x + y * y); // is r a member of point?
}

Promise Rules

No New Statics

Calling a Promise static method with new is invalid, resulting in a TypeError at runtime.

https://github.com/xjamundx/eslint-plugin-promise/blob/development/docs/rules/no-new-statics.md

👍 Examples of correct code

Promise.resolve(value)
Promise.reject(error)
Promise.race([p1, p2])
Promise.all([p1, p2])

👎 Examples of incorrect code

new Promise.resolve(value)
new Promise.reject(error)
new Promise.race([p1, p2])
new Promise.all([p1, p2])

No Return Wrap

Ensure that inside a then() or a catch() we always return or throw a raw value instead of wrapping in Promise.resolve or Promise.reject

https://github.com/xjamundx/eslint-plugin-promise/blob/development/docs/rules/no-new-statics.md

👍 Examples of correct code

myPromise.then(function (val) {
  return val * 2
})
myPromise.then(function (val) {
  throw new Exception("Message");
})

👎 Examples of incorrect code

myPromise.then(function (val) {
  return Promise.resolve(val * 2)
})
myPromise.then(function (val) {
  return Promise.reject('bad thing')
})

Param Name

Ensure that inside a then() or a catch() we always return or throw a raw value instead of wrapping in Promise.resolve or Promise.reject

https://github.com/xjamundx/eslint-plugin-promise/blob/development/docs/rules/param-names.md

👍 Examples of correct code

new Promise(function (resolve) { ... })
new Promise(function (resolve, reject) { ... })
new Promise(function (_resolve, _reject) { ... }) // Unused marker for parameters are allowed

👎 Examples of incorrect code

new Promise(function (reject, resolve) { ... }) // incorrect order
new Promise(function (ok, fail) { ... }) // non-standard parameter names
new Promise(function (_, reject) { ... }) // a simple underscore is not allowed

Always Return

Ensure that inside a then() or a catch() we always return or throw a raw value instead of wrapping in Promise.resolve or Promise.reject

https://github.com/xjamundx/eslint-plugin-promise/blob/development/docs/rules/always-return.md

👍 Examples of correct code

myPromise.then((val) => val * 2);
myPromise.then(function(val) { return val * 2; });
myPromise.then(doSomething); // could be either
myPromise.then((b) => { if (b) { return "yes" } else { return "no" } });

👎 Examples of incorrect code

myPromise.then(function (val) {})
myPromise.then(() => {
  doSomething()
})
myPromise.then((b) => {
  if (b) {
    return 'yes'
  } else {
    forgotToReturn()
  }
})

No Nesting

Warning Avoid nested then() or catch() statements (no-nesting)

https://github.com/xjamundx/eslint-plugin-promise/blob/development/docs/rules/no-nesting.md

👍 Examples of correct code

myPromise.then(doSomething).then(doSomethingElse).catch(errors)

👎 Examples of incorrect code

myPromise.then((val) => doSomething(val).then(doSomethingElse))

myPromise.then((val) => doSomething(val).catch(errors))

myPromise.catch((err) => doSomething(err).then(doSomethingElse))

myPromise.catch((err) => doSomething(err).catch(errors))

No Return Finally

Disallow return statements inside a callback passed to finally(), since nothing would consume what's returned.

https://github.com/xjamundx/eslint-plugin-promise/blob/development/docs/rules/no-return-in-finally.md

👍 Examples of correct code

myPromise.finally(function (val) {
  console.log('value:', val)
})

👎 Examples of incorrect code

myPromise.finally(function (val) {
  return val
})

Valid Params

Disallow return statements inside a callback passed to finally(), since nothing would consume what's returned.

https://github.com/xjamundx/eslint-plugin-promise/blob/development/docs/rules/valid-params.md

👍 Examples of correct code

// Promise.all() requires 1 argument
Promise.all([p1, p2, p3])
Promise.all(iterable)

// Promise.race() requires 1 argument
Promise.race([p1, p2, p3])
Promise.race(iterable)

// Promise.resolve() requires 0 or 1 arguments
Promise.resolve()
Promise.resolve({})
Promise.resolve([1, 2, 3])
Promise.resolve(referenceToObject)

// Promise.reject() requires 0 or 1 arguments
Promise.reject()
Promise.reject(Error())
Promise.reject(referenceToError)

// Promise.then() requires 1 or 2 arguments
somePromise().then((value) => doSomething(value))
somePromise().then(successCallback, errorCallback)

// Promise.catch() requires 1 argument
somePromise().catch((error) => {
  handleError(error)
})
somePromise().catch(console.error)

// Promise.finally() requires 1 argument
somePromise().finally(() => {
  console.log('done!')
})
somePromise().finally(console.log)

👎 Examples of incorrect code

Promise.all() // is called with 0 or 2+ arguments
Promise.race() // is called with 0 or 2+ arguments
Promise.resolve(a, b) // is called with 2+ arguments
Promise.reject(a, b) // is called with 2+ arguments
Promise.then() // is called with 0 or 3+ arguments
Promise.catch() // is called with 0 or 2+ arguments
Promise.finally() // is called with 0 or 2+ arguments

No Async Promise Executor

Disallows using an async function as a Promise executor.

https://eslint.org/docs/rules/no-async-promise-executor

👍 Examples of correct code

const foo = new Promise((resolve, reject) => {
  readFile('foo.txt', function(err, result) {
    if (err) {
      reject(err);
    } else {
      resolve(result);
    }
  });
});

const result = Promise.resolve(foo);

👎 Examples of incorrect code

const foo = new Promise(async (resolve, reject) => {
  readFile('foo.txt', function(err, result) {
    if (err) {
      reject(err);
    } else {
      resolve(result);
    }
  });
});

const result = new Promise(async (resolve, reject) => {
  resolve(await foo);
});

No Misused Promises

Block misused of promise

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/no-misused-promises.md

👍 Examples of correct code

const promise = Promise.resolve('value');

// Always `await` the Promise in a conditional
if (await promise) {
  // Do something
}

const val = (await promise) ? 123 : 456;

while (await promise) {
  // Do something
}

👎 Examples of incorrect code

const promise = Promise.resolve('value');

// always true
if (promise) {
  // Do something
}

const val = promise ? 123 : 456;

while (promise) {
  // Do something
}

Import

New Line After Import

Enforces having one empty lines after the last top-level import statement or require call.

https://github.com/import-js/eslint-plugin-import/blob/main/docs/rules/newline-after-import.md

👍 Examples of correct code

import defaultExport from './foo'

const FOO = 'BAR'

// OR
import defaultExport from './foo'
import { bar }  from 'bar-lib'

const FOO = 'BAR'

// OR
const FOO = require('./foo')
const BAR = require('./bar')

const BAZ = 1

👎 Examples of incorrect code

import * as foo from 'foo'
const FOO = 'BAR'

// OR
import * as foo from 'foo'
const FOO = 'BAR'

import { bar }  from 'bar-lib'

// OR
const FOO = require('./foo')
const BAZ = 1
const BAR = require('./bar')

Import Deprecated

Reports use of a deprecated name, as indicated by a JSDoc block with a @deprecated tag or TomDoc Deprecated: comment.

https://github.com/import-js/eslint-plugin-import/blob/HEAD/docs/rules/no-deprecated.md

👍 Examples of correct code

👎 Examples of incorrect code

// @file: ./answer.js

/**
 * this is what you get when you trust a mouse talk show
 * @deprecated need to restart the experiment
 * @returns {Number} nonsense
 */
export function multiply(six, nine) {
  return 42
}

import { multiply } from './answer';

No Mutable Export

Forbids the use of mutable exports with var or let.

https://github.com/import-js/eslint-plugin-import/blob/HEAD/docs/rules/no-mutable-exports.md

👍 Examples of correct code

export const count = 1
export function getCount() {}
export class Counter {}

👎 Examples of incorrect code

export let count = 2
export var count = 3

let count = 4
export { count } // reported here

No Amd

Reports require([array], ...) and define([array], ...) function calls at the module scope.

https://github.com/import-js/eslint-plugin-import/blob/HEAD/docs/rules/no-amd.md

👍 Examples of correct code

👎 Examples of incorrect code

define(["a", "b"], function (a, b) { /* ... */ })

require(["b", "c"], function (b, c) { /* ... */ })

Prefer Default Export

When there is only a single export from a module, prefer using default export over named export.

https://github.com/import-js/eslint-plugin-import/blob/HEAD/docs/rules/prefer-default-export.md

👍 Examples of correct code

// There is a default export.
export const foo = 'foo';
const bar = 'bar';
export default bar;

// or

// good2.js
// There is more than one named export in the module.
export const foo = 'foo';
export const bar = 'bar';

// or

// good3.js
// There is more than one named export in the module
const foo = 'foo';
const bar = 'bar';
export { foo, bar }

// or

// good4.js
// There is a default export.
const foo = 'foo';
export { foo as default }

// or

// export-star.js
// Any batch export will disable this rule. The remote module is not inspected.
export * from './other-module'

👎 Examples of incorrect code

// There is only a single module export and it's a named export.
export const foo = 'foo';

Max Imports

Forbid modules to have too many dependencies (import or require statements).

https://github.com/import-js/eslint-plugin-import/blob/HEAD/docs/rules/max-dependencies.md

👍 Examples of correct code

import a from './a'; // 1
const b = require('./b'); // 2
// ...
import y from './y'; // 25 - No exceeds!

👎 Examples of incorrect code

import a from './a'; // 1
const b = require('./b'); // 2
// ...
import z from './z'; // 26 - exceeds max!

No CommanJs Export

Reports the use of import declarations with CommonJS exports in any module except for the main module.

https://github.com/import-js/eslint-plugin-import/blob/HEAD/docs/rules/no-import-module-exports.md

👍 Examples of correct code

import thing from 'other-thing'
export default thing

const thing = require('thing')
module.exports = thing

const thing = require('thing')
exports.foo = bar

import thing from 'otherthing'
console.log(thing.module.exports)

// in lib/index.js
import foo from 'path';
module.exports = foo;

👎 Examples of incorrect code

import { stuff } from 'starwars'
module.exports = thing

import * as allThings from 'starwars'
exports.bar = thing

import thing from 'other-thing'
exports.foo = bar

import thing from 'starwars'
const baz = module.exports = thing
console.log(baz)

No Useless Path Import

Use this rule to prevent unnecessary path segments in import and require statements.

https://github.com/import-js/eslint-plugin-import/blob/HEAD/docs/rules/no-useless-path-segments.md

👍 Examples of correct code

import "./header.js";
import "./pages";
import "./pages/about";
import ".";
import "..";
import fs from "fs";

👎 Examples of incorrect code

import "./../my-project/pages/about.js"; // should be "./pages/about.js"
import "./../my-project/pages/about"; // should be "./pages/about"
import "../my-project/pages/about.js"; // should be "./pages/about.js"
import "../my-project/pages/about"; // should be "./pages/about"
import "./pages//about"; // should be "./pages/about"
import "./pages/"; // should be "./pages"
import "./pages/index"; // should be "./pages" (except if there is a ./pages.js file)
import "./pages/index.js"; // should be "./pages" (except if there is a ./pages.js file)

No Extraneous Dependencies

Disable import dependencies if no exists in package.json dependencies

https://github.com/import-js/eslint-plugin-import/blob/HEAD/docs/rules/no-extraneous-dependencies.md

👍 Examples of correct code

import anything from "anything"; // exists in dependencies

👎 Examples of incorrect code

import typescript from "typescript"; // exists in dev dependency package.json

Import Order

Enforce a convention in the order of require() / import statements Use: Alphabetic order import

https://github.com/import-js/eslint-plugin-import/blob/HEAD/docs/rules/order.md

👍 Examples of correct code

import fs from "fs"
import file from "./file";
import file2 from "./file2";

👎 Examples of incorrect code

import file from "./file";
import fs from "fs"
import file2 from "./file2";

No Anonymous Default Export

Reports if a module's default export is unnamed.

https://github.com/import-js/eslint-plugin-import/blob/HEAD/docs/rules/no-anonymous-default-export.md

👍 Examples of correct code

const foo = 123;
export default foo;

export default class MyClass() {};

export default function foo() {};

👎 Examples of incorrect code

export default []

export default () => {}

export default class {}

export default function () {}

export default 123

export default {}

export default new Foo()

Prefer Node Protocol

Prefer using the node: protocol when importing Node.js builtin modules

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/prefer-node-protocol.md

👍 Examples of correct code

import dgram from 'node:dgram';
export {strict as default} from 'node:assert';
import fs from 'node:fs/promises';
const fs = require('fs');
import _ from 'lodash';
import fs from './fs.js';
const fs = require('node:fs/promises');

👎 Examples of incorrect code

import dgram from 'dgram';
export {strict as default} from 'assert';
import fs from 'fs/promises';
const fs = require('fs/promises');

Prefer export from

Prefer export…from when re-exporting

When re-exporting from a module, it's unnecessary to import and then export. It can be done in a single export…from declaration.

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/prefer-export-from.md

👍 Examples of correct code

export {default} from './foo.js';

export {named} from './foo.js';

export * as namespace from './foo.js';
export {
    default,
    default as renamedDefault,
    named,
    named as renamedNamed,
} from './foo.js';

// There is no substitution
import * as namespace from './foo.js';
export default namespace;

👎 Examples of incorrect code

import defaultExport from './foo.js';
export default defaultExport;

import {named} from './foo.js';
export {named};

import * as namespace from './foo.js';
export {namespace};

import defaultExport, {named} from './foo.js';
export default defaultExport;
export {
    defaultExport as renamedDefault,
    named,
    named as renamedNamed,
};

No Empty Import

Prevent import empty

https://github.com/ODGodinho/ODG-Linter-Js

👍 Examples of correct code

import { a } from './foo.js';
import a from './foo.js';

👎 Examples of incorrect code

import { } from './foo.js';
import t, { } from './foo.js';
import type { } from './foo.js';

Export End File

Ensure all exports appear after other statements.

https://github.com/import-js/eslint-plugin-import/blob/main/docs/rules/exports-last.md

👍 Examples of correct code

import { a } from './foo.js';
import a from './foo.js';

👎 Examples of incorrect code

import { } from './foo.js';
import t, { } from './foo.js';

Import First

This rule reports any imports that come after non-import statements.

https://github.com/import-js/eslint-plugin-import/blob/main/docs/rules/first.md

👍 Examples of correct code

import foo from './foo'
import bar from './bar' // <- reported

initWith(foo)

👎 Examples of incorrect code

import foo from './foo'

// some module-level initializer
initWith(foo)

import bar from './bar' // <- reported

No Named Default

Reports use of a default export as a locally named import.

https://github.com/import-js/eslint-plugin-import/blob/main/docs/rules/no-named-default.md

👍 Examples of correct code

import foo from './foo.js';
import foo, { bar } from './foo.js';

👎 Examples of incorrect code

// message: Using exported name 'bar' as identifier for default export.
import { default as foo } from './foo.js';
import { default as foo, bar } from './foo.js';

Documentation

Spaced Comment

Require space after comment block.

Many style guides require empty lines before or after comments. The primary goal of these rules is to make the comments easier to read and improve readability of the code.

https://eslint.org/docs/rules/spaced-comment https://eslint.org/docs/latest/rules/lines-around-comment

👍 Examples of correct code

// This is a comment with a whitespace at the beginning

/* This is a comment with a whitespace at the beginning */

/*
 * This is a comment with a whitespace at the beginning
 */

/**
 * valid
 */
function() {}

👎 Examples of incorrect code

//This is a comment with no whitespace at the beginning

/*This is a comment with no whitespace at the beginning */

/**
 * invalid after space
 */

function() {}

Capitalized Comments

Require capitalization of the first letter of a comment.

https://eslint.org/docs/rules/capitalized-comments

👍 Examples of correct code

// Capitalized comment

// 1. Non-letter at beginning of comment

// 丈 Non-Latin character at beginning of comment

👎 Examples of incorrect code

// lowercase comment

Comment Align

Require alignment of JSDoc block asterisks.

https://github.com/gajus/eslint-plugin-jsdoc#user-content-eslint-plugin-jsdoc-rules-check-alignment

👍 Examples of correct code

/**
 * @param {Number} foo
 */

👎 Examples of incorrect code

/**
  * @param {Number} foo
 */

/**
* @param {Number} foo
*/

Disallow Space Align

Disallow use space for align dockblock

https://github.com/gajus/eslint-plugin-jsdoc#user-content-eslint-plugin-jsdoc-rules-check-alignment

👍 Examples of correct code

/**
 * @param {string} lorem Description.
 * @param {int} sit Description multi words.
 */

👎 Examples of incorrect code

/**
 * @param {string} lorem Description.
 * @param {int}    sit Description multi words.
 */

Validate Param

Check is valid @param and exists

https://github.com/gajus/eslint-plugin-jsdoc#check-param-names

👍 Examples of correct code

/**
 * @param {string} a Description.
 */
foo(a) {
}

/**
 * @param {string} a Description.
 * @param {string} b Description.
 */
foo(a, b) {
}

👎 Examples of incorrect code

/**
 * @param {string} b B iss not exists.
 */
foo(a) {
}

/**
 * @param {string} b order is not correct
 * @param {string} a
 */
foo(a, b) {
}

/**
 * @param {string} a
 * required b
 */
foo(a, b) {
}

Validate Syntax

Check is valid syntax docblock

https://github.com/gajus/eslint-plugin-jsdoc#check-syntax

👍 Examples of correct code

/**
 * @param {string} foo
 */
function quux (foo) {

}

👎 Examples of incorrect code

/**
 * @param {string=} foo
 */
function quux (foo) {

}

/**
 * @param {string} [foo]
 */
function quux (foo) {

}

Validate Tag Name

Check is valid tag docblock

https://github.com/gajus/eslint-plugin-jsdoc#check-tag-names

👍 Examples of correct code

/**
 * @param
 */

👎 Examples of incorrect code

/**
 * @notExistTag
 */

Validate Types

Check is valid type in docblock

https://github.com/gajus/eslint-plugin-jsdoc#check-types

👍 Examples of correct code

/**
 * @param {string} a
 */

👎 Examples of incorrect code

/**
 * @param {strings} a strings is not valid
 */

Validate Values

Check is valid values in docblock

https://github.com/gajus/eslint-plugin-jsdoc#check-values

👍 Examples of correct code

/**
 * @version 1.0.0
 */

/**
 * @version v1.0.0
 */

👎 Examples of incorrect code

/**
 * @version v 1.0.0
 */

/**
 * @version it's my version
 */

Empty Tags

Require tags is empty

https://github.com/gajus/eslint-plugin-jsdoc#user-content-eslint-plugin-jsdoc-rules-empty-tags

👍 Examples of correct code

/**
 * @global
 */

👎 Examples of incorrect code

/**
 * @global this is global
 */

Disallow Extra Asterisk

Disallow Extra asterisk in docblock

https://github.com/gajus/eslint-plugin-jsdoc#no-multi-asterisks

👍 Examples of correct code

/**
 * bold text
 */

👎 Examples of incorrect code

/**
 * *bold* text
 */

Disallow Default Value

Disallow Extra asterisk in docblock

https://github.com/gajus/eslint-plugin-jsdoc#user-content-eslint-plugin-jsdoc-rules-no-defaults

👍 Examples of correct code

/**
 * @param {number} foo
 */
function quux(foo) {

}

👎 Examples of incorrect code

/**
 * @param {number} [foo="7"]
 */
function quux(foo) {

}

Regex Rules

No Optional With Assertion

This rule reports elements that contradict an assertion. All elements reported by this rule fall into one of two categories:

https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-contradiction-with-assertion.html

👍 Examples of correct code

var foo = /a\b-a/;
var foo = /a\ba/; // handled by regexp/no-useless-assertions

👎 Examples of incorrect code

var foo = /a\b-?a/;
var foo = /a\b(a|-)/;
var foo = /a\ba*-/;

No Control Character

Control characters are special, invisible characters in the ASCII range 0-31. These characters are rarely used in JavaScript strings so a regular expression containing elements that explicitly match these characters is most likely a mistake.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-control-character.html

👍 Examples of correct code

var foo = /\n/;
var foo = RegExp("\n");

var pattern1 = /\x20/;
var pattern2 = /\u0020/;
var pattern3 = /\u{20}/u;
var pattern4 = /\t/;
var pattern5 = /\n/;
var pattern6 = new RegExp("\x20");
var pattern7 = new RegExp("\\t");
var pattern8 = new RegExp("\\n");

👎 Examples of incorrect code

var foo = /\x1f/;
var foo = /\x0a/;
var foo = RegExp('\x0a');

var pattern1 = /\x00/;
var pattern2 = /\x0C/;
var pattern3 = /\x1F/;
var pattern4 = /\u000C/;
var pattern5 = /\u{C}/u;
var pattern6 = new RegExp("\x0C"); // raw U+000C character in the pattern
var pattern7 = new RegExp("\\x0C"); // \x0C pattern

No Dupe Condition

This rule disallows duplicate disjunctions.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-dupe-disjunctions.html

👍 Examples of correct code

const foo = /a|b/
const foo = /(a|b)/
const foo = /(?:a|b)/

👎 Examples of incorrect code

const foo = /a|a/
const foo = /(a|a)/
const foo = /(?:a|a)/
const foo = /abc|abc/
const foo = /[ab]|[ba]/
const foo = /a|abc/
const foo = /.|abc/
const foo = /.|a|b|c/

No Empty Alternative

While (re-)writing long regular expressions, it can happen that one forgets to remove the | character of a former alternative. This rule tries to point out these potential mistakes by reporting all empty alternatives.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-empty-alternative.html

👍 Examples of correct code

var foo = /(?:)/
var foo = /a+|b*/

👎 Examples of incorrect code

var foo = /a+|b+|/
var foo = /\|\||\|||\|\|\|/
var foo = /a(?:a|bc|def|h||ij|k)/

No Empty Capturing Group

This rule reports capturing group that captures assertions.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-empty-capturing-group.html

👍 Examples of correct code

var foo = /(?:)/
var foo = /a+|b*/

👎 Examples of incorrect code

var foo = /a+|b+|/
var foo = /\|\||\|||\|\|\|/
var foo = /a(?:a|bc|def|h||ij|k)/

No Empty Character Class

This rule reports character classes that cannot match any characters.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-empty-character-class.html

👍 Examples of correct code

var foo = /abc[d]/;
var foo = /abc[a-z]/;
var foo = /[^]/;
var foo = /[\s\S]/;

👎 Examples of incorrect code

var foo = /abc[]/;
var foo = /[^\s\S]/;

No Empty Group

This rule reports empty groups.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-empty-group.html

👍 Examples of correct code

var foo = /(a)/;
var foo = /(?:a)/;

👎 Examples of incorrect code

var foo = /()/;
var foo = /(|)/;
// non-capturing group
var foo = /(?:)/;
var foo = /(?:|)/;

No Empty Look Rounds Assertion

This rule reports empty lookahead assertion or empty lookbehind assertion.

What are empty look around?

An empty look around is a look around for which at least one path in the look around expression contains only elements that do not consume characters and do not assert characters. This means that the look around expression will trivially accept any input string.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-empty-lookarounds-assertion.html

👍 Examples of correct code

var foo = /x(?=y)/;
var foo = /x(?!y)/;
var foo = /(?<=y)x/;
var foo = /(?<!y)x/;

👎 Examples of incorrect code

var foo = /x(?=)/;
var foo = /x(?!)/;
var foo = /(?<=)x/;
var foo = /(?<!)x/;
var foo = /(?=b?)\w/;
var foo = /(?!b?)\w/;

No Escape Backspace

This rule reports [\b]. The word boundaries (\b) and the escape backspace ([\b]) are indistinguishable at a glance. This rule does not allow backspace ([\b]). Use unicode escapes (\u0008) instead.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-escape-backspace.html

👍 Examples of correct code

var foo = /\b/;
var foo = /\u0008/;
var foo = /\cH/;
var foo = /\x08/;

👎 Examples of incorrect code

var foo = /[\b]/;

No Invalid Regexp

This rule reports invalid regular expression patterns given to RegExp constructors.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-invalid-regexp.html

👍 Examples of correct code

RegExp('foo')
RegExp('[a' + ']')

👎 Examples of incorrect code

RegExp('\\')
RegExp('[a-Z]*')
RegExp('\\p{Foo}', 'u')

const space = '\\s*'
RegExp('=' + space + '+(\\w+)', 'u')

No Lazy Ends

If a lazily quantified element is the last element matched by an expression (e.g. the a{2,3}? in b+a{2,3}?), we know that the lazy quantifier will always only match the element the minimum number of times. The maximum is completely ignored because the expression can accept after the minimum was reached.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-lazy-ends.html

👍 Examples of correct code

var foo = /a+?b*/.test(str)
var foo = /a??(?:ba+?|c)*/.test(str)
var foo = /ba*?$/.test(str)

👎 Examples of incorrect code

var foo = /a??/.test(str)
var foo = /a+b+?/.test(str)
var foo = /a(?:c|ab+?)?/.test(str)

No Misleading Unicode Character

This rule reports misleading Unicode characters.

Some Unicode characters like '❇️', '🇧🇷', and '👨‍👩‍👦' consist of multiple code points. This causes problems in character classes and around quantifiers. E.g.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-misleading-unicode-character.html

👍 Examples of correct code

var foo = /👍+/u;
var foo = /👨‍👩‍👦/;

👎 Examples of incorrect code

var foo = /👍+/;
var foo = /[❇️👨‍👩‍👦]❤️/;

No Optional Assertion

Assertions that are quantified (directly or indirectly) can be considered optional if the quantifier has a minimum of zero.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-optional-assertion.html

👍 Examples of correct code

var foo = /\w+(?::|\b)/;

👎 Examples of incorrect code

var foo = /a(?:$)*b/;
var foo = /a(?:foo|(?<!-)(?:-|\b))*b/; // The `\b` is optional.
var foo = /(?:^)?\w+/;   // warns about `^`
var foo = /\w+(?::|$)?/; // warns about `$`

No Potentially Useless Back Reference

If the referenced group of a backreference is not matched because some other path leads to the back-reference, the back-reference will trivially accept (e.g. /(?:(a)|b)\1/). The same will happen if the captured text of the referenced group was reset before reaching the back-reference.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-potentially-useless-backreference.html https://eslint.org/docs/latest/rules/no-useless-backreference

👍 Examples of correct code

var foo = /(a+)b\1/;
var foo = /(a+)b|\1/;

👎 Examples of incorrect code

var foo = /(?:(a)|b)\1/;
var foo = /(a)?b\1/;
var foo = /((a)|c)+b\2/;

No Useless Assertions

Some assertion are unnecessary because the rest of the pattern forces them to always be accept (or reject).

https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-useless-assertions.html

👍 Examples of correct code

var foo = /\bfoo\b/;

👎 Examples of incorrect code

var foo = /#\bfoo/;    // \b will always accept
var foo = /foo\bbar/;  // \b will always reject
var foo = /$foo/;      // $ will always reject
var foo = /(?=\w)\d+/; // (?=\w) will always accept

No Useless Back Reference

Back references that will always trivially accept serve no function and can be removed.

This rule is a based on the ESLint core no-useless-back-reference rule. It reports all the ESLint core rule reports and some more.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-useless-backreference.html

👍 Examples of correct code

var foo = /(a)b\1/;
var foo = /(a?)b\1/;
var foo = /(\b|a)+b\1/;
var foo = /(a)?(?:a|\1)/;

👎 Examples of incorrect code

var foo = /\1(a)/;
var foo = /(a\1)/;
var foo = /(a)|\1/;
var foo = /(?:(a)|\1)+/;
var foo = /(?<=(a)\1)/;
var foo = /(\b)a\1/;

No Useless Dollar Replacements

This rule aims to detect and disallow useless $ replacements in regular expression replacements.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-useless-dollar-replacements.html

👍 Examples of correct code

var newStr = str.replace(/(\w+)\s(\w+)/, '$2, $1');
// newStr = "Smith, John"

var newStr = str.replace(/(?<first>\w+)\s(?<last>\w+)/, '$<last>, $<first>');
// newStr = "Smith, John"

'123456789012'.replaceAll(/(.)../g, '$1**'); // "1**4**7**0**"

👎 Examples of incorrect code

var newStr = str.replace(/(\w+)\s(\w+)/, '$3, $1 $2');
// newStr = "$3, John Smith"

var newStr = str.replace(/(?<first>\w+)\s(?<last>\w+)/, '$<last>, $<first> $<middle>');
// newStr = "Smith, John "

var newStr = str.replace(/(\w+)\s(\w+)/, '$<last>, $<first>');
// newStr = "$<last>, $<first>"

Strict Regex

This rule disallows not strictly valid regular expressions.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/strict.html

👍 Examples of correct code

var foo = /\}/
var foo = /\{/
var foo = /\]/
var foo = /\u{42}/u; // It matches "B".
var foo = /u{42}/; // It matches a string followed by 42 "u"s.

👎 Examples of incorrect code

var foo = /}/
var foo = /{/
var foo = /]/
var foo = /\u{42}/; // It matches a string followed by 42 "u"s.

Confusing Quantifier

Confusing quantifiers are ones which imply one thing but don't deliver on that.

An example of this is (?:a?b*|c+){4}. The group is quantified with {4} which implies that at least 4 characters will be matched but this is not the case. The whole pattern will match the empty string. It does that because in the a?b* alternative, it's possible to choose 0 many a and b. So rather than {4}, {0,4} should be used to reflect the fact that the empty string can be matched.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/confusing-quantifier.html

👍 Examples of correct code

var foo = /a*/;
var foo = /(a|b|c)+/;
var foo = /a?/;

👎 Examples of incorrect code

var foo = /(a?){4}/; // warns about `{4}`
var foo = /(a?b*)+/; // warns about `+`

Control Character Escape

This rule reports control characters that were not escaped using a control escape (\0, t, \n, \v, f, \r).

https://ota-meshi.github.io/eslint-plugin-regexp/rules/control-character-escape.html

👍 Examples of correct code

var foo = /[\n\r]/;
var foo = /\t/;
var foo = RegExp("\t+\n");

👎 Examples of incorrect code

var foo = / /;
var foo = /\u0009/;
var foo = /\u{a}/u;
var foo = RegExp("\\u000a");

Negation

This rule enforces use of \D, \W, \S and \P on negation.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/negation.html

👍 Examples of correct code

var foo = /\D/
var foo = /\W/
var foo = /\S/
var foo = /\P{ASCII}/u

var foo = /\d/
var foo = /\w/
var foo = /\s/
var foo = /\p{ASCII}/u

👎 Examples of incorrect code

var foo = /[^\d]/
var foo = /[^\w]/
var foo = /[^\s]/
var foo = /[^\p{ASCII}]/u

var foo = /[^\D]/
var foo = /[^\W]/
var foo = /[^\S]/
var foo = /[^\P{ASCII}]/u

No Dupe Characters Character Class

Because multiple same character classes in regular expressions only one is useful, they might be typing mistakes.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-dupe-characters-character-class.html

👍 Examples of correct code

var foo = /[\(\)]/;

var foo = /[a-z\s]/;

var foo = /[\w]/;

👎 Examples of incorrect code

var foo = /[\\(\\)]/;
//          ^^ ^^        "\\" are duplicated
var foo = /[a-z\\s]/;
//          ^^^  ^       "s" are duplicated
var foo = /[\w0-9]/;
//          ^^^^^        "0-9" are duplicated

No Invisible Character

This rule disallows using invisible characters other than SPACE (U+0020) without using escapes.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-invisible-character.html

👍 Examples of correct code

var foo = /\t/;
var foo = /\v/;
var foo = /\f/;
var foo = /\u3000/;
var foo = / /; // SPACE (`U+0020`)

👎 Examples of incorrect code

// \u000B - Line Tabulation (\v) - <VT>
// \u000C - Form Feed (\f) - <FF>
// \u00A0 - No-Break Space - <NBSP>
// \u0085 - Next Line
// \u1680 - Ogham Space Mark
// \u180E - Mongolian Vowel Separator - <MVS>
// \ufeff - Zero Width No-Break Space - <BOM>
// \u2000 - En Quad
// \u2001 - Em Quad
// \u2002 - En Space - <ENSP>
// \u2003 - Em Space - <EMSP>
// \u2004 - Three-Per-Em
// \u2005 - Four-Per-Em
// \u2006 - Six-Per-Em
// \u2007 - Figure Space
// \u2008 - Punctuation Space - <PUNCSP>
// \u2009 - Thin Space
// \u200A - Hair Space
// \u200B - Zero Width Space - <ZWSP>
// \u2028 - Line Separator
// \u2029 - Paragraph Separator
// \u202F - Narrow No-Break Space
// \u205f - Medium Mathematical Space
// \u3000 - Ideographic Space

No Legacy Features

This rule disallow legacy RegExp features.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-legacy-features.html

👍 Examples of correct code

👎 Examples of incorrect code

RegExp.input
RegExp.$_
RegExp.lastMatch
RegExp["$&"]
RegExp.lastParen
RegExp["$+"]
RegExp.leftContext
RegExp["$`"]
RegExp.rightContext
RegExp["$'"]
RegExp.$1
RegExp.$2
RegExp.$3
RegExp.$4
RegExp.$5
RegExp.$6
RegExp.$7
RegExp.$8
RegExp.$9

const regexObj = new RegExp('foo', 'gi');
regexObj.compile('new foo', 'g');

No Non Standard Flag

This rule reports non-standard flags.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-non-standard-flag.html

👍 Examples of correct code

var foo = /a*b*c/guy;

👎 Examples of incorrect code

var foo = RegExp("a*b*c", "l"); // L don1t exists

No Obscure Range

The character range operator (the - inside character classes) can easily be misused (mostly unintentionally) to construct non-obvious character class. This rule will disallow all non-obvious uses of the character range operator.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-obscure-range.html

👍 Examples of correct code

var foo = /[a-z]/;
var foo = /[J-O]/;
var foo = /[1-9]/;
var foo = /[\x00-\x40]/;
var foo = /[\0-\uFFFF]/;
var foo = /[\0-\u{10FFFF}]/u;
var foo = /[\1-\5]/;
var foo = /[\cA-\cZ]/;

👎 Examples of incorrect code

var foo = /[A-\x43]/;
var foo = /[\41-\x45]/;
var foo = /[!-$]/;
var foo = /[😀-😄]/u;

No Trivially Nested Quantifier

The character range operator (the - inside character classes) can easily be misused (mostly unintentionally) to construct non-obvious character class. This rule will disallow all non-obvious uses of the character range operator.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-obscure-range.html

👍 Examples of correct code

var foo = /[a-z]/;
var foo = /[J-O]/;
var foo = /[1-9]/;
var foo = /[\x00-\x40]/;
var foo = /[\0-\uFFFF]/;
var foo = /[\0-\u{10FFFF}]/u;
var foo = /[\1-\5]/;
var foo = /[\cA-\cZ]/;

👎 Examples of incorrect code

var foo = /[A-\x43]/;
var foo = /[\41-\x45]/;
var foo = /[!-$]/;
var foo = /[😀-😄]/u;

No Unused Capturing Group

This rule reports unused capturing groups.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-unused-capturing-group.html

👍 Examples of correct code

var matches = '2000-12-31 2001-01-01'.match(/(\d{4})-(\d{2})-(\d{2})/)
var y = matches[1] // "2000"
var m = matches[2] // "12"
var d = matches[3] // "31"

👎 Examples of incorrect code

var isDate = /(\d{4})-(\d{2})-(\d{2})/.test('2000-12-31') // true But group never used

No Useless Character Class

This rule reports character classes that defines only one character.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-useless-character-class.html

👍 Examples of correct code

var foo = /abc/;

👎 Examples of incorrect code

var foo = /a[b]c/;

No Useless Flag

This will point out present regex flags that do not change the pattern.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-useless-character-class.html

👍 Examples of correct code

var foo = /a|b/i;

var foo = /^foo$/m;

var foo = /a.*?b/s;

const regex1 = /foo/y;
const str = 'table football, football';
regex1.lastIndex = 6
var array = regex1.exec(str)

👎 Examples of incorrect code

var foo = /\.{3}/i;
var foo = /\w+/i;

var foo = /foo|[^\r\n]*/m;
var foo = /a|b/m;

var foo = /[.:]/s;
var foo = /^foo$/s;

str.split(/foo/y);

No Useless Lazy

This rule reports lazy quantifiers that don't need to by lazy.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-useless-lazy.html

👍 Examples of correct code

var foo = /a*?/;
var foo = /a+?/;
var foo = /a{4,}?/;
var foo = /a{2,4}?/;
var foo = /a[\s\S]*?bar/;

👎 Examples of incorrect code

var foo = /a{1}?/;
var foo = /a{4}?/;
var foo = /a{2,2}?/;
var foo = /ab+?c/;

No Useless Quantifier

This rule reports quantifiers that can trivially be removed without affecting the pattern.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-useless-quantifier.html

👍 Examples of correct code

var foo = /a*/;
var foo = /(?:a|b?)??/;
var foo = /(?:\b|(?!a))*/;

👎 Examples of incorrect code

var foo = /a{1}/;
var foo = /(?:\b)+/;
var foo = /(?:a+b*|c*)?/;

No Useless Range

This rule reports unnecessary range of characters by using a hyphen. e.g. [a-a]

https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-useless-range.html

👍 Examples of correct code

var foo = /[a]/
var foo = /[ab]/

👎 Examples of incorrect code

var foo = /[a-a]/
var foo = /[a-b]/

No Useless Two Num Quantifier

This rule reports unnecessary {n,m} quantifiers.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-useless-two-nums-quantifier.html

👍 Examples of correct code

var foo = /a{0,1}/;
var foo = /a{1,5}/;
var foo = /a{1,}/;
var foo = /a{2}/;

👎 Examples of incorrect code

var foo = /a{0,0}/;
var foo = /a{1,1}/;
var foo = /a{2,2}/;

No Zero Quantifier

This rule reports quantifiers with a maximum of zero. These quantifiers trivially do not affect the pattern is any way and can be removed.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-zero-quantifier.html

👍 Examples of correct code

var foo = /a?/;
var foo = /a{0,}/;
var foo = /a{0,1}/;

👎 Examples of incorrect code

var foo = /a{0}/;
var foo = /a{0,0}?/;
var foo = /(a){0}/;

Optimal LookAround Quantifier

Non-constant quantifiers are quantifiers that describe a range.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/optimal-lookaround-quantifier.html

👍 Examples of correct code

// lookaheads
var foo = /\w+(?=\s*:)/;

// lookbehinds
var foo = /(?<=ab+)/;

👎 Examples of incorrect code

// lookaheads
var foo = /(?=ab+)/; // == /(?=ab)/
var foo = /(?=ab*)/; // == /(?=a)/
var foo = /(?!ab?)/; // == /(?!a)/
var foo = /(?!ab{6,})/; // == /(?!ab{6})/

// lookbehinds
var foo = /(?<=a+b)/; // == /(?<=ab)/
var foo = /(?<!\w*\s*,)/; // == /(?<!,)/

Optimal Quantifier Concatenation

If two quantified characters, character classes, or characters are concatenated, the quantifiers can be optimized if either of the characters elements is a subset of the other.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/optimal-lookaround-quantifier.html

👍 Examples of correct code

var foo = /\w+\d{4}/;
var foo = /\w{3,5}\d*/;
var foo = /a+b+c+d+[abc]+/;
var foo = /a\w*/;

👎 Examples of incorrect code

var foo = /\w+\d+/;
var foo = /\w+\d?/;
var foo = /[ab]*(?:a|b)/;
var foo = /\w+(?:(a)|b)*/;

Prefer Quantifier

This rule is aimed to use quantifiers instead of consecutive characters in regular expressions.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/optimal-lookaround-quantifier.html

👍 Examples of correct code

var foo = /\d{4}-\d{2}-\d{2}/;

👎 Examples of incorrect code

var foo = /\d\d\d\d-\d\d-\d\d/;

Prefer Range

This rule is aimed to use ranges instead of multiple adjacent characters in character class.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/prefer-range.html

👍 Examples of correct code

var foo = /[0-9]/
var foo = /[a-f]/

👎 Examples of incorrect code

var foo = /[123456789]/
var foo = /[a-cd-f]/

Sort Alternatives

This rule will only sort alternatives if reordering the alternatives doesn't affect the pattern.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/sort-alternatives.html

👍 Examples of correct code

var foo = /\b(1|2|3)\b/;
var foo = /\b(alpha|beta|gamma)\b/;

👎 Examples of incorrect code

var foo = /\b(2|1|3)\b/;
var foo = /__(?:Foo|Bar)__/;
var foo = /\((?:TM|R|C)\)/;

Hexadecimal Escape

This rule aims is enforces the consistent use of hexadecimal escapes.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/hexadecimal-escape.html

👍 Examples of correct code

var foo = /\x0a/;

👎 Examples of incorrect code

var foo = /\u000a/;
var foo = /\u{a}/u;

Match Any

This rule enforces the regular expression notation to match any character. e.g. [\s\S], [^], /./s (dotAll) and more.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/match-any.html

👍 Examples of correct code

var foo = /[\s\S]/;
var foo = /./s;

👎 Examples of incorrect code

var foo = /[\S\s]/;
var foo = /[^]/;
var foo = /[\d\D]/;
var foo = /[\w\W]/;

No Useless Escape

This rule reports unnecessary escape characters in RegExp. You may be able to find another mistake by finding unnecessary escapes.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-useless-escape.html

👍 Examples of correct code

var foo = /\[/
var foo = /\\/

👎 Examples of incorrect code

var foo = /\a/
var foo = /\x7/
var foo = /\u41/
var foo = /\u{[41]}/

No Useless Non Capturing Group

This rule reports unnecessary non-capturing group

https://ota-meshi.github.io/eslint-plugin-regexp/rules/prefer-character-class.html

👍 Examples of correct code

var foo = /(?:abcd)?/.test(str)
var foo = /a(?:ab|cd)/.test(str)

👎 Examples of incorrect code

var foo = /(?:ab|cd)/.test(str)
var foo = /(?:abcd)/.test(str)
var foo = /(?:[a-d])/.test(str)
var foo = /(?:[a-d])|e/.test(str)
var foo = /(?:a|(?:b|c)|d)/.test(str)

Prefer Character Class

Instead of single-character alternatives (e.g. (?:a|b|c)),character classes (e.g. [abc]) should be preferred.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-useless-non-capturing-group.html

👍 Examples of correct code

var foo = /[abc]/
var foo = /(?:a|b)/

👎 Examples of incorrect code

var foo = /a|b|c/
var foo = /(a|b|c)c/
var foo = /.|\s/
var foo = /(\w|\d)+:/

Prefer D

This rule is aimed at using \d instead of [0-9] in regular expressions.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/prefer-d.html

👍 Examples of correct code

var foo = /\d/;
var foo = /\D/;

👎 Examples of incorrect code

var foo = /[0-9]/;
var foo = /[^0-9]/;

Prefer Plus

This rule is aimed at using + quantifier instead of {1,} in regular expressions.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/prefer-plus-quantifier.html

👍 Examples of correct code

var foo = /a+/;

👎 Examples of incorrect code

var foo = /a{1,}/;

Prefer Question Quantifier

This rule is aimed at using ? quantifier instead of {0,1} in regular expressions.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/prefer-question-quantifier.html

👍 Examples of correct code

var foo = /a?/;

👎 Examples of incorrect code

var foo = /a{0,1}/;

Prefer Star Quantifier

This rule is aimed at using * quantifier instead of {0,} in regular expressions.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/prefer-star-quantifier.html

👍 Examples of correct code

var foo = /a*/

👎 Examples of incorrect code

var foo = /a{0,}/;

Prefer Unicode Code Point Escapes

This rule enforces the use of Unicode codepoint escapes instead of Unicode escapes using surrogate pairs.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/prefer-unicode-codepoint-escapes.html

👍 Examples of correct code

var foo = /\u{1f600}/u
var foo = /😀/u

👎 Examples of incorrect code

var foo = /\ud83d\ude00/u

Prefer W

This rule is aimed at using \w in regular expressions.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/prefer-w.html

👍 Examples of correct code

var foo = /\w/;
var foo = /\W/;

👎 Examples of incorrect code

var foo = /[0-9a-zA-Z_]/;
var foo = /[^0-9a-zA-Z_]/;
var foo = /[0-9a-z_]/i;
var foo = /[0-9a-z_-]/i;

Sort Character Class Elements

This rule checks elements of character classes are sorted.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/sort-character-class-elements.html

👍 Examples of correct code

var foo = /[abcdef]/
var foo = /[ab-f]/

👎 Examples of incorrect code

var foo = /[bcdefa]/
var foo = /[b-fa]/

Sort Flags

The flags of JavaScript regular expressions should be sorted alphabetically because the flags of the .flags property of RegExp objects are always sorted. Not sorting flags in regex literals misleads readers into thinking that the order may have some purpose which it doesn't.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/sort-flags.html

👍 Examples of correct code

var foo = /abc/
var foo = /abc/iu
var foo = /abc/gimsuy

👎 Examples of incorrect code

var foo = /abc/mi
var foo = /abc/us

Prefer Named Capture Group

This rule reports capturing groups without a name.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/prefer-named-capture-group.html

👍 Examples of correct code

var foo = /(?<foo>ba+r)/;
var foo = /\b(?:foo)+\b/;

👎 Examples of incorrect code

var foo = /\b(foo)+\b/;

Prefer Regexp Exec

RegExp#exec is faster than String#match and both work the same when not using the /g flag.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/prefer-regexp-exec.html https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/prefer-regexp-exec.md

👍 Examples of correct code

/thing/.exec('something');

'some things are just things'.match(/thing/g);

const text = 'something';
const search = /thing/;
search.exec(text);

👎 Examples of incorrect code

'something'.match(/thing/);

'some things are just things'.match(/thing/);

const text = 'something';
const search = /thing/;
text.match(search);

Existing Groups

Replacement strings should reference existing regular expression groups

https://sonarsource.github.io/rspec/#/rspec/S6328/javascript

👍 Examples of correct code

const str = 'James Bond';
console.log(str.replace(/(\w+)\s(\w+)/, '$1, $0 $1'));
console.log(str.replace(/(?<firstName>\w+)\s(?<lastName>\w+)/, '$<surname>, $<firstName> $<surname>'));

👎 Examples of incorrect code

const str = 'James Bond';
console.log(str.replace(/(\w+)\s(\w+)/, '$2, $1 $2'));
console.log(str.replace(/(?<firstName>\w+)\s(?<lastName>\w+)/, '$<lastName>, $<firstName> $<lastName>'));

No Misleading Capturing Group

This rule reports capturing groups that capture less text than their pattern might suggest.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-misleading-capturing-group.html

👍 Examples of correct code

var foo = /a+(b*)/

👎 Examples of incorrect code

var foo = /a+(a*)/
var foo = /\w+(\d*)/
var foo = /^(a*).+/

No Extra Lookaround Assertions

The last positive lookahead assertion within a lookahead assertion is the same without lookahead assertions. Also, The first positive lookbehind assertion within a lookbehind assertion is the same without lookbehind assertions. They can be inlined or converted to group.

https://ota-meshi.github.io/eslint-plugin-regexp/rules/no-extra-lookaround-assertions.html

👍 Examples of correct code

var ts = 'JavaScript'.replace(/Java(?=Script)/u, 'Type');
var java = 'JavaScript'.replace(/(?<=Java)Script/u, '');
var re1 = /a(?=bc)/u;
var re2 = /a(?=b(?:c|C))/u;
var re3 = /(?<=ab)c/u;
var re4 = /(?<=(?:a|A)b)c/u;

👎 Examples of incorrect code

var ts = 'JavaScript'.replace(/Java(?=Scrip(?=t))/u, 'Type');
var java = 'JavaScript'.replace(/(?<=(?<=J)ava)Script/u, '');
var re1 = /a(?=b(?=c))/u;
var re2 = /a(?=b(?=c|C))/u;
var re3 = /(?<=(?<=a)b)c/u;
var re4 = /(?<=(?<=a|A)b)c/u;

Security

Eval Disabled

JavaScript's eval() function is potentially dangerous and is often misused. Using eval() on untrusted code can open a program up to several different injection attacks. The use of eval() in most contexts can be substituted for a better, alternative approach to a problem. Disallow the use of eval()-like methods

https://eslint.org/docs/rules/no-eval#no-eval https://eslint.org/docs/rules/no-implied-eval https://sonarsource.github.io/rspec/#/rspec/S1523/javascript https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/no-implied-eval.md

👍 Examples of correct code

var obj = { x: "foo" },
    key = "x",
    value = obj[key];

class A {
    foo() {
        // This is a user-defined method.
        this.eval("var a = 0");
    }

    eval() {
    }

    static {
        // This is a user-defined static method.
        this.eval("var a = 0");
    }

    static eval() {
    }
}

👎 Examples of incorrect code

var obj = { x: "foo" },
    key = "x",
    value = eval("obj." + key);

(0, eval)("var a = 0");

var foo = eval;
foo("var a = 0");

// This `this` is the global object.
this.eval("var a = 0");

Detect Unsafe Regex

There are many ways to block the event loop, one way an attacker can do that is with Regular Expression Denial of Service (ReDoS).

https://github.com/nodesecurity/eslint-plugin-security/blob/main/docs/regular-expression-dos-and-node.md

👍 Examples of correct code

 var expression = /^[\w+\-.]+@[\d\-A-Za-z]+\.[\d\-.A-Za-z]+$/

👎 Examples of incorrect code

var expression = /^([a-zA-Z0-9_\.\-])+\@(([a-zA-Z0-9\-])+\.)+([a-zA-Z0-9]{2,4})+$/;

Buffer No Assert

Detect calls to buffer with noAssert flag set.

From the Node.js API docs: "Setting noAssert to true skips validation of the offset. This allows the offset to be beyond the end of the Buffer."

https://github.com/nodesecurity/eslint-plugin-security#detect-buffer-noassert

👍 Examples of correct code

👎 Examples of incorrect code

/** https://nodejs.org/api/buffer.html */

No Exec Child Process

Detect instances of child_process & non-literal exec()

https://github.com/nodesecurity/eslint-plugin-security/blob/main/docs/avoid-command-injection-node.md

👍 Examples of correct code

var child_process = require('child_process');

var path = '.';
child_process.execFile('/bin/ls', ['-l', path], function (err, result) {
  console.log(result);
});

// Or

var child_process = require('child_process');

var path = '.';
var ls = child_process.spawn('/bin/ls', ['-l', path]);
ls.stdout.on('data', function (data) {
  console.log(data.toString());
});

👎 Examples of incorrect code

var path = 'user input';
child_process.exec('ls -l ' + path, function (err, data) {
  console.log(data);
});

No Mustache Scape

Detects object.escapeMarkup = false, which can be used with some template engines to disable escaping of HTML entities. This can lead to Cross-Site Scripting (XSS) vulnerabilities.

More information: OWASP XSS

https://github.com/nodesecurity/eslint-plugin-security#detect-disable-mustache-escape

👍 Examples of correct code

// No remove object.escapeMarkup = false

👎 Examples of incorrect code

object.escapeMarkup = false

No Csrf Before Method Override

As the declaration order of middlewares determines the execution stack in Connect, it is possible to abuse this functionality in order to bypass the standard Connect's anti-CSRF protection.

https://github.com/nodesecurity/eslint-plugin-security/blob/main/docs/bypass-connect-csrf-protection-by-abusing.md

👍 Examples of correct code

app.use(express.csrf())
app.use(express.methodOverride())

👎 Examples of incorrect code

app.use(express.methodOverride())
app.use(express.csrf())

No Literal Fs Filename

Detects variable in filename argument of fs calls, which might allow an attacker to access anything on your system.

https://github.com/nodesecurity/eslint-plugin-security#detect-non-literal-fs-filename

👍 Examples of correct code

👎 Examples of incorrect code

No Pseudo Random Bytes

Detects if pseudoRandomBytes() is in use, which might not give you the randomness you need and expect.

https://github.com/nodesecurity/eslint-plugin-security#detect-non-literal-fs-filename https://sonarsource.github.io/rspec/#/rspec/S2245/javascript

👍 Examples of correct code

var crypto = require("crypto")

const random = crypto.randomBytes(60);

👎 Examples of incorrect code

const random = Math.random();

Prevent Secret Token

Prevent commit with token, passwords, keys etc.

👍 Examples of correct code

const apiKey = process.env.apiKey;

👎 Examples of incorrect code

const apiKey = "123456"

Prevent Literal Code

Prevent attack in your code

https://github.com/nodesecurity/eslint-plugin-security/blob/HEAD/docs/regular-expression-dos-and-node.md

👍 Examples of correct code

require('../name');
require(`../name`);

👎 Examples of incorrect code

require(name);
require('../' + name);
require(`../${name}`);
require(name());

No Import Dynamic

This rule checks every call to require() that uses expressions for the module name argument.

https://github.com/import-js/eslint-plugin-import/blob/HEAD/docs/rules/no-dynamic-require.md

👍 Examples of correct code

require('../name');
require(`../name`);

👎 Examples of incorrect code

require(name);
require('../' + name);
require(`../${name}`);
require(name());

Security SSL

Authorizing HTTP communications with S3 buckets is security-sensitive

https://sonarsource.github.io/rspec/#/rspec/S6249/javascript

👍 Examples of correct code

const s3 = require('aws-cdk-lib/aws-s3');

const bucket = new s3.Bucket(this, 'example', {
    bucketName: 'example',
    versioned: true,
    publicReadAccess: false,
    enforceSSL: true
});

👎 Examples of incorrect code

const s3 = require('aws-cdk-lib/aws-s3');

const bucket = new s3.Bucket(this, 'example'); // Sensitive

Security Encrypt Access

Allowing public ACLs or policies on a S3 bucket is security-sensitive Disabling server-side encryption of S3 buckets is security-sensitive

By default S3 buckets are private, it means that only the bucket owner can access it. This access control can be relaxed with ACLs or policies. To prevent permissive policies or ACLs to be set on a S3 bucket the following booleans settings can be enabled:

https://sonarsource.github.io/rspec/#/rspec/S6281/javascript https://sonarsource.github.io/rspec/#/rspec/S6245/javascript

👍 Examples of correct code

const s3 = require('aws-cdk-lib/aws-s3');

new s3.Bucket(this, 'id', {
    bucketName: 'bucket',
    blockPublicAccess: s3.BlockPublicAccess.BLOCK_ALL
});

new s3.Bucket(this, 'id', {
    blockPublicAccess: s3.BlockPublicAccess.BLOCK_ALL,
    encryption: s3.BucketEncryption.KMS_MANAGED
});

# Alternatively with a KMS key managed by the user.

new s3.Bucket(this, 'id', {
    blockPublicAccess: s3.BlockPublicAccess.BLOCK_ALL,
    encryption: s3.BucketEncryption.KMS_MANAGED,
    encryptionKey: access_key
});

👎 Examples of incorrect code

const s3 = require('aws-cdk-lib/aws-s3');

new s3.Bucket(this, 'id', {
    bucketName: 'bucket'
}); // Sensitive

Certificate Transparency

Disabling Certificate Transparency monitoring is security-sensitive

Implement Expect-CT HTTP header which instructs the web browser to check public CT logs in order to verify if the website appears inside and if it is not, the browser will block the request and display a warning to the user.

https://sonarsource.github.io/rspec/#/rspec/S5742/javascript

👍 Examples of correct code

const express = require('express');
const helmet = require('helmet');

let app = express();

app.use(helmet.expectCt({
  enforce: true,
  maxAge: 86400
})); // Compliant

👎 Examples of incorrect code

const express = require('express');
const helmet = require('helmet');

let app = express();

app.use(
    helmet({
      expectCt: false // Sensitive
    })
);

No Log confidential information

Log management is an important topic, especially for the security of a web application, to ensure user activity, including potential attackers, is recorded and available for an analyst to understand what’s happened on the web application in case of malicious activities.

https://sonarsource.github.io/rspec/#/rspec/S5757/javascript

👍 Examples of correct code

const { Signale } = require('signale');

const CREDIT_CARD_NUMBERS = fetchFromWebForm()
// here we suppose the credit card numbers are retrieved somewhere and CREDIT_CARD_NUMBERS looks like
// ["1234-5678-0000-9999", "1234-5678-0000-8888"]; for instance

const options = {
  secrets: ["([0-9]{4}-?)+"]
};

const logger = new Signale(options); // Compliant

CREDIT_CARD_NUMBERS.forEach(function(CREDIT_CARD_NUMBER) {
  logger.log('The customer ordered products with the credit card number = %s', CREDIT_CARD_NUMBER);
});

👎 Examples of incorrect code

const { Signale } = require('signale');

const CREDIT_CARD_NUMBERS = fetchFromWebForm()
// here we suppose the credit card numbers are retrieved somewhere and CREDIT_CARD_NUMBERS looks like
//  ["1234-5678-0000-9999", "1234-5678-0000-8888"]; for instance

const options = {
  secrets: []         // empty list of secrets
};

const logger = new Signale(options); // Sensitive

CREDIT_CARD_NUMBERS.forEach(function(CREDIT_CARD_NUMBER) {
  logger.log('The customer ordered products with the credit card number = %s', CREDIT_CARD_NUMBER);
});

Content Length

Rejecting requests with significant content length is a good practice to control the network traffic intensity and thus resource consumption in order to prevents DoS attacks.

https://sonarsource.github.io/rspec/#/rspec/S5693/javascript

👍 Examples of correct code

const form = new Formidable();
form.maxFileSize = 8000000; // Compliant: 8MB

let diskUpload = multer({
  storage: diskStorage,
  limits: {
     fileSize: 8000000 // Compliant: 8MB
  }
});

let jsonParser = bodyParser.json(); // Compliant, when the limit is not defined, the default value is set to 100kb
let urlencodedParser = bodyParser.urlencoded({ extended: false, limit: "2mb" }); // Compliant

👎 Examples of incorrect code

const form = new Formidable();
form.maxFileSize = 10000000; // Sensitive: 10MB is more than the recommended limit of 8MB

const formDefault = new Formidable(); // Sensitive, the default value is 200MB

let diskUpload = multer({
  storage: diskStorage,
  limits: {
    fileSize: 10000000; // Sensitive: 10MB is more than the recommended limit of 8MB
  }
});

let diskUploadUnlimited = multer({ // Sensitive: the default value is no limit
  storage: diskStorage,
});

// 4MB is more than the recommended limit of 2MB for non-file-upload requests
let jsonParser = bodyParser.json({ limit: "4mb" }); // Sensitive
let urlencodedParser = bodyParser.urlencoded({ extended: false, limit: "4mb" }); // Sensitive

Anti Trojan Source No Bidi

The trick is to use Unicode control characters to reorder tokens in source code at the encoding level. These visually reordered tokens can be used to display logic that, while semantically correct, diverges from the logic presented by the logical ordering of source code tokens.

https://trojansource.codes/ https://github.com/lirantal/eslint-plugin-anti-trojan-source https://github.com/eslint-community/eslint-plugin-security/blob/main/docs/rules/detect-bidi-characters.md

Cookie Httponly

When a cookie is configured with the HttpOnly attribute set to true, the browser guaranties that no client-side script will be able to read it. In most cases, when a cookie is created, the default value of HttpOnly is false and it’s up to the developer to decide whether or not the content of the cookie can be read by the client-side script. As a majority of Cross-Site Scripting (XSS) attacks target the theft of session-cookies, the HttpOnly attribute can help to reduce their impact as it won’t be possible to exploit the XSS vulnerability to steal session-cookies.

https://sonarsource.github.io/rspec/#/rspec/S3330/javascript https://sonarsource.github.io/rspec/#/rspec/S2255/javascript

let session = cookieSession({
  httpOnly: true,// Compliant
});  // Compliant

const express = require('express');
const session = require('express-session');

let app = express();
app.use(session({
  cookie:
  {
    httpOnly: true // Compliant
  }
}));

let cookies = new Cookies(req, res, { keys: keys });

cookies.set('LastVisit', new Date().toISOString(), {
  httpOnly: true // Compliant
}); // Compliant

const cookieParser = require('cookie-parser');
const csrf = require('csurf');
const express = require('express');

let csrfProtection = csrf({ cookie: { httpOnly: true }}); // Compliant

👎 Examples of incorrect code

let session = cookieSession({
  httpOnly: false,// Sensitive
});  // Sensitive

const express = require('express');
const session = require('express-session');

let app = express()
app.use(session({
  cookie:
  {
    httpOnly: false // Sensitive
  }
}));

let cookies = new Cookies(req, res, { keys: keys });

cookies.set('LastVisit', new Date().toISOString(), {
  httpOnly: false // Sensitive
}); // Sensitive

const cookieParser = require('cookie-parser');
const csrf = require('csurf');
const express = require('express');

let csrfProtection = csrf({ cookie: { httpOnly: false }}); // Sensitive

Protect XSS Render

Disabling auto-escaping in template engines is security-sensitive

https://sonarsource.github.io/rspec/#/rspec/S5247/javascript

let Mustache = require("mustache");

let rendered = Mustache.render(template, { name: inputName });

// or

const Handlebars = require('handlebars');

let source = "<p>attack {{name}}</p>";
let data = { "name": "<b>Alan</b>" };

let template = Handlebars.compile(source);

// or

let md = require('markdown-it')();

let result = md.render('# <b>attack</b>');

// or

const marked = require('marked');

marked.setOptions({
  renderer: new marked.Renderer()
}); // Compliant by default sanitize is set to true

console.log(marked("# test <b>attack/b>"));

// or

let kramed = require('kramed');

let options = {
  renderer: new kramed.Renderer({
    sanitize: true // Compliant
  })
};

console.log(kramed('Attack [xss?](javascript:alert("xss")).', options));

👎 Examples of incorrect code

let Mustache = require("mustache");

Mustache.escape = function(text) {return text;}; // Sensitive

let rendered = Mustache.render(template, { name: inputName });

// or

const Handlebars = require('handlebars');
let source = "<p>attack {{name}}</p>";
let template = Handlebars.compile(source, { noEscape: true }); // Sensitive

// or

const markdownIt = require('markdown-it');
let md = markdownIt({
  html: true // Sensitive
});

let result = md.render('# <b>attack</b>');

// or

const marked = require('marked');

marked.setOptions({
  renderer: new marked.Renderer(),
  sanitize: false // Sensitive
});

console.log(marked("# test <b>attack/b>"));

// or

let kramed = require('kramed');

var options = {
  renderer: new kramed.Renderer({
    sanitize: false // Sensitive
  })
};

Force Integrity

Disabling resource integrity features is security-sensitive

Fetching external resources, for example from a CDN, without verifying their integrity could impact the security of an application if the CDN gets compromised and resources are replaced by malicious ones. Resources integrity feature will block resources inclusion into an application if the pre-computed digest of the expected resource doesn’t match with the digest of the retrieved resource.

https://sonarsource.github.io/rspec/#/rspec/S5725/javascript

let script = document.createElement("script");
script.src = "https://cdnexample.com/script-v1.2.3.js";
script.integrity = "sha384-oqVuAfXRKap7fdgcCY5uykM6+R9GqQ8K/uxy9rx7HNQlGYl1kPzQho1wx4JwY8wC"; // Compliant
script.crossOrigin = "anonymous";
document.head.appendChild(script);

👎 Examples of incorrect code

let script = document.createElement("script");
script.src = "https://cdnexample.com/script-latest.js";
 // Sensitive no integrity
script.crossOrigin = "anonymous";
document.head.appendChild(script);

DNS prefetching

Allowing browsers to perform DNS prefetching is security-sensitive

https://sonarsource.github.io/rspec/#/rspec/S5743/javascript

const express = require('express');
const helmet = require('helmet');

let app = express();

app.use(
  helmet.dnsPrefetchControl({
    allow: false // Compliant
  })
);

👎 Examples of incorrect code

const express = require('express');
const helmet = require('helmet');

let app = express();

app.use(
  helmet.dnsPrefetchControl({
    allow: true // Sensitive: allowing DNS prefetching is security-sensitive
  })
);

No Prototype Builtins

Additionally, objects can have properties that shadow the builtins on Object.prototype, potentially causing unintended behavior or denial-of-service security vulnerabilities. For example, it would be unsafe for a webserver to parse JSON input from a client and call hasOwnProperty directly on the resulting object, because a malicious client could send a JSON value like {"hasOwnProperty": 1} and cause the server to crash.

To avoid subtle bugs like this, it’s better to always call these methods from Object.prototype. For example, foo.hasOwnProperty("bar") should be replaced with Object.prototype.hasOwnProperty.call(foo, "bar").

https://eslint.org/docs/latest/rules/No-Prototype-Builtins

var hasBarProperty = Object.prototype.hasOwnProperty.call(foo, "bar");

var isPrototypeOfBar = Object.prototype.isPrototypeOf.call(foo, bar);

var barIsEnumerable = {}.propertyIsEnumerable.call(foo, "bar");

👎 Examples of incorrect code

var hasBarProperty = foo.hasOwnProperty("bar");

var isPrototypeOfBar = foo.isPrototypeOf(bar);

var barIsEnumerable = foo.propertyIsEnumerable("bar");

File Permissions

Setting loose POSIX file permissions is security-sensitive Dont use 777

https://sonarsource.github.io/rspec/#/rspec/S2612/javascript

const fs = require('fs');

fs.chmodSync("/tmp/fs", 0o770); // Compliant

// OR

const fs = require('fs');
const fsPromises = fs.promises;

fsPromises.chmod("/tmp/fsPromises", 0o770); // Compliant

👎 Examples of incorrect code

const fs = require('fs');

fs.chmodSync("/tmp/fs", 0o777); // Sensitive

// OR

const fs = require('fs');
const fsPromises = fs.promises;

fsPromises.chmod("/tmp/fsPromises", 0o777); // Sensitive

File Upload

These minimum restrictions should be applied when handling file uploads:

the file upload folder to restrict untrusted files to a specific folder.
the file extension of the uploaded file to prevent remote code execution.

https://sonarsource.github.io/rspec/#/rspec/S2598/javascript

const Formidable = require('formidable');

const form = new Formidable(); // Compliant
form.uploadDir = "./uploads/";
form.keepExtensions = false;

👎 Examples of incorrect code

const Formidable = require('formidable');

const form = new Formidable(); // Noncompliant, this form is not safe
form.uploadDir = ""; // because upload dir is not defined (by default os temp dir: /var/tmp or /tmp)
form.keepExtensions = true; // and file extensions are kept

Frame Ancestors

Disabling content security policy frame-ancestors directive is security-sensitive

Clickjacking attacks occur when an attacker try to trick an user to click on certain buttons/links of a legit website. This attack can take place with malicious HTML frames well hidden in an attacker website.

https://sonarsource.github.io/rspec/#/rspec/S5732/javascript

const express = require('express');
const helmet = require('helmet');

let app = express();

app.use(
  helmet.contentSecurityPolicy({
    directives: {
      // other directives
      frameAncestors: ["'example.com'"] // Compliant
    }
  })
);

👎 Examples of incorrect code

const express = require('express');
const helmet = require('helmet');

let app = express();

app.use(
  helmet.contentSecurityPolicy({
    directives: {
      // other directives
      frameAncestors: ["'none'"] // Sensitive: frameAncestors  is set to none
    }
  })
);

Hashing Insecurity

Using weak hashing algorithms is security-sensitive

https://sonarsource.github.io/rspec/#/rspec/S4790/javascript

const crypto = require("crypto");

const hash = crypto.createHash('sha512'); // Compliant

👎 Examples of incorrect code

const crypto = require("crypto");

const hash = crypto.createHash('sha1'); // Sensitive

Hidden Files

Hidden files are created automatically by many tools to save user-preferences, well-known examples are .profile, .bashrc, .bash_history or .git. To simplify the view these files are not displayed by default using operating system commands like ls.

https://sonarsource.github.io/rspec/#/rspec/S5691/javascript

👍 Examples of correct code

let serveStatic = require("serve-static");
let app = express();
// Compliant: ignore or deny are recommended values
let serveStaticMiddleware = serveStatic('public', { 'index': false, 'dotfiles': 'ignore'});

// Compliant: by default, "dotfiles" (file or directory that begins with a dot) are not served
// (with the exception that files within a directory that begins with a dot are not ignored),
// see serve-static module documentation
let serveStaticDefault = serveStatic('public', { 'index': false});
app.use(serveStaticMiddleware);

👎 Examples of incorrect code

let serveStatic = require("serve-static");
let app = express();
let serveStaticMiddleware = serveStatic('public', { 'index': false, 'dotfiles': 'allow'});   // Sensitive
app.use(serveStaticMiddleware);

No Dynamic Delete

Deleting dynamically computed keys can be dangerous and in some cases not well optimized. Using the delete operator on keys that aren't runtime constants could be a sign that you're using the wrong data structures. Using Objects with added and removed keys can cause occasional edge case bugs, such as if a key is named "hasOwnProperty".

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/no-dynamic-delete.md

👍 Examples of correct code

const container: { [i: string]: number } = {
  /* ... */
};

// Constant runtime lookups by string index
delete container.aaa;

// Constants that must be accessed by []
delete container[7];
delete container['-Infinity'];

const name = 'aaa';
switch (name) {
    case "aaa":
        delete container.aaa
        break;
    case "bbb":
        delete container.bbb
        break;
    default:
        throw new Error("Not Authorized");
        break;
}

👎 Examples of incorrect code

// Can be replaced with the constant equivalents, such as container.aaa
delete container['aaa'];
delete container['Infinity'];

// Dynamic, difficult-to-reason-about lookups
const name = 'name';
delete container[name];
delete container[name.toUpperCase()];

Cors

Having a permissive Cross-Origin Resource Sharing policy is security-sensitive

https://sonarsource.github.io/rspec/#/rspec/S5122/javascript

👍 Examples of correct code

const http = require('http');
const srv = http.createServer((req, res) => {
  res.writeHead(200, { 'Access-Control-Allow-Origin': 'trustedwebsite.com' }); // Compliant
  res.end('ok');
});
srv.listen(3000);

const cors = require('cors');

let corsOptions = {
  origin: 'trustedwebsite.com' // Compliant
};

let app = express();
app.use(cors(corsOptions));

function (req, res) {
  const origin = req.header('Origin');

  if (trustedOrigins.indexOf(origin) >= 0) {
    res.setHeader('Access-Control-Allow-Origin', origin);
  }
};

👎 Examples of incorrect code

function (req, res) {
  const origin = req.header('Origin');
  res.setHeader('Access-Control-Allow-Origin', origin); // Sensitive
};

const cors = require('cors');

let app1 = express();
app1.use(cors()); // Sensitive: by default origin is set to *

let corsOptions = {
  origin: '*' // Sensitive
};

let app2 = express();
app2.use(cors(corsOptions));

const http = require('http');
const srv = http.createServer((req, res) => {
  res.writeHead(200, { 'Access-Control-Allow-Origin': '*' }); // Sensitive
  res.end('ok');
});
srv.listen(3000);

Csrf

Force CSRF protections is security-sensitive

https://sonarsource.github.io/rspec/#/rspec/S4502/javascript

👍 Examples of correct code

let csrf = require('csurf');
let express = require('express');

let csrfProtection = csrf({ cookie:  true });

let app = express();

app.post('/money_transfer', parseForm, csrfProtection, function (req, res) { // Compliant
  res.send('Money transferred')
});

let csrf = require('csurf');
let express = require('express');

app.use(csrf({ cookie: true, ignoreMethods: ["GET"] })); // Compliant

👎 Examples of incorrect code

let csrf = require('csurf');
let express = require('express');

let csrfProtection = csrf({ cookie: true });

let app = express();

// Sensitive: this operation doesn't look like protected by CSURF middleware (csrfProtection is not used)
app.post('/money_transfer', parseForm, function (req, res) {
  res.send('Money transferred');
});

// or

let csrf = require('csurf');
let express = require('express');

app.use(csrf({ cookie: true, ignoreMethods: ["POST", "GET"] })); // Sensitive as POST is unsafe method

Aws Security

Aws General Security

👍 Examples of correct code

import {aws_apigateway as apigateway} from "aws-cdk-lib"

const resource = api.root.addResource("example",{
    defaultMethodOptions:{
        authorizationType: apigateway.AuthorizationType.IAM
    }
})
resource.addMethod(
    "POST",
    new apigateway.HttpIntegration("https://example.org"),
    {
        authorizationType: apigateway.AuthorizationType.IAM
    }
)
resource.addMethod(  // authorizationType is inherited from the Resource's configured defaultMethodOptions
    "GET"
)

// or

import {aws_apigatewayv2 as apigateway} from "aws-cdk-lib"

new apigateway.CfnRoute(this, "auth", {
    apiId: api.ref,
    routeKey: "POST /auth",
    authorizationType: "AWS_IAM",
    target: exampleIntegration
})

// or

import { aws_opensearchservice as opensearchservice } from 'aws-cdk-lib';

const exampleDomain = new opensearchservice.Domain(this, 'ExampleDomain', {
  version: EngineVersion.OPENSEARCH_1_3,
  encryptionAtRest: {
    enabled: true,
  },
});

// or

import { Topic } from 'aws-cdk-lib/aws-sns';

const encryptionKey = new Key(this, 'exampleKey', {
    enableKeyRotation: true,
});

new Topic(this, 'exampleTopic', {
    masterKey: encryptionKey
});

// or

import { CfnNotebookInstance } from 'aws-cdk-lib/aws-sagemaker';

const encryptionKey = new Key(this, 'example', {
    enableKeyRotation: true,
});
new CfnNotebookInstance(this, 'example', {
    instanceType: 'instanceType',
    roleArn: 'roleArn',
    kmsKeyId: encryptionKey.keyId
});

// or

const s3 = require('aws-cdk-lib/aws-s3');

new s3.Bucket(this, 'id', {
    bucketName: 'bucket',
    versioned: true
});

// or

new s3.Bucket(this, 'id', {
    encryption: s3.BucketEncryption.KMS_MANAGED,
    encryptionKey: access_key
});

// or

const s3 = require('aws-cdk-lib/aws-s3');

const bucket = new s3.Bucket(this, 'example', {
    bucketName: 'example',
    versioned: true,
    publicReadAccess: false,
    enforceSSL: true
});

👎 Examples of incorrect code

import {aws_apigateway as apigateway} from "aws-cdk-lib"

const resource = api.root.addResource("example")
resource.addMethod(
    "GET",
    new apigateway.HttpIntegration("https://example.org"),
    {
        authorizationType: apigateway.AuthorizationType.NONE // Sensitive
    }
)

// or

import {aws_apigatewayv2 as apigateway} from "aws-cdk-lib"

new apigateway.CfnRoute(this, "no-auth", {
    apiId: api.ref,
    routeKey: "GET /no-auth",
    authorizationType: "NONE", // Sensitive
    target: exampleIntegration
})

// or

import { aws_opensearchservice as opensearchservice } from 'aws-cdk-lib';

const exampleCfnDomain = new opensearchservice.CfnDomain(this, 'ExampleCfnDomain', {
  engineVersion: 'OpenSearch_1.3',
}); // Sensitive, encryption must be explicitly enabled

// or

import { Topic, CfnTopic } from 'aws-cdk-lib/aws-sns';

new CfnTopic(this, 'exampleCfnTopic'); // Sensitive

// or

import { CfnNotebookInstance } from 'aws-cdk-lib/aws-sagemaker';

new CfnNotebookInstance(this, 'example', {
      instanceType: 'instanceType',
      roleArn: 'roleArn'
}); // Sensitive

// or

const s3 = require('aws-cdk-lib/aws-s3');

new s3.Bucket(this, 'id', {
    bucketName: 'bucket',
    versioned: false // Sensitive
});

// or

const s3 = require('aws-cdk-lib/aws-s3');

new s3.Bucket(this, 'id', {
    bucketName: 'default'
}); // Sensitive

Security Cookie

When a cookie is protected with the secure attribute set to true it will not be send by the browser over an unencrypted HTTP request and thus cannot be observed by an unauthorized person during a man-in-the-middle attack.

https://sonarsource.github.io/rspec/#/rspec/S2092/javascript

👍 Examples of correct code

let session = cookieSession({
  secure: true,// Compliant
});  // Compliant

// or

const express = require('express');
const session = require('express-session');

let app = express();
app.use(session({
  cookie:
  {
    secure: true // Compliant
  }
}));

// or

let cookies = new Cookies(req, res, { keys: keys });

cookies.set('LastVisit', new Date().toISOString(), {
  secure: true // Compliant
}); // Compliant

// or

const cookieParser = require('cookie-parser');
const csrf = require('csurf');
const express = require('express');

let csrfProtection = csrf({ cookie: { secure: true }}); // Compliant

👎 Examples of incorrect code

let session = cookieSession({
  secure: false,// Sensitive
});  // Sensitive

// or

const express = require('express');
const session = require('express-session');

let app = express();
app.use(session({
  cookie:
  {
    secure: false // Sensitive
  }
}));

// or

let cookies = new Cookies(req, res, { keys: keys });

cookies.set('LastVisit', new Date().toISOString(), {
  secure: false // Sensitive
}); // Sensitive

// or

const cookieParser = require('cookie-parser');
const csrf = require('csurf');
const express = require('express');

let csrfProtection = csrf({ cookie: { secure: false }}); // Sensitive

Insecure Cookie

Creating cookies without the "secure" flag is security-sensitive

https://sonarsource.github.io/rspec/#/rspec/S2092/javascript

👍 Examples of correct code

let session = cookieSession({
  secure: true,// Compliant
});  // Compliant

// Or

const express = require('express');
const session = require('express-session');

let app = express();
app.use(session({
  cookie:
  {
    secure: true // Compliant
  }
}));

// or

let cookies = new Cookies(req, res, { keys: keys });

cookies.set('LastVisit', new Date().toISOString(), {
  secure: true // Compliant
}); // Compliant

// or

const cookieParser = require('cookie-parser');
const csrf = require('csurf');
const express = require('express');

let csrfProtection = csrf({ cookie: { secure: true }}); // Compliant

👎 Examples of incorrect code

const cookieParser = require('cookie-parser');
const csrf = require('csurf');
const express = require('express');

let csrfProtection = csrf({ cookie: { secure: false }}); // Sensitive

// or

let cookies = new Cookies(req, res, { keys: keys });

cookies.set('LastVisit', new Date().toISOString(), {
  secure: false // Sensitive
}); // Sensitive

// or

const express = require('express');
const session = require('express-session');

let app = express();
app.use(session({
  cookie:
  {
    secure: false // Sensitive
  }
}));

// or

let session = cookieSession({
  secure: false,// Sensitive
});  // Sensitive

No Clear Text Protocols

Using clear-text protocols is security-sensitive

https://sonarsource.github.io/rspec/#/rspec/S5332/javascript

👍 Examples of correct code

url = "https://example.com"; // use enviroment or config class
url = "sftp://anonymous@example.com"; // use enviroment or config class
url = "ssh://anonymous@example.com"; // use enviroment or config class

// or

const nodemailer = require("nodemailer");
let transporter = nodemailer.createTransport({
  secure: true,
  requireTLS: true,
  port: 465,
  secured: true
});

// or

var Client = require('ftp');
var c = new Client();
c.connect({
  'secure': true
});

👎 Examples of incorrect code

url = "http://example.com"; // Sensitive
url = "ftp://anonymous@example.com"; // Sensitive
url = "telnet://anonymous@example.com"; // Sensitive

const nodemailer = require("nodemailer");
let transporter = nodemailer.createTransport({
  secure: false, // Sensitive
  requireTLS: false // Sensitive
});

var Client = require('ftp');
var c = new Client();
c.connect({
  'secure': false // Sensitive
});

const Telnet = require('telnet-client'); // Sensitive

Insecure Jwt Token

If a JSON Web Token (JWT) is not signed with a strong cipher algorithm (or not signed at all) an attacker can forge it and impersonate user identities.

https://sonarsource.github.io/rspec/#/rspec/S5659/javascript

👍 Examples of correct code

const jwt = require('jsonwebtoken');

let token = jwt.sign({ foo: 'bar' }, key, { algorithm: 'HS256' }); // Compliant

jwt.verify(token, key, { expiresIn: 360000 * 5, algorithms: ['HS256'] }, callbackcheck); // Compliant

👎 Examples of incorrect code

const jwt = require('jsonwebtoken');

// Noncompliant: 'none' cipher doesn't sign the JWT (no signature will be included)
let token = jwt.sign({ foo: 'bar' }, key, { algorithm: 'none' });

// Noncompliant: 'none' cipher should not be used when verifying JWT signature
jwt.verify(token, key, { expiresIn: 360000 * 5, algorithms: ['RS256', 'none'] }, callbackcheck);

No Hardcoded Credentials

Hard-coded credentials are security-sensitive

https://sonarsource.github.io/rspec/#/rspec/S2068/javascript

👍 Examples of correct code

var mysql = require('mysql');

var connection = mysql.createConnection({
  host: process.env.MYSQL_URL,
  user: process.env.MYSQL_USERNAME,
  password: process.env.MYSQL_PASSWORD,
  database: process.env.MYSQL_DATABASE
});
connection.connect();

👎 Examples of incorrect code

var mysql = require('mysql');

var connection = mysql.createConnection(
{
  host:'localhost',
  user: "admin",
  database: "project",
  password: "mypassword", // sensitive
  multipleStatements: true
});

No Hardcoded Ip

Using hardcoded IP addresses is security-sensitive

https://sonarsource.github.io/rspec/#/rspec/S1313/javascript

👍 Examples of correct code

ip = process.env.IP_ADDRESS; // Compliant

const net = require('net');
var client = new net.Socket();
client.connect(80, ip, function() {
  // ...
});

👎 Examples of incorrect code

ip = "192.168.12.42"; // Sensitive

const net = require('net');
var client = new net.Socket();
client.connect(80, ip, function() {
  // ...
});

No Os Command From Path

Searching OS commands in PATH is security-Sensitive

https://sonarsource.github.io/rspec/#/rspec/S4036/javascript

👍 Examples of correct code

const cp = require('child_process');
cp.exec('/usr/bin/file.exe'); // Compliant

👎 Examples of incorrect code

const cp = require('child_process');
cp.exec('file.exe'); // Sensitive

No Unsafe Unzip

Expanding archive files without controlling resource consumption is security-sensitive

https://sonarsource.github.io/rspec/#/rspec/S5042/javascript

👍 Examples of correct code

const tar = require('tar');
const MAX_FILES = 10000;
const MAX_SIZE = 1000000000; // 1 GB

let fileCount = 0;
let totalSize = 0;

tar.x({
  file: 'foo.tar.gz',
  filter: (path, entry) => {
    fileCount++;
    if (fileCount > MAX_FILES) {
      throw 'Reached max. number of files';
    }

    totalSize += entry.size;
    if (totalSize > MAX_SIZE) {
      throw 'Reached max. size';
    }

    return true;
  }
});

// or

const AdmZip = require('adm-zip');
const MAX_FILES = 10000;
const MAX_SIZE = 1000000000; // 1 GB
const THRESHOLD_RATIO = 10;

let fileCount = 0;
let totalSize = 0;
let zip = new AdmZip("./foo.zip");
let zipEntries = zip.getEntries();
zipEntries.forEach(function(zipEntry) {
    fileCount++;
    if (fileCount > MAX_FILES) {
        throw 'Reached max. number of files';
    }

    let entrySize = zipEntry.getData().length;
    totalSize += entrySize;
    if (totalSize > MAX_SIZE) {
        throw 'Reached max. size';
    }

    let compressionRatio = entrySize / zipEntry.header.compressedSize;
    if (compressionRatio > THRESHOLD_RATIO) {
        throw 'Reached max. compression ratio';
    }

    if (!zipEntry.isDirectory) {
        zip.extractEntryTo(zipEntry.entryName, ".");
    }
});

// or

const fs = require("fs");
const pathmodule = require("path");
const JSZip = require("jszip");

const MAX_FILES = 10000;
const MAX_SIZE = 1000000000; // 1 GB

let fileCount = 0;
let totalSize = 0;
let targetDirectory = __dirname + '/archive_tmp';

fs.readFile("foo.zip", function(err, data) {
  if (err) throw err;
  JSZip.loadAsync(data).then(function (zip) {
    zip.forEach(function (relativePath, zipEntry) {
      fileCount++;
      if (fileCount > MAX_FILES) {
        throw 'Reached max. number of files';
      }

      // Prevent ZipSlip path traversal (S6096)
      const resolvedPath = pathmodule.join(targetDirectory, zipEntry.name);
      if (!resolvedPath.startsWith(targetDirectory)) {
        throw 'Path traversal detected';
      }

      if (!zip.file(zipEntry.name)) {
        fs.mkdirSync(resolvedPath);
      } else {
        zip.file(zipEntry.name).async('nodebuffer').then(function (content) {
          totalSize += content.length;
          if (totalSize > MAX_SIZE) {
            throw 'Reached max. size';
          }

          fs.writeFileSync(resolvedPath, content);
        });
      }
    });
  });
});

// or

const yauzl = require('yauzl');

const MAX_FILES = 10000;
const MAX_SIZE = 1000000000; // 1 GB
const THRESHOLD_RATIO = 10;

yauzl.open('foo.zip', function (err, zipfile) {
  if (err) throw err;

  let fileCount = 0;
  let totalSize = 0;

  zipfile.on("entry", function(entry) {
    fileCount++;
    if (fileCount > MAX_FILES) {
      throw 'Reached max. number of files';
    }

    // The uncompressedSize comes from the zip headers, so it might not be trustworthy.
    // Alternatively, calculate the size from the readStream.
    let entrySize = entry.uncompressedSize;
    totalSize += entrySize;
    if (totalSize > MAX_SIZE) {
      throw 'Reached max. size';
    }

    if (entry.compressedSize > 0) {
      let compressionRatio = entrySize / entry.compressedSize;
      if (compressionRatio > THRESHOLD_RATIO) {
        throw 'Reached max. compression ratio';
      }
    }

    zipfile.openReadStream(entry, function(err, readStream) {
      if (err) throw err;
      // TODO: extract
    });
  });
});

// or
const extract = require('extract-zip')

const MAX_FILES = 10000;
const MAX_SIZE = 1000000000; // 1 GB
const THRESHOLD_RATIO = 10;

async function main() {
  let fileCount = 0;
  let totalSize = 0;

  let target = __dirname + '/foo';
  await extract('foo.zip', {
    dir: target,
    onEntry: function(entry, zipfile) {
      fileCount++;
      if (fileCount > MAX_FILES) {
        throw 'Reached max. number of files';
      }

      // The uncompressedSize comes from the zip headers, so it might not be trustworthy.
      // Alternatively, calculate the size from the readStream.
      let entrySize = entry.uncompressedSize;
      totalSize += entrySize;
      if (totalSize > MAX_SIZE) {
        throw 'Reached max. size';
      }

      if (entry.compressedSize > 0) {
        let compressionRatio = entrySize / entry.compressedSize;
        if (compressionRatio > THRESHOLD_RATIO) {
          throw 'Reached max. compression ratio';
        }
      }
    }
  });
}
main();

👎 Examples of incorrect code

const tar = require('tar');

tar.x({ // Sensitive
  file: 'foo.tar.gz'
});

// or

const AdmZip = require('adm-zip');

let zip = new AdmZip("./foo.zip");
zip.extractAllTo("."); // Sensitive

// or

const fs = require("fs");
const JSZip = require("jszip");

fs.readFile("foo.zip", function(err, data) {
  if (err) throw err;
  JSZip.loadAsync(data).then(function (zip) { // Sensitive
    zip.forEach(function (relativePath, zipEntry) {
      if (!zip.file(zipEntry.name)) {
        fs.mkdirSync(zipEntry.name);
      } else {
        zip.file(zipEntry.name).async('nodebuffer').then(function (content) {
          fs.writeFileSync(zipEntry.name, content);
        });
      }
    });
  });
});

// or

const yauzl = require('yauzl');

yauzl.open('foo.zip', function (err, zipfile) {
  if (err) throw err;

  zipfile.on("entry", function(entry) {
    zipfile.openReadStream(entry, function(err, readStream) {
      if (err) throw err;
      // TODO: extract
    });
  });
});

// or

const extract = require('extract-zip')

async function main() {
  let target = __dirname + '/test';
  await extract('test.zip', { dir: target }); // Sensitive
}
main();

Sockets

Using sockets is security-sensitive. It has led in the past to the following vulnerabilities:

https://sonarsource.github.io/rspec/#/rspec/S4818/javascript

👍 Examples of correct code

👎 Examples of incorrect code

const net = require('net');

var socket = new net.Socket(); // Sensitive
socket.connect(80, 'google.com');

// net.createConnection creates a new net.Socket, initiates connection with socket.connect(),
// then returns the net.Socket that starts the connection
net.createConnection({ port: port }, () => {}); // Sensitive

// net.connect is an alias to net.createConnection
net.connect({ port: port }, () => {}); // Sensitive

Helmet Security

MIME confusion attacks occur when an attacker successfully tricks a web-browser to interpret a resource as a different type than the one expected. To correctly interpret a resource (script, image, stylesheet ...) web browsers look for the Content-Type header defined in the HTTP response received from the server, but often this header is not set or is set with an incorrect value. To avoid content-type mismatch and to provide the best user experience, web browsers try to deduce the right content-type, generally by inspecting the content of the resources (the first bytes). This "guess mechanism" is called MIME type sniffing.

https://sonarsource.github.io/rspec/#/rspec/S5734/javascript

👍 Examples of correct code

const express = require('express');
const helmet= require('helmet');

let app = express();

app.use(helmet.noSniff());

👎 Examples of incorrect code

const express = require('express');
const helmet = require('helmet');

let app = express();

app.use(
  helmet({
    noSniff: false, // Sensitive
  })
);

Unverified Certificate

The certificate chain validation includes these steps: The certificate is issued by its parent Certificate Authority or the root CA trusted by the system. Each CA is allowed to issue certificates. Each certificate in the chain is not expired.

https://sonarsource.github.io/rspec/#/rspec/S4830/javascript

👍 Examples of correct code

let options = {
    hostname: "www.example.com",
    port: 443,
    path: "/",
    method: "GET",
    secureProtocol: "TLSv1_2_method",
};

let req = https.request(options, (res) => {
  res.on('data', (d) => {
    process.stdout.write(d);
  });
}); // Compliant: by default rejectUnauthorized  is set to true

// or

let options = {
    secureProtocol: 'TLSv1_2_method'
};

let socket = tls.connect(443, "www.example.com", options, () => {
  process.stdin.pipe(socket);
  process.stdin.resume();
}); // Compliant: by default rejectUnauthorized  is set to true

// or

let socket = request.get({
    url: 'https://www.example.com/',
    secureProtocol: 'TLSv1_2_method' // Compliant
}); // Compliant: by default rejectUnauthorized  is set to true

👎 Examples of incorrect code

let options = {
  hostname: 'www.example.com',
  port: 443,
  path: '/',
  method: 'GET',
  secureProtocol: 'TLSv1_2_method',
  rejectUnauthorized: false ; // Noncompliant
};

let req = https.request(options, (res) => {
  res.on('data', (d) => {
    process.stdout.write(d);
  });
}); // Noncompliant

// or

let options = {
    secureProtocol: 'TLSv1_2_method',
    rejectUnauthorized: false ; // Noncompliant
};

let socket = tls.connect(443, "www.example.com", options, () => {
  process.stdin.pipe(socket);
  process.stdin.resume();
});  // Noncompliant

// or

let socket = request.get({
    url: 'www.example.com',
    secureProtocol: 'TLSv1_2_method',
    rejectUnauthorized: false ; // Noncompliant
});

Sql Queries

Formatted SQL queries can be difficult to maintain, debug and can increase the risk of SQL injection when concatenating untrusted values into the query. However, this rule doesn’t detect SQL injections

https://sonarsource.github.io/rspec/#/rspec/S2077/javascript

👍 Examples of correct code

// === MySQL ===
const mysql = require('mysql');
const mycon = mysql.createConnection({ host: host, user: user, password: pass, database: db });
mycon.connect(function(err) {
  mycon.query('SELECT name FROM users WHERE id = ?', [userinput], (err, res) => {});
});

// === PostgreSQL ===
const pg = require('pg');
const pgcon = new pg.Client({ host: host, user: user, password: pass, database: db });
pgcon.connect();
pgcon.query('SELECT name FROM users WHERE id = $1', [userinput], (err, res) => {});

👎 Examples of incorrect code

// === MySQL ===
const mysql = require('mysql');
const mycon = mysql.createConnection({ host: host, user: user, password: pass, database: db });
mycon.connect(function(err) {
  mycon.query('SELECT * FROM users WHERE id = ' + userinput, (err, res) => {}); // Sensitive
});

// === PostgreSQL ===
const pg = require('pg');
const pgcon = new pg.Client({ host: host, user: user, password: pass, database: db });
pgcon.connect();
pgcon.query('SELECT * FROM users WHERE id = ' + userinput, (err, res) => {}); // Sensitive

Catch Error Name

Enforce a specific parameter name in catch clauses

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/catch-error-name.md

👍 Examples of correct code

try {} catch (exception) {}
promise.catch(exception => {});
promise.catch(error => {});
promise.catch(error1 => {});
promise.catch(error2 => {});

👎 Examples of incorrect code

try {} catch (badName) {}
// `_` is not allowed if it's used
try {} catch (_) {
    console.log(_);
}
promise.catch(badName => {});
promise.then(undefined, badName => {});

Consistent Destructured

Enforces the use of already destructured objects and their variables over accessing each property individually. Previous destructurings are easily missed which leads to an inconsistent code style.

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/consistent-destructuring.md

👍 Examples of correct code

const {a} = foo;
console.log(a);
console.log(foo.a, foo.b);
const {a} = foo;
console.log(a, foo.b());
const {a} = foo.bar;
console.log(foo.bar);

👎 Examples of incorrect code

const {a} = foo;
console.log(a, foo.b);
const {a} = foo;
console.log(foo.a);
const {
    a: {
        b
    }
} = foo;
console.log(foo.a.c);
const {bar} = foo;
const {a} = foo.bar;

Consistent Function Scope

A function definition should be placed as close to the top-level scope as possible without breaking its captured values. This improves readability, directly improves performance and allows JavaScript engines to better optimize performance.

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/consistent-function-scoping.md

👍 Examples of correct code

function doBar(bar) {
    return bar === 'bar';
}

export function doFoo(foo) {
    return doBar;
}

export function doFoo(foo) {
    function doBar(bar) {
        return bar === 'bar' && foo.doBar(bar);
    }

    return doBar;
}

👎 Examples of incorrect code

export function doFoo(foo) {
    // Does not capture anything from the scope, can be moved to the outer scope
    function doBar(bar) {
        return bar === 'bar';
    }

    return doBar;
}

function doFoo(foo) {
    const doBar = bar => {
        return bar === 'bar';
    };
}

Exception With Message

This rule enforces a message value to be passed in when creating an instance of a built-in Error object, which leads to more readable and debuggable code.

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/error-message.md

👍 Examples of correct code

throw new TypeError('Array expected.');
const error = new AggregateError(errors, 'Promises rejected.');

👎 Examples of incorrect code

throw Error();
throw Error('');
throw new TypeError();
const error = new AggregateError(errors);

Escape Case

Enforces defining escape sequence values with uppercase characters rather than lowercase ones. This promotes readability by making the escaped value more distinguishable from the identifier.

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/escape-case.md

👍 Examples of correct code

const foo = '\xA9';
const foo = '\uD834';
const foo = '\u{1D306}';
const foo = '\cA';

👎 Examples of incorrect code

const foo = '\xa9';
const foo = '\ud834';
const foo = '\u{1d306}';
const foo = '\ca';

New For Builtin

Enforce the use of new for all builtin's, except String, Number, Boolean, Symbol and BigInt

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/new-for-builtins.md

👍 Examples of correct code

const list = new Array(10);
const now = new Date();
const map = new Map([
    ['foo', 'bar']
]);

👎 Examples of incorrect code

const list = Array(10);
const now = Date();
const map = Map([
    ['foo', 'bar']
]);

No Abusive Eslint Disable

This rule makes you specify the rules you want to disable when using eslint-disable, eslint-disable-line or eslint-disable-next-line comments.

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/no-abusive-eslint-disable.md

👍 Examples of correct code

/* eslint-disable no-console */
console.log(message);

console.log(message); // eslint-disable-line no-console

// eslint-disable-next-line no-console
console.log(message);

👎 Examples of incorrect code

/* eslint-disable */
console.log(message);

console.log(message); // eslint-disable-line

// eslint-disable-next-line
console.log(message);

Prefer for-of

Prefer for…of over the forEach method Do not use a for loop that can be replaced with a for-of loop

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/no-array-for-each.md https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/no-for-loop.md

👍 Examples of correct code

for (const element of array) {
    bar(element);
}
for (const [index, element] of array.entries()) {
    bar(element, index);
}
for (const [index, element] of array.entries()) {
    bar(element, index, array);
}

👎 Examples of incorrect code

array.forEach(element => {
    bar(element);
});
array?.forEach(element => {
    bar(element);
});
array.forEach((element, index) => {
    bar(element, index);
});
array.forEach((element, index, array) => {
    bar(element, index, array);
});

for (let index = 0; index < array.length; index++) {
    const element = array[index];
    console.log(index, element);
}

No Array Push Push

Enforce combining multiple Array#push() into one call

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/no-array-push-push.md

👍 Examples of correct code

foo.push(1, 2, 3);

// or
const length = foo.push(1);
foo.push(2);

// or
foo.push(1);
bar();
foo.push(2);

👎 Examples of incorrect code

foo.push(1);
foo.push(2, 3);

No Await Chased

Disallow member access from await expression

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/no-await-expression-member.md

👍 Examples of correct code

const {default: foo} = await import('./foo.js');
const [, secondElement] = await getArray();
const {property} = await getObject();
const response = await fetch('/foo');
const data = await response.json();

👎 Examples of incorrect code

const foo = (await import('./foo.js')).default;
const secondElement = (await getArray())[1];
const property = (await getObject()).property;
const data = await (await fetch('/foo')).json();

No Document Cookie

Do not use document.cookie directly It's not recommended to use document.cookie directly as it's easy to get the string wrong. Instead, you should use the Cookie Store API or a cookie library.

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/no-document-cookie.md

👍 Examples of correct code

await cookieStore.set({
    name: 'foo',
    value: 'bar',
    expires: Date.now() + 24 * 60 * 60 * 1000,
    domain: 'example.com'
});
const array = document.cookie.split('; ');
import Cookies from 'js-cookie';

Cookies.set('foo', 'bar');

👎 Examples of incorrect code

document.cookie =
    'foo=bar' +
    '; Path=/' +
    '; Domain=example.com' +
    '; expires=Fri, 31 Dec 9999 23:59:59 GMT' +
    '; Secure';

document.cookie += '; foo=bar';

No Empty File

Disallow any files only containing the following: Whitespace Comments Directives Empty statements Empty block statements Hashbang

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/no-empty-file.md

👍 Examples of correct code

await cookieStore.set({
    name: 'foo',
    value: 'bar',
    expires: Date.now() + 24 * 60 * 60 * 1000,
    domain: 'example.com'
});
const array = document.cookie.split('; ');
import Cookies from 'js-cookie';

Cookies.set('foo', 'bar');

👎 Examples of incorrect code

const x = 0;

/* Comment */

No Instance Of Array

Require Array.isArray() instead of instanceof Array

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/no-instanceof-array.md

👍 Examples of correct code

Array.isArray(array);
Array.isArray([ 1, 2, 3 ]);

👎 Examples of incorrect code

array instanceof Array;
[ 1, 2, 3 ] instanceof Array;

No Invalid Remove Event Listener

The removeEventListener function must be called with a reference to the same function that was passed to addEventListener. Calling removeEventListener with an inline function or the result of an inline .bind() call is indicative of an error, and won't actually remove the listener.

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/no-invalid-remove-event-listener.md

👍 Examples of correct code

window.removeEventListener('click', listener);
window.removeEventListener('click', getListener());
class MyElement extends HTMLElement {
    constructor() {
        super();
        this.handler = this.handler.bind(this);
    }

    handler() {}

    disconnectedCallback() {
        this.removeEventListener('click', this.handler);
    }
}

👎 Examples of incorrect code

window.removeEventListener('click', fn.bind(window));
window.removeEventListener('click', () => {});
window.removeEventListener('click', function () {});
class MyElement extends HTMLElement {
    handler() {}

    disconnectedCallback() {
        this.removeEventListener('click', this.handler.bind(this));
    }
}

No Lonely If

This rule adds onto the built-in no-lonely-if rule, which only disallows if statements in else, not in if.

https://eslint.org/docs/latest/rules/no-lonely-if https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/no-lonely-if.md

👍 Examples of correct code

if (foo && bar) {
    // …
}
if (foo) {
    // …
} else if (bar && baz) {
    // …
}
if (foo) {
    // …
} else if (bar) {
    if (baz) {
        // …
    }
} else {
    // …
}

👎 Examples of incorrect code

if (foo) {
    if (bar) {
        // …
    }
}
if (foo) {
    // …
} else if (bar) {
    if (baz) {
        // …
    }
}

No Nested Ternary

Improved version of the no-nested-ternary ESLint rule, which allows cases where the nested ternary is only one level and wrapped in Parentheses.

https://eslint.org/docs/latest/rules/no-nested-ternary https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/no-nested-ternary.md

👍 Examples of correct code

const foo = i > 5 ? (i < 100 ? true : false) : true;
const foo = i > 5 ? (i < 100 ? true : false) : (i < 100 ? true : false);

👎 Examples of incorrect code

const foo = i > 5 ? i < 100 ? true : false : true;
const foo = i > 5 ? true : (i < 100 ? true : (i < 1000 ? true : false));

No New Buffer

Enforce the use of Buffer.from() and Buffer.alloc() instead of the deprecated new Buffer()

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/no-new-buffer.md

👍 Examples of correct code

const buffer = Buffer.from('7468697320697320612074c3a97374', 'hex');
const buffer = Buffer.from([0x62, 0x75, 0x66, 0x66, 0x65, 0x72])
const buffer = Buffer.alloc(10);

👎 Examples of incorrect code

const buffer = new Buffer('7468697320697320612074c3a97374', 'hex');
const buffer = new Buffer([0x62, 0x75, 0x66, 0x66, 0x65, 0x72]);
const buffer = new Buffer(10);

No This Assignment

this should be used directly. If you want a reference to this from a higher scope, consider using arrow function expression or Function#bind().

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/no-this-assignment.md

👍 Examples of correct code

setTimeout(() => {
    this.bar();
}, 1000);
setTimeout(function () {
    this.bar();
}.bind(this), 1000);
class Bar {
    constructor(fooInstance) {
        this.fooInstance = fooInstance;
    }
    method() {
        this.fooInstance.baz();
    }
}

new Bar(this).method();

👎 Examples of incorrect code

const foo = this;

setTimeout(function () {
    foo.bar();
}, 1000);
const foo = this;

class Bar {
    method() {
        foo.baz();
    }
}

new Bar().method();

No Useless Fallback In Spread

Disallow useless fallback when spreading in object literals

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/no-useless-fallback-in-spread.md

👍 Examples of correct code

const object = {...foo};
const object = {...(foo && {})};
const array = [...(foo || [])];

👎 Examples of incorrect code

const object = {...(foo || {})};
const object = {...(foo ?? {})};

No Useless Length Check

Disallow useless array length check

Array#some() returns false for an empty array. There is no need to check if the array is not empty. Array#every() returns true for an empty array. There is no need to check if the array is empty.

We only check .length === 0, .length !== 0, and .length > 0.

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/no-useless-length-check.md

👍 Examples of correct code

if (array.every(Boolean));
if (array.some(Boolean));
const isAllTrulyOrEmpty = array.every(Boolean);
if (array.length === 0 || anotherCheck() || array.every(Boolean));
const isNonEmptyAllTrulyArray = array.length > 0 && array.every(Boolean);
const isEmptyArrayOrAllTruly = array.length === 0 || array.some(Boolean);

👎 Examples of incorrect code

if (array.length === 0 || array.every(Boolean));
if (array.length !== 0 && array.some(Boolean));
if (array.length > 0 && array.some(Boolean));
const isAllTrulyOrEmpty = array.length === 0 || array.every(Boolean);

No Useless Spread

Disallow unnecessary spread

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/no-useless-spread.md

👍 Examples of correct code

const array = [firstElement, secondElement, thirdElement];
const object = {firstProperty, secondProperty, thirdProperty};
foo(firstArgument, secondArgument, thirdArgument);
const object = new Foo(firstArgument, secondArgument, thirdArgument);
const array = [...foo, bar];
const object = {...foo, bar};
foo(foo, ...bar);
const object = new Foo(...foo, bar);
const set = new Set(iterable);
const results = await Promise.all(iterable);
for (const foo of set);
function * foo() {
    yield * anotherGenerator();
}

👎 Examples of incorrect code

const array = [firstElement, ...[secondElement], thirdElement];
const object = {firstProperty, ...{secondProperty}, thirdProperty};
foo(firstArgument, ...[secondArgument], thirdArgument);
const object = new Foo(firstArgument, ...[secondArgument], thirdArgument);
const set = new Set([...iterable]);
const results = await Promise.all([...iterable]);
for (const foo of [...set]);
function * foo() {
    yield * [...anotherGenerator()];
}

No Useless Switch Case

Disallow useless case in switch statements

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/no-useless-switch-case.md

👍 Examples of correct code

switch (foo) {
    case 1:
    case 2:
        handleCase1And2();
        break;
}
switch (foo) {
    case 1:
        handleCase1();
        break;
    default:
        handleDefaultCase();
        break;
}
switch (foo) {
    case 1:
        handleCase1();
        // Fallthrough
    default:
        handleDefaultCase();
        break;
}
switch (foo) {
    // This is actually useless, but we only check cases where the last case is the `default` case
    case 1:
    default:
        handleDefaultCase();
        break;
    case 2:
        handleCase2();
        break;
}

👎 Examples of incorrect code

switch (foo) {
    case 1:
    default:
        handleDefaultCase();
        break;
}

No Useless Undefined

Disallow useless undefined

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/no-useless-undefined.md

👍 Examples of correct code

let foo;
const {foo} = bar;
const noop = () => {};
function foo() {
    return;
}
function* foo() {
    yield;
}
function foo(bar) {
}
function foo({bar}) {
}
foo();

👎 Examples of incorrect code

let foo = undefined;
const {foo = undefined} = bar;
const noop = () => undefined;
function foo() {
    return undefined;
}
function* foo() {
    yield undefined;
}
function foo(bar = undefined) {
}
function foo({bar = undefined}) {
}
foo(undefined);

Prefer Add Event Listener

Prefer .addEventListener() and .removeEventListener() over on-functions

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/prefer-add-event-listener.md

👍 Examples of correct code

foo.addEventListener('click', () => {});
foo.addEventListener('keydown', () => {});
foo.bar.addEventListener('click', onClick);
foo.removeEventListener('click', onClick);

👎 Examples of incorrect code

foo.onclick = () => {};
foo.onkeydown = () => {};
foo.bar.onclick = onClick;
foo.onclick = null;

Prefer Array Find

Prefer .find(…) over the first element from .filter(…)

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/prefer-array-find.md

👍 Examples of correct code

const item = array.find(x => callback(x));
item = array.find(x => callback(x));

👎 Examples of incorrect code

const item = array.filter(x => callback(x))[0];
const item = array.filter(x => callback(x)).shift();
const [item] = array.filter(x => callback(x));
[item] = array.filter(x => callback(x));

Prefer Array Index Of

Prefer Array#indexOf() over Array#findIndex() when looking for the index of an item

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/prefer-array-index-of.md

👍 Examples of correct code

const index = foo.indexOf('foo');
const index = foo.findIndex(x => x == undefined);
const index = foo.findIndex(x => x !== 'foo');
const index = foo.findIndex((x, index) => x === index);
const index = foo.findIndex(x => (x === 'foo') && isValid());
const index = foo.findIndex(x => y === 'foo');
const index = foo.findIndex(x => y.x === 'foo');
const index = foo.findIndex(x => {
    const bar = getBar();
    return x === bar;
});

👎 Examples of incorrect code

const index = foo.findIndex(x => x === 'foo');
const index = foo.findIndex(x => 'foo' === x);
const index = foo.findIndex(x => {
    return x === 'foo';
});

Prefer Array Some

Prefer .some(…) over .filter(…).length check and .find(…)

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/prefer-array-some.md

👍 Examples of correct code

const hasUnicorn = array.some(element => callback(element));
if (array.some(element => callback(element))) {
    // …
}
const foo = array.find(element => callback(element)) || bar;
<template>
    <div v-if="array.some(element => callback(element))">Vue</div>
</template>

👎 Examples of incorrect code

const hasUnicorn = array.filter(element => callback(element)).length > 0;
const hasUnicorn = array.filter(element => callback(element)).length !== 0;
const hasUnicorn = array.filter(element => callback(element)).length >= 1;
if (array.find(element => callback(element))) {
    // …
}
const foo = array.find(element => callback(element)) ? bar : baz;
const hasUnicorn = array.find(element => callback(element) !== undefined);
const hasUnicorn = array.find(element => callback(element) != null);
<template>
    <div v-if="array.find(element => callback(element))">Vue</div>
</template>
<template>
    <div v-if="array.filter(element => callback(element)).length > 0">Vue</div>
</template>

Prefer Dom Node Append

Prefer Node#append() over Node#appendChild()

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/prefer-dom-node-append.md

👍 Examples of correct code

foo.append(bar);
foo.append('bar');
foo.append(bar, 'baz');

👎 Examples of incorrect code

foo.appendChild(bar);

Prefer Dom Node Remove

Prefer childNode.remove() over parentNode.removeChild(childNode)

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/prefer-dom-node-dataset.md

👍 Examples of correct code

foo.remove();
this.remove();

👎 Examples of incorrect code

parentNode.removeChild(foo);
parentNode.removeChild(this);

Prefer Include

Prefer .includes() over .indexOf() and Array#some() when checking for existence or non-existence

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/prefer-includes.md https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/prefer-includes.md

👍 Examples of correct code

const str = 'foobar';
str.indexOf('foo') !== -n;
str.indexOf('foo') !== 1;
!str.indexOf('foo') === 1;
!str.indexOf('foo') === -n;
str.includes('foo');
[1,2,3].includes(4);
const isFound = foo.includes('foo');
const isFound = foo.some(x => x == undefined);
const isFound = foo.some(x => x !== 'foo');
const isFound = foo.some((x, index) => x === index);
const isFound = foo.some(x => (x === 'foo') && isValid());
const isFound = foo.some(x => y === 'foo');
const isFound = foo.some(x => y.x === 'foo');
const isFound = foo.some(x => {
    const bar = getBar();
    return x === bar;
});

👎 Examples of incorrect code

[].indexOf('foo') !== -1;
x.indexOf('foo') != -1;
str.indexOf('foo') > -1;
'foobar'.indexOf('foo') >= 0;
x.indexOf('foo') === -1
const isFound = foo.some(x => x === 'foo');
const isFound = foo.some(x => 'foo' === x);
const isFound = foo.some(x => {
    return x === 'foo';
});

Prefer Keyboard Event Key

Enforces the use of KeyboardEvent#key over KeyboardEvent#keyCode which is deprecated. The .key property is also more semantic and readable.

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/prefer-keyboard-event-key.md

👍 Examples of correct code

window.addEventListener("click", event => {
    console.log(event.key);
});
window.addEventListener("keydown", event => {
    if (event.key === "Backspace") {
        console.log("Backspace was pressed");
    }
});

👎 Examples of incorrect code

window.addEventListener("keydown", event => {
    console.log(event.keyCode);
});
window.addEventListener("keydown", event => {
    if (event.keyCode === 8) {
        console.log("Backspace was pressed");
    }
});

Prefer Negative Index

Prefer negative index over .length - index for {String,Array,TypedArray}#slice(), Array#splice() and Array#at()

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/prefer-negative-index.md

👍 Examples of correct code

foo.slice(-2, -1);
foo.splice(-1, 1);
foo.at(-1);
Array.prototype.slice.call(foo, -2, -1);
Array.prototype.slice.apply(foo, [-2, -1]);

👎 Examples of incorrect code

foo.slice(foo.length - 2, foo.length - 1);
foo.splice(foo.length - 1, 1);
foo.at(foo.length - 1);
Array.prototype.slice.call(foo, foo.length - 2, foo.length - 1);
Array.prototype.slice.apply(foo, [foo.length - 2, foo.length - 1]);

Prefer Optional Catch Binding

Prefer omitting the catch binding parameter

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/prefer-optional-catch-binding.md

👍 Examples of correct code

try {} catch {}
try {} catch (error) {
    console.error(error);
}

👎 Examples of incorrect code

try {} catch (notUsedError) {}
try {} catch ({message}) {}

Prefer Prototype Methods

Prefer borrowing methods from the prototype instead of the instance When “borrowing” a method from Array or Object, it's clearer to get it from the prototype than from an instance.

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/prefer-prototype-methods.md

👍 Examples of correct code

const array = Array.prototype.slice.apply(bar);
const hasProperty = Object.prototype.hasOwnProperty.call(foo, 'property');
Reflect.apply(Array.prototype.forEach, arrayLike, [callback]);
const maxValue = Math.max.apply(Math, numbers);

👎 Examples of incorrect code

const array = [].slice.apply(bar);
const hasProperty = {}.hasOwnProperty.call(foo, 'property');
Reflect.apply([].forEach, arrayLike, [callback]);

Prefer Query Selector

Prefer .querySelector() over .getElementById(), .querySelectorAll() over .getElementsByClassName() and .getElementsByTagName()

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/prefer-query-selector.md

👍 Examples of correct code

document.querySelector('#foo');
document.querySelector('.bar');
document.querySelector('main #foo .bar');
document.querySelectorAll('.foo .bar');
document.querySelectorAll('li a');
document.querySelector('li').querySelectorAll('a');

👎 Examples of incorrect code

document.getElementById('foo');
document.getElementsByClassName('foo bar');
document.getElementsByTagName('main');
document.getElementsByClassName(fn());

Prefer Regexp Test

Prefer RegExp#test() over String#match() and RegExp#exec()

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/prefer-regexp-test.md

👍 Examples of correct code

if (/unicorn/.test(string)) {}
<template>
    <div v-if="/unicorn/.test(string)">Vue</div>
</template>

👎 Examples of incorrect code

if (string.match(/unicorn/)) {}
if (/unicorn/.exec(string)) {}
<template>
    <div v-if="/unicorn/.exec(string)">Vue</div>
</template>

Prefer Spread

Prefer the spread operator over Array.from(…), Array#concat(…), Array#slice() and String#split('')

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/prefer-spread.md

👍 Examples of correct code

[ ...set ].map(element => foo(element));
const array = [ ...array1, ...array2 ];
const tail = array.slice(1);
const copy = [ ...array ];

👎 Examples of incorrect code

Array.from(set).map(element => foo(element));
const array = array1.concat(array2);
const copy = array.slice();
const characters = string.split('');

Prefer String Replace All

Prefer String#replaceAll() over regex searches with the global flag The String#replaceAll() method is both faster and safer as you don't have to escape the regex if the string is not a literal.

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/prefer-string-replace-all.md

👍 Examples of correct code

string.replace(/Non-literal characters .*/g, "");
string.replace(/Extra flags/gi, "");
string.replace("Not a regex expression", "")
string.replaceAll("Literal characters only", "");

👎 Examples of incorrect code

string.replace(/This has no special regex symbols/g, "");
string.replace(/\(It also checks for escaped regex symbols\)/g, "");
string.replace(/Works for u flag too/gu, "");

Prefer String Starts Ends With

Prefer String#startsWith() & String#endsWith() over RegExp#test()

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/prefer-string-starts-ends-with.md https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/prefer-string-starts-ends-with.md

👍 Examples of correct code

const foo = baz.startsWith("bar");
const foo = baz.endsWith("bar");
const foo = baz?.startsWith("bar");
const foo = (baz ?? "").startsWith("bar");
const foo = String(baz).startsWith("bar");
const foo = /^bar/i.test(baz);

👎 Examples of incorrect code

const foo = /^bar/.test(baz);
const foo = /bar$/.test(baz);

// or

declare const foo: string;

// starts with
foo[0] === 'b';
foo.charAt(0) === 'b';
foo.indexOf('bar') === 0;
foo.slice(0, 3) === 'bar';
foo.substring(0, 3) === 'bar';
foo.match(/^bar/) != null;
/^bar/.test(foo);

// ends with
foo[foo.length - 1] === 'b';
foo.charAt(foo.length - 1) === 'b';
foo.lastIndexOf('bar') === foo.length - 3;
foo.slice(-3) === 'bar';
foo.substring(foo.length - 3) === 'bar';
foo.match(/bar$/) != null;
/bar$/.test(foo);

Prefer String Trim Start End

Prefer String#trimStart() / String#trimEnd() over String#trimLeft() / String#trimRight()

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/prefer-string-trim-start-end.md

👍 Examples of correct code

const foo = bar.trimStart();
const foo = bar.trimEnd();

👎 Examples of incorrect code

const foo = bar.trimLeft();
const foo = bar.trimRight();

Prefer Switch

Prefer switch over multiple else-if

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/prefer-switch.md

👍 Examples of correct code

if (foo === 1) {
    // 1
} else if (foo === 2) {
    // 2
}
switch (foo) {
    case 1: {
        // 1
        break;
    }
    case 2: {
        // 2
        break;
    }
    case 3: {
        // 3
        break;
    }
    default: {
        // default
    }
}

👎 Examples of incorrect code

if (foo === 1) {
    // 1
} else if (foo === 2) {
    // 2
} else if (foo === 3) {
    // 3
} else {
    // default
}

Prefer Ternary

This rule enforces the use of ternary expressions over 'simple' if-else statements, where 'simple' means the consequent and alternate are each one line and have the same basic type and form.

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/prefer-ternary.md

👍 Examples of correct code

function func() {
    return test ? a : b;
}
function* func() {
    yield (test ? a : b);
}
async function func() {
    await (test ? a() : b());
}
const error = test ? new Error('foo') : new Error('bar');
throw error;
let foo;
foo = test ? 1 : 2;
// Multiple expressions
let foo;
let bar;
if (test) {
    foo = 1;
    bar = 2;
} else{
    foo = 2;
}
// Different expressions
function func() {
    if (test) {
        return a;
    } else {
        throw new Error('error');
    }
}
// Assign to different variable
let foo;
let bar;
if (test) {
    foo = 1;
} else{
    baz = 2;
}

👎 Examples of incorrect code

function func() {
    if (test) {
        return a;
    } else {
        return b;
    }
}
function* func() {
    if (test) {
        yield a;
    } else {
        yield b;
    }
}
async function func() {
    if (test) {
        await a();
    } else {
        await b();
    }
}
if (test) {
    throw new Error('foo');
} else {
    throw new Error('bar');
}
let foo;
if (test) {
    foo = 1;
} else {
    foo = 2;
}

Multiline Ternary Multiple

Enforces newlines between the operands of a ternary expression if the expression spans multiple lines.

https://eslint.org/docs/latest/rules/multiline-ternary

👍 Examples of correct code

foo > bar ? value1 : value2;

foo > bar ?
    value1 :
    value2;

foo > bar ?
    (baz > qux ? value1 : value2) :
    value3;

foo > bar ?
    (baz > qux ?
        value1 :
        value2) :
    value3;

foo > bar &&
    bar > baz ?
        value1 :
        value2;

👎 Examples of incorrect code

foo > bar ? value1 :
    value2;

foo > bar ?
    value1 : value2;

foo > bar &&
    bar > baz ? value1 : value2;

Prevent Abbreviations

Using complete words results in more readable code. Not everyone knows all your abbreviations. Code is written only once, but read many times.

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/prevent-abbreviations.md

👍 Examples of correct code

const error = new Error();
const event = document.createEvent('Event');
const levels = {
    error: 0
};
this.event = 'click';
class Button {}
// Property is not checked by default
const levels = {
    err: 0
};
// Property is not checked by default
this.evt = 'click';

👎 Examples of incorrect code

const e = new Error();
const e = document.createEvent('Event');
class Btn {}

Consistent Relative URL

Enforce consistent relative URL style

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/relative-url-style.md

👍 Examples of correct code

const url = new URL('foo', base);

👎 Examples of incorrect code

const url = new URL('./foo', base)

Require Array Join Separator

Enforce using the separator argument with Array#join() It's better to make it clear what the separator is when calling Array#join(), instead of relying on the default comma (',') separator.

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/require-array-join-separator.md

👍 Examples of correct code

const string = array.join(',');
const string = array.join('|');
const string = Array.prototype.join.call(arrayLike, '');
const string = [].join.call(arrayLike, '\n');

👎 Examples of incorrect code

const string = array.join();
const string = Array.prototype.join.call(arrayLike);
const string = [].join.call(arrayLike);

Require Number To Fixed Digits Argument

Enforce using the digits argument with Number#toFixed()

It's better to make it clear what the value of the digits argument is when calling Number#toFixed(), instead of relying on the default value of 0.

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/require-number-to-fixed-digits-argument.md

👍 Examples of correct code

const string = foo.toFixed(0);
const string = foo.toFixed(2);
const integer = Math.floor(foo);
const integer = Math.ceil(foo);
const integer = Math.round(foo);
const integer = Math.trunc(foo);

👎 Examples of incorrect code

const string = number.toFixed();

Template String Indent

Fix whitespace-insensitive template indentation

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/template-indent.md

👍 Examples of correct code

function foo() {
    const sqlQuery = sql`
        select *
        from students
        where first_name = ${x}
        and last_name = ${y}
    `;

    const gqlQuery = gql`
        query user(id: 5) {
            firstName
            lastName
        }
    `;

    const html = /* HTML */ `
        <div>
            <span>hello</span>
        </div>
    `;
}

👎 Examples of incorrect code

function foo() {
    const sqlQuery = sql`
select *
from students
where first_name = ${x}
and last_name = ${y}
    `;

    const gqlQuery = gql`
                                    query user(id: 5) {
                                        firstName
                                        lastName
                                    }
                                `;

    const html = /* HTML */ `
                <div>
                        <span>hello</span>
                </div>
    `;
}

Adjacent Overload Signatures

Function overload signatures represent multiple ways a function can be called, potentially with different return types. It's typical for an interface or type alias describing a function to place all overload signatures next to each other. If Signatures placed elsewhere in the type are easier to be missed by future developers reading the code.

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/adjacent-overload-signatures.md

👍 Examples of correct code

declare namespace Foo {
  export function foo(s: string): void;
  export function foo(n: number): void;
  export function foo(sn: string | number): void;
  export function bar(): void;
}

type Foo = {
  foo(s: string): void;
  foo(n: number): void;
  foo(sn: string | number): void;
  bar(): void;
};

interface Foo {
  foo(s: string): void;
  foo(n: number): void;
  foo(sn: string | number): void;
  bar(): void;
}

class Foo {
  foo(s: string): void;
  foo(n: number): void;
  foo(sn: string | number): void {}
  bar(): void {}
}

export function bar(): void;
export function foo(s: string): void;
export function foo(n: number): void;
export function foo(sn: string | number): void;

👎 Examples of incorrect code

declare namespace Foo {
  export function foo(s: string): void;
  export function foo(n: number): void;
  export function bar(): void;
  export function foo(sn: string | number): void;
}

type Foo = {
  foo(s: string): void;
  foo(n: number): void;
  bar(): void;
  foo(sn: string | number): void;
};

interface Foo {
  foo(s: string): void;
  foo(n: number): void;
  bar(): void;
  foo(sn: string | number): void;
}

class Foo {
  foo(s: string): void;
  foo(n: number): void;
  bar(): void {}
  foo(sn: string | number): void {}
}

export function foo(s: string): void;
export function foo(n: number): void;
export function bar(): void;
export function foo(sn: string | number): void;

Ban Ts Comment

TypeScript provides several directive comments that can be used to alter how it processes files. Using these to suppress TypeScript compiler errors reduces the effectiveness of TypeScript overall. Instead, it's generally better to correct the types of code, to make directives unnecessary.

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/ban-ts-comment.md

👍 Examples of correct code

if (false) {
  // Compiler warns about unreachable code error
  console.log('hello');
}

if (false) {
  // @ts-expect-error: Unreachable code error
  console.log('hello');
}
if (false) {
  /*
  @ts-expect-error: Unreachable code error
  */
  console.log('hello');
}

👎 Examples of incorrect code

if (false) {
  // @ts-ignore: Unreachable code error
  console.log('hello');
}
if (false) {
  /*
  @ts-ignore: Unreachable code error
  */
  console.log('hello');
}

Ban Tslint Comment

Useful when migrating from TSLint to ESLint. Once TSLint has been removed, this rule helps locate TSLint annotations (e.g. // tslint:disable).

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/ban-tslint-comment.md

👍 Examples of correct code

// This is a comment that just happens to mention tslint
/* This is a multiline comment that just happens to mention tslint */
someCode(); // This is a comment that just happens to mention tslint

👎 Examples of incorrect code

/* tslint:disable */
/* tslint:enable */
/* tslint:disable:rule1 rule2 rule3... */
/* tslint:enable:rule1 rule2 rule3... */
// tslint:disable-next-line
someCode(); // tslint:disable-line
// tslint:disable-next-line:rule1 rule2 rule3...

Class Literal Property Style

This style checks for any getter methods that return literal values, and requires them to be defined using fields with the readonly modifier instead.

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/class-literal-property-style.md

👍 Examples of correct code

class Mx {
  public readonly myField1 = 1;

  // not a literal
  public readonly myField2 = [1, 2, 3];

  private readonly ['myField3'] = 'hello world';

  public get myField4() {
    return `hello from ${window.location.href}`;
  }
}

👎 Examples of incorrect code

class Mx {
  public static get myField1() {
    return 1;
  }

  private get ['myField2']() {
    return 'hello world';
  }
}

Consistent Generic Constructors

When constructing a generic class, you can specify the type arguments on either the left-hand side (as a type annotation) or the right-hand side (as part of the constructor call):

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/consistent-generic-constructors.md

👍 Examples of correct code

const map = new Map<string, number>();
const map: Map<string, number> = new MyMap();
const set = new Set<string>();
const set = new Set();
const set: Set<string> = new Set<string>();

👎 Examples of incorrect code

const map: Map<string, number> = new Map();
const set: Set<string> = new Set();

Consistent Indexed Object Style

TypeScript supports defining arbitrary object keys using an index signature. TypeScript also has a builtin type named Record to create an empty object defining only an index signature. For example, the following types are equal:

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/consistent-indexed-object-style.md

👍 Examples of correct code

type Foo = Record<string, unknown>;

👎 Examples of incorrect code

interface Foo {
  [key: string]: unknown;
}

type Foo = {
  [key: string]: unknown;
};

Consistent Type Assertions

This rule aims to standardize the use of type assertion style across the codebase. Keeping to one syntax consistently helps with code readability.

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/consistent-type-assertions.md

👍 Examples of correct code

const x: T = { ... };
const y = { ... } as any;
const z = { ... } as unknown;
foo({ ... } as T);
new Clazz({ ... } as T);
function foo() { throw { bar: 5 } as Foo }
const foo = <Foo props={{ ... } as Bar}/>;

👎 Examples of incorrect code

const x = { ... } as T;

function foo() {
  return { ... } as T;
}

Consistent Type Definitions

TypeScript provides two common ways to define an object type: interface and type.

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/consistent-type-definitions.md

👍 Examples of correct code

type T = string;
type Foo = string | {};

interface T {
  x: number;
}

👎 Examples of incorrect code

type T = { x: number };

Consistent Type Exports

TypeScript allows specifying a keyword on exports to indicate that the export exists only in the type system, not at runtime. This allows transpilers to drop exports without knowing the types of the dependencies.type

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/consistent-type-exports.md

👍 Examples of correct code

const x = 1;
type T = number;

export { x, type T };

👎 Examples of incorrect code

const x = 1;
type T = number;

export { x, T };

Consistent Type Imports

TypeScript allows specifying a type keyword on imports to indicate that the export exists only in the type system, not at runtime. This allows transpilers to drop imports without knowing the types of the dependencies.

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/consistent-type-imports.md

👍 Examples of correct code

import { type Foo } from 'Foo';
import type Bar from 'Bar';
type T = Foo;
const x: Bar = 1;

👎 Examples of incorrect code

import type { Foo } from 'Foo';

// or

import { Foo } from 'Foo';
import Bar from 'Bar';
type T = Foo;
const x: Bar = 1;

Member Delimiter Style

TypeScript allows three delimiters between members in interfaces and type aliases:

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/member-delimiter-style.md

👍 Examples of correct code

interface Foo {
    name: string;
    greet(): string;
}

interface Foo { name: string }

type Bar = {
    name: string;
    greet(): string;
}

type Bar = { name: string }

type FooBar = { name: string; greet(): string }

👎 Examples of incorrect code

// missing semicolon delimiter
interface Foo {
    name: string
    greet(): string
}

// using incorrect delimiter
interface Bar {
    name: string,
    greet(): string,
}

// missing last member delimiter
interface Baz {
    name: string;
    greet(): string
}

// incorrect delimiter
type FooBar = { name: string, greet(): string }

// last member should not have delimiter
type FooBar = { name: string; greet(): string; }

Member Ordering

This rule aims to standardize the way classes, interfaces, and type literals are structured and ordered.

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/member-ordering.md

👍 Examples of correct code

[
  // Index signature
  "signature",

  // Fields
  "public-static-field",
  "protected-static-field",
  "private-static-field",
  "#private-static-field",

  "public-decorated-field",
  "protected-decorated-field",
  "private-decorated-field",

  "public-instance-field",
  "protected-instance-field",
  "private-instance-field",
  "#private-instance-field",

  "public-abstract-field",
  "protected-abstract-field",

  "public-field",
  "protected-field",
  "private-field",
  "#private-field",

  "static-field",
  "instance-field",
  "abstract-field",

  "decorated-field",

  "field",

  // Static initialization
  "static-initialization",

  // Constructors
  "public-constructor",
  "protected-constructor",
  "private-constructor",

  // Getters
  "public-static-get",
  "protected-static-get",
  "private-static-get",
  "#private-static-get",

  "public-decorated-get",
  "protected-decorated-get",
  "private-decorated-get",

  "public-instance-get",
  "protected-instance-get",
  "private-instance-get",
  "#private-instance-get",

  "public-abstract-get",
  "protected-abstract-get",

  "public-get",
  "protected-get",
  "private-get",
  "#private-get",

  "static-get",
  "instance-get",
  "abstract-get",

  "decorated-get",

  "get",

  // Setters
  "public-static-set",
  "protected-static-set",
  "private-static-set",
  "#private-static-set",

  "public-decorated-set",
  "protected-decorated-set",
  "private-decorated-set",

  "public-instance-set",
  "protected-instance-set",
  "private-instance-set",
  "#private-instance-set",

  "public-abstract-set",
  "protected-abstract-set",

  "public-set",
  "protected-set",
  "private-set",

  "static-set",
  "instance-set",
  "abstract-set",

  "decorated-set",

  "set",

  // Methods
  "public-static-method",
  "protected-static-method",
  "private-static-method",
  "#private-static-method",
  "public-decorated-method",
  "protected-decorated-method",
  "private-decorated-method",
  "public-instance-method",
  "protected-instance-method",
  "private-instance-method",
  "#private-instance-method",
  "public-abstract-method",
  "protected-abstract-method"
]

👎 Examples of incorrect code

// another order

No Confusing Non Null Assertion

Using a non-null assertion (!) next to an assign or equals check (= or == or ===) creates code that is confusing as it looks similar to a not equals check (!= !==).

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/no-confusing-non-null-assertion.md

👍 Examples of correct code

interface Foo {
  bar?: string;
  num?: number;
}

const foo: Foo = getFoo();
const isEqualsBar = foo.bar === "hello";
const isEqualsNum = 1 + foo.num! === 2;

👎 Examples of incorrect code

interface Foo {
  bar?: string;
  num?: number;
}

const foo: Foo = getFoo();
const isEqualsBar = foo.bar! == 'hello';
const isEqualsNum = 1 + foo.num! == 2;

No Extra Non Null Assertion

The ! non-null assertion operator in TypeScript is used to assert that a value's type does not include null or undefined.

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/no-extra-non-null-assertion.md

👍 Examples of correct code

const foo: { bar: number } | null = null;
const bar = foo!.bar;
function foo(bar: number | undefined) {
  const bar: number = bar!;
}
function foo(bar?: { n: number }) {
  return bar?.n;
}

👎 Examples of incorrect code

const foo: { bar: number } | null = null;
const bar = foo!!!.bar;
function foo(bar: number | undefined) {
  const bar: number = bar!!!;
}
function foo(bar?: { n: number }) {
  return bar!?.n;
}

No For In Array

A for-in loop (for (var i in o)) iterates over the properties of an Object. While it is legal to use for-in loops with array types, it is not common. for-in will iterate over the indices of the array as strings, omitting any "holes" in the array.

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/no-for-in-array.md

👍 Examples of correct code

for (const value of array) {
  console.log(value);
}

for (const [ index, value ] of array.entries()) {
    console.log(index, value);
}

👎 Examples of incorrect code

for (const i in array) {
  console.log(array[i]);
}

for (const i in array) {
  console.log(i, array[i]);
}

No Inferrable Type

TypeScript is able to infer the types of parameters, properties, and variables from their default or initial values. There is no need to use an explicit : type annotation on one of those constructs initialized to a boolean, number, or string.

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/no-inferrable-types.md

👍 Examples of correct code

const a = 10n;
const a = BigInt(10);
const a = !0;
const a = Boolean(null);
const a = true;
const a = null;
const a = 10;
const a = Infinity;
const a = NaN;
const a = Number('1');
const a = /a/;
const a = new RegExp('a');
const a = `str`;
const a = String(1);
const a = Symbol('a');
const a = undefined;
const a = void someValue;

👎 Examples of incorrect code

const a: bigint = 10n;
const a: bigint = BigInt(10);
const a: boolean = !0;
const a: boolean = Boolean(null);
const a: boolean = true;
const a: null = null;
const a: number = 10;
const a: number = Infinity;
const a: number = NaN;
const a: number = Number('1');
const a: RegExp = /a/;
const a: RegExp = new RegExp('a');
const a: string = `str`;
const a: string = String(1);
const a: symbol = Symbol('a');
const a: undefined = undefined;
const a: undefined = void someValue;

No This Alias

Assigning a variable to this instead of properly using arrow lambdas may be a symptom of pre-ES6 practices or not managing scope well.

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/no-this-alias.md

👍 Examples of correct code

setTimeout(() => {
  this.doWork();
});

👎 Examples of incorrect code

const self = this;

setTimeout(function () {
  self.doWork();
});

Non Nullable Type Assertion Style

!: Non-null assertion as: Traditional type assertion with a coincidentally equivalent type

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/non-nullable-type-assertion-style.md

👍 Examples of correct code

const maybe = Math.random() > 0.5 ? '' : undefined;

const definitely = maybe!;
const alsoDefinitely = maybe!;

👎 Examples of incorrect code

const maybe = Math.random() > 0.5 ? '' : undefined;

const definitely = maybe as string;
const alsoDefinitely = <string>maybe;

Parameter Properties

The constructor parameter is assigned to the class property at the beginning of the constructor

http://link.com

👍 Examples of correct code

class Foo {
  constructor(readonly name: string) {}
}

class Foo {
  constructor(private name: string) {}
}

class Foo {
  constructor(protected name: string) {}
}

class Foo {
  constructor(public name: string) {}
}

class Foo {
  constructor(private readonly name: string) {}
}

class Foo {
  constructor(protected readonly name: string) {}
}

class Foo {
  constructor(public readonly name: string) {}
}

👎 Examples of incorrect code

class Foo {
  constructor(name: string) {
    this.name = name;
  }
}

Prefer Enum Initializers

TypeScript enums are a practical way to organize semantically related constant values. Members of enums thatdon't have explicit values are by default given sequentially increasing numbers.

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/prefer-enum-initializers.md

👍 Examples of correct code

enum Status {
  Open = 'Open',
  Close = 'Close',
}

enum Direction {
  Up = 1,
  Down = 2,
}

enum Color {
  Red = 'Red',
  Green = 'Green',
  Blue = 'Blue',
}

👎 Examples of incorrect code

enum Status {
  Open = 1,
  Close,
}

enum Direction {
  Up,
  Down,
}

enum Color {
  Red,
  Green = 'Green'
  Blue = 'Blue',
}

Prefer Function Type

TypeScript allows for two common ways to declare a type for a function:

Function type: () => string Object type with a signature: { (): string }

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/prefer-function-type.md

👍 Examples of correct code

type Example = () => string;
function foo(example: () => number): number {
  return bar();
}
// returns the function itself, not the `this` argument.
type ReturnsSelf = (arg: string) => ReturnsSelf;
function foo(bar: { (): string; baz: number }): string {
  return bar();
}
interface Foo {
  bar: string;
}
interface Bar extends Foo {
  (): void;
}
// multiple call signatures (overloads) is allowed:
interface Overloaded {
  (data: string): number;
  (id: number): string;
}
// this is equivelent to Overloaded interface.
type Intersection = ((data: string) => number) & ((id: number) => string);

👎 Examples of incorrect code

interface Example {
  (): string;
}
function foo(example: { (): number }): number {
  return example();
}
interface ReturnsSelf {
  // returns the function itself, not the `this` argument.
  (arg: string): this;
}

Prefer Namespace Keyword

TypeScript historically allowed a form of code organization called "custom modules" (module Example {}), later renamed to "namespaces" (namespace Example).

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/prefer-namespace-keyword.md

👍 Examples of correct code

namespace Example {}

declare module 'foo' {}

👎 Examples of incorrect code

module Example {}

Prefer Nullish Coalescing

The ?? nullish coalescing runtime operator allows providing a default value when dealing with null or undefined. Because the nullish coalescing operator only coalesces when the original value is null or undefined, it is much safer than relying upon logical OR operator chaining ||, which coalesces on any falsy value.

This rule reports when an || operator can be safely replaced with a ??.

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/prefer-nullish-coalescing.md

👍 Examples of correct code

const foo: any = 'bar';
foo ?? 'a string';
foo ?? 'a string';
foo ?? 'a string';
foo ?? 'a string';

const foo: string | undefined = 'bar';
foo ?? 'a string';
foo ?? 'a string';

const foo: string | null = 'bar';
foo ?? 'a string';
foo ?? 'a string';

👎 Examples of incorrect code

const foo: any = 'bar';
foo !== undefined && foo !== null ? foo : 'a string';
foo === undefined || foo === null ? 'a string' : foo;
foo == undefined ? 'a string' : foo;
foo == null ? 'a string' : foo;

const foo: string | undefined = 'bar';
foo !== undefined ? foo : 'a string';
foo === undefined ? 'a string' : foo;

const foo: string | null = 'bar';
foo !== null ? foo : 'a string';
foo === null ? 'a string' : foo;

Prefer Optional Chain

This rule reports on code where an && operator can be safely replaced with ?. optional chaining.

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/prefer-optional-chain.md

👍 Examples of correct code

foo?.a?.b?.c;
foo?.['a']?.b?.c;
foo?.a?.b?.method?.();

foo?.a?.b?.c?.d?.e;

!foo?.bar;
!foo?.[bar];
!foo?.bar?.baz?.();

👎 Examples of incorrect code

foo && foo.a && foo.a.b && foo.a.b.c;
foo && foo['a'] && foo['a'].b && foo['a'].b.c;
foo && foo.a && foo.a.b && foo.a.b.method && foo.a.b.method();

// With empty objects
(((foo || {}).a || {}).b || {}).c;
(((foo || {})['a'] || {}).b || {}).c;

// With negated `or`s
!foo || !foo.bar;
!foo || !foo[bar];
!foo || !foo.bar || !foo.bar.baz || !foo.bar.baz();

// this rule also supports converting chained strict nullish checks:
foo &&
  foo.a != null &&
  foo.a.b !== null &&
  foo.a.b.c != undefined &&
  foo.a.b.c.d !== undefined &&
  foo.a.b.c.d.e;

Prefer Readonly

This rule reports on private members are marked as readonly if they're never modified outside of the constructor.

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/prefer-readonly.md

👍 Examples of correct code

class Container {

  // Public members might be modified externally
  public publicMember: boolean;

  // Protected members might be modified by child classes
  protected protectedMember: number;

  // This is modified later on by the class
  private modifiedLater = 'unchanged';

  private readonly myProp = 'readonly';

  public mutate() {
    this.modifiedLater = 'mutated';
  }
}

👎 Examples of incorrect code

class Container {

  // These member variables could be marked as readonly
  private neverModifiedMember = true;
  private onlyModifiedInConstructor: number;

  public constructor(
    onlyModifiedInConstructor: number,
    // Private parameter properties can also be marked as readonly
    private neverModifiedParameter: string,
  ) {
    this.onlyModifiedInConstructor = onlyModifiedInConstructor;
  }
}

Prefer Reduce Type Parameter

It's common to call Array#reduce with a generic type, such as an array or object, as the initial value. Since these values are empty, their types are not usable:

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/prefer-reduce-type-parameter.md

👍 Examples of correct code

[1, 2, 3].reduce<number[]>((arr, num) => arr.concat(num * 2), []);

['a', 'b'].reduce<Record<string, boolean>>(
  (accum, name) => ({
    ...accum,
    [name]: true,
  }),
  {},
);

👎 Examples of incorrect code

[1, 2, 3].reduce((arr, num) => arr.concat(num * 2), [] as number[]);

['a', 'b'].reduce(
  (accum, name) => ({
    ...accum,
    [name]: true,
  }),
  {} as Record<string, boolean>,
);

Promise Function Async

In contrast, non-async Promise - returning functions are technically capable of either. Code that handles the results of those functions will often need to handle both cases, which can get complex. This rule's practice removes a requirement for creating code to handle both cases.

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/promise-function-async.md

👍 Examples of correct code

const arrowFunctionReturnsPromise = async () => Promise.resolve('value');

async function functionReturnsPromise() {
  return Promise.resolve('value');
}

👎 Examples of incorrect code

const arrowFunctionReturnsPromise = () => Promise.resolve('value');

function functionReturnsPromise() {
  return Promise.resolve('value');
}

Require Array Sort Compare

When called without a compare function, Array#sort() converts all non-undefined array elements into strings and then compares said strings based off their UTF-16

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/require-array-sort-compare.md

👍 Examples of correct code

const array: any[];
const userDefinedType: { sort(): void };

array.sort((a, b) => a - b);
array.sort((a, b) => a.localeCompare(b));

userDefinedType.sort();

const one = '1';
const two = '2';
const three = '3';
[one, two, three].sort();

👎 Examples of incorrect code

const array: any[];
const stringArray: string[];

array.sort();

// String arrays should be sorted using `String#localeCompare`.
stringArray.sort();

Sort Type Constituents

Sorting union (|) and intersection (&) types can help:

keep your codebase standardized find repeated types reduce diff churn This rule reports on any types that aren't sorted alphabetically.

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/sort-type-constituents.md

👍 Examples of correct code

type T1 = A | B;

type T2 = { a: string } & { b: string };

type T3 = [1, 2, 3] & [1, 2, 4];

type T4 =
  | any
  | string
  | A
  | B
  | number[]
  | string[]
  | readonly number[]
  | readonly string[]
  | 'a'
  | 'b'
  | 'a'
  | 'b'
  | (() => string)
  | (() => void)
  | { a: string }
  | { b: string }
  | [1, 2, 3]
  | [1, 2, 4];

👎 Examples of incorrect code

type T1 = B | A;

type T2 = { b: string } & { a: string };

type T3 = [1, 2, 4] & [1, 2, 3];

type T4 =
  | [1, 2, 4]
  | [1, 2, 3]
  | { b: string }
  | { a: string }
  | (() => void)
  | (() => string)
  | 'b'
  | 'a'
  | 'b'
  | 'a'
  | readonly string[]
  | readonly number[]
  | string[]
  | number[]
  | B
  | A
  | string
  | any;

Space Before Blocks

Consistency is an important part of any style guide. While it is a personal preference where to put the opening brace of blocks, it should be consistent across a whole project. Having an inconsistent style distracts the reader from seeing the important parts of the code.

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/space-before-blocks.md

👍 Examples of correct code

if (a) {
    b();
}

if (a) {
    b();
} else{ /*no error. this is checked by `keyword-spacing` rule.*/
    c();
}

class C {
    static{} /*no error. this is checked by `keyword-spacing` rule.*/
}

function a() {}

for (;;) {
    b();
}

try {} catch(a) {}

enum Breakpoint {
  Large, Medium;
}

interface State {
  currentBreakpoint: Breakpoint;
}

👎 Examples of incorrect code

if (a){
    b();
}

function a(){}

for (;;){
    b();
}

try {} catch(a){}

class Foo{
  constructor(){}
}

enum Breakpoint{
  Large, Medium;
}

interface State{
  currentBreakpoint: Breakpoint;
}

Performance

No Alert

Disallow the use of alert, confirm, and prompt

https://eslint.org/docs/latest/rules/no-alert

👍 Examples of correct code

// Example Switch Alerts
Swal.fire(
  'Good job!',
  'You clicked the button!',
  'success'
)

customAlert("Something happened!");

customConfirm("Are you sure?");

customPrompt("Who are you?");

function foo() {
    var alert = myCustomLib.customAlert;
    alert();
}

👎 Examples of incorrect code

alert("here!");

confirm("Are you sure?");

prompt("What's your name?", "John Doe");

No Loop Func

Disallow function declarations that contain unsafe references inside loop statements

https://eslint.org/docs/latest/rules/no-loop-func https://sonarsource.github.io/rspec/#/rspec/S1515/javascript

👍 Examples of correct code

var a = function() {};

for (var i=10; i; i--) {
    a();
}

👎 Examples of incorrect code

for (var i=10; i; i--) {
    (function() { return i; })();
}

while(i) {
    var a = function() { return i; };
    a();
}

do {
    function a() { return i; };
    a();
} while (i);

let foo = 0;
for (let i = 0; i < 10; ++i) {
    //Bad, `foo` is not in the loop-block's scope and `foo` is modified in/after the loop
    setTimeout(() => console.log(foo));
    foo += 1;
}

for (let i = 0; i < 10; ++i) {
    //Bad, `foo` is not in the loop-block's scope and `foo` is modified in/after the loop
    setTimeout(() => console.log(foo));
}
foo = 100;

Errors

Construtor Super Invalid

Verifies calls of super() in constructors.

https://eslint.org/docs/latest/rules/constructor-super

👍 Examples of correct code

class A {
    constructor() { }
}

class A extends B {
    constructor() {
        super();
    }
}

👎 Examples of incorrect code

class A {
    constructor() {
        super();  // This is a SyntaxError.
    }
}

class A extends B {
    constructor() { }  // Would throw a ReferenceError.
}

// Classes which inherits from a non constructor are always problems.
class A extends null {
    constructor() {
        super();  // Would throw a TypeError.
    }
}

class A extends null {
    constructor() { }  // Would throw a ReferenceError.
}

Getter Return

Enforces that a return statement is present in property getters.

https://eslint.org/docs/latest/rules/getter-return

👍 Examples of correct code

p = {
    get name(){
        return "nicholas";
    }
};

Object.defineProperty(p, "age", {
    get: function (){
        return 18;
    }
});

class P{
    get name(){
        return "nicholas";
    }
}

👎 Examples of incorrect code

p = {
    get name(){
        // no returns.
    }
};

Object.defineProperty(p, "age", {
    get: function (){
        // no returns.
    }
});

class P{
    get name(){
        // no returns.
    }
}

No Class Assign

Disallows modifying variables of class declarations.

https://eslint.org/docs/latest/rules/no-class-assign

👍 Examples of correct code

let A = class {
    b() {
        A = 0; // A is a variable.
    }
}

// OR

class A {
    b(A) {
        A = 0; // A is a parameter.
    }
}

👎 Examples of incorrect code

class A { }
A = 0;

// OR

A = 0;
class A { }

// OR

class A {
    b() {
        A = 0;
    }
}

// OR

let A = class A {
    b() {
        A = 0;
        // `let A` is shadowed by the class name.
    }
}

No Compare Neg Zero

Disallows comparing against -0.

https://eslint.org/docs/latest/rules/no-compare-neg-zero

👍 Examples of correct code

if (x === 0) {
    // doSomething()...
}

👎 Examples of incorrect code

if (x === -0) {
    // doSomething()...
}

No Const Assign

Disallow reassigning const variables

https://eslint.org/docs/latest/rules/no-const-assign

👍 Examples of correct code

const a = 0;
console.log(a);

// or

let a = 0;
a = 1;

// or

// `a` is re-defined (not modified) on each loop step.
for (const a in [ 1, 2, 3 ]) {
    console.log(a);
}

👎 Examples of incorrect code

const a = 0;
a = 1;

// or

const a = 0;
a += 1;

// or

const a = 0;
++a;

No Dupe Class Members

Disallow duplicate class members

https://eslint.org/docs/latest/rules/no-dupe-class-members

👍 Examples of correct code

class Foo {
  bar() { }
  qux() { }
}

class Foo {
  get bar() { }
  set bar(value) { }
}

class Foo {
  bar;
  qux;
}

class Foo {
  bar;
  qux() { }
}

class Foo {
  static bar() { }
  bar() { }
}

👎 Examples of incorrect code

class Foo {
  bar() { }
  bar() { }
}

class Foo {
  bar() { }
  get bar() { }
}

class Foo {
  bar;
  bar;
}

class Foo {
  bar;
  bar() { }
}

class Foo {
  static bar() { }
  static bar() { }
}

No Dupe Keys

No Duplicate Key in Object

https://eslint.org/docs/rules/no-dupe-keys#no-dupe-keys

👍 Examples of correct code

var foo = {
    bar: "baz",
    quxx: "qux"
};

👎 Examples of incorrect code

var foo = {
    bar: "baz",
    bar: "qux"
};

var foo = {
    "bar": "baz",
    bar: "qux"
};

var foo = {
    0x1: "baz",
    1: "qux"
};

No Dupe Args

Disallow duplicate arguments in function definitions

https://eslint.org/docs/latest/rules/no-dupe-args#rule-details

👍 Examples of correct code

function foo(a, b, c) {
    console.log(a, b, c);
}

var bar = function (a, b, c) {
    console.log(a, b, c);
};

👎 Examples of incorrect code

function foo(a, b, a) {
    console.log("value of the second a:", a);
}

var bar = function (a, b, a) {
    console.log("value of the second a:", a);
};

No Unsafe Finally

Disallow control flow statements in finally blocks

https://eslint.org/docs/latest/rules/no-unsafe-finally

👍 Examples of correct code

let foo = function() {
    try {
        return 1;
    } catch(err) {
        return 2;
    } finally {
        console.log("hola!");
    }
};

let foo = function() {
    try {
        return 1;
    } catch(err) {
        return 2;
    } finally {
        let a = function() {
            return "hola!";
        }
    }
};

let foo = function(a) {
    try {
        return 1;
    } catch(err) {
        return 2;
    } finally {
        switch(a) {
            case 1: {
                console.log("hola!")
                break;
            }
        }
    }
};

👎 Examples of incorrect code

let foo = function() {
    try {
        return 1;
    } catch(err) {
        return 2;
    } finally {
        return 3;
    }
};

let foo = function() {
    try {
        return 1;
    } catch(err) {
        return 2;
    } finally {
        throw new Error;
    }
};

No Import Assign

Disallow control flow statements in finally blocks

https://eslint.org/docs/latest/rules/no-unsafe-finally

👍 Examples of correct code

import mod, { named } from "./mod.mjs"
import * as mod_ns from "./mod.mjs"

mod.prop = 1
named.prop = 2
mod_ns.named.prop = 3

// Known Limitation
function test(obj) {
    obj.named = 4 // Not errored because 'obj' is not namespace objects.
}
test(mod_ns) // Not errored because it doesn't know that 'test' updates the member of the argument.

👎 Examples of incorrect code

import mod, { named } from "./mod.mjs"
import * as mod_ns from "./mod.mjs"

mod = 1          // ERROR: 'mod' is readonly.
named = 2        // ERROR: 'named' is readonly.
mod_ns.named = 3 // ERROR: The members of 'mod_ns' are readonly.
mod_ns = {}      // ERROR: 'mod_ns' is readonly.
// Can't extend 'mod_ns'
Object.assign(mod_ns, { foo: "foo" }) // ERROR: The members of 'mod_ns' are readonly.

No Self Import

Forbid a module from importing itself.

https://github.com/import-js/eslint-plugin-import/blob/HEAD/docs/rules/no-self-import.md

👍 Examples of correct code

// foo.js
import bar from './bar';

const bar = require('./bar');

👎 Examples of incorrect code

// foo.js
import foo from './foo';
const foo = require('./foo');

// index.js
import index from '.';
const index = require('.');

Export Name

Reports funny business with exports, like repeated exports of names or defaults.

https://github.com/import-js/eslint-plugin-import/blob/HEAD/docs/rules/export.md

👍 Examples of correct code

// foo.js
export default class MyClass { /*...*/ } // Multiple default exports.

function makeFunction() { return new MyClass(...arguments) }

export default makeFunction // Multiple default exports.

👎 Examples of incorrect code

export default class MyClass { /*...*/ } // Multiple default exports.

function makeClass() { return new MyClass(...arguments) }

export default makeClass // Multiple default exports.

// OR

export const foo = function () { /*...*/ } // Multiple exports of name 'foo'.

function bar() { /*...*/ }
export { bar as foo } // Multiple exports of name 'foo'.

Useless Loop

No useless loop

https://eslint.org/docs/rules/no-unreachable-loop

👍 Examples of correct code

do {
    doSomething();
} while (false)

for (let i = 0; i < 1; i++) {
    doSomething(i);
}

for (const a of [1]) {
    doSomething(a);
}

👎 Examples of incorrect code

while (foo) {
    doSomething(foo);
    foo = foo.parent;
    break;
}

function verifyList(head) {
    let item = head;
    do {
        if (verify(item)) {
            return true;
        } else {
            return false;
        }
    } while (item);
}

function findSomething(arr) {
    for (var i = 0; i < arr.length; i++) {
        if (isSomething(arr[i])) {
            return arr[i];
        } else {
            throw new Error("Doesn't exist.");
        }
    }
}

for (key in obj) {
    if (key.startsWith("_")) {
        break;
    }
    firstKey = key;
    firstValue = obj[key];
    break;
}

for (foo of bar) {
    if (foo.id === id) {
        doSomething(foo);
    }
    break;
}

No Loss Of Precision

Disallow literal numbers that lose precision

https://eslint.org/docs/latest/rules/no-loss-of-precision

👍 Examples of correct code

const x = 12345
const x = 123.456
const x = 123e34
const x = 12300000000000000000000000
const x = 0x1FFFFFFFFFFFFF
const x = 9007199254740991
const x = 9007_1992547409_91

👎 Examples of incorrect code

const x = 9007199254740993
const x = 5123000000000000000000000000001
const x = 1230000000000000000000000.0
const x = .1230000000000000000000000
const x = 0X20000000000001
const x = 0X2_000000000_0001;

No Unsafe Argument

Disallow calling a function with a value with type any.

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/no-unsafe-argument.md

👍 Examples of correct code

declare function foo(arg1: string, arg2: number, arg3: string): void;

foo('a', 1, 'b');

const tuple1 = ['a', 1, 'b'] as const;
foo(...tuple1);

declare function bar(arg1: string, arg2: number, ...rest: string[]): void;
const array: string[] = ['a'];
bar('a', 1, ...array);

declare function baz(arg1: Set<string>, arg2: Map<string, string>): void;
foo(new Set<string>(), new Map<string, string>());

👎 Examples of incorrect code

declare function foo(arg1: string, arg2: number, arg3: string): void;

const anyTyped = 1 as any;

foo(...anyTyped);
foo(anyTyped, 1, 'a');

const anyArray: any[] = [];
foo(...anyArray);

const tuple1 = ['a', anyTyped, 'b'] as const;
foo(...tuple1);

const tuple2 = [1] as const;
foo('a', ...tuple, anyTyped);

declare function bar(arg1: string, arg2: number, ...rest: string[]): void;
const x = [1, 2] as [number, ...number[]];
foo('a', ...x, anyTyped);

declare function baz(arg1: Set<string>, arg2: Map<string, string>): void;
foo(new Set<any>(), new Map<any, string>());

Import Default Not Fount

If a default import is requested, report if there is no default export in the imported module.

https://github.com/import-js/eslint-plugin-import/blob/main/docs/rules/default.md

👍 Examples of correct code

// ./foo.js
export class Example {}

import foo from './foo'

// assuming 'node_modules' are ignored (true by default)
import someModule from 'some-module'

👎 Examples of incorrect code

import bar from './bar' // no default export found in ./bar
import baz from './baz' // no default export found in ./baz

Import Namespace

No Import Resolve

Ensures an imported module can be resolved to a module on the local filesystem, as defined by standard Node require.resolve behavior.

https://github.com/import-js/eslint-plugin-import/blob/HEAD/docs/rules/no-unresolved.md

👍 Examples of correct code

import x from './bar' // file found

👎 Examples of incorrect code

import x from './foo' // File not found

Import Not Found

Verifies that all named imports are part of the set of named exports in the referenced module.

https://github.com/import-js/eslint-plugin-import/blob/HEAD/docs/rules/named.md

👍 Examples of correct code

// ./foo.js
export const foo = "I'm so foo"

// -------------------

// ./bar.js
import { foo } from './foo';

// ES7 proposal
export { foo as bar } from './foo';

// node_modules without jsnext:main are not analyzed by default
// (import/ignore setting)
import { SomeNonsenseThatDoesntExist } from 'react';

👎 Examples of incorrect code

// ./baz.js
import { notFoo } from './foo';

// ES7 proposal
export { notFoo as defNotBar } from './foo';

// will follow 'jsnext:main', if available
import { dontCreateStore } from 'redux';

JS Type

The types of the arguments to built-in functions are specified in the JavaScript language specifications

https://sonarsource.github.io/rspec/#/rspec/S3782/javascript

👍 Examples of correct code

const isTooSmall = Math.abs(x) < 0.0042;

👎 Examples of incorrect code

const isTooSmall = Math.abs(x < 0.0042); // Type require number no boolean

No Invalid This

Under the strict mode, this keywords outside of classes or class-like objects might be undefined and raise a TypeError.

https://eslint.org/docs/latest/rules/no-invalid-this#rule-details https://typescript-eslint.io/rules/no-invalid-this

👍 Examples of correct code

"use strict";

this.a = 0;
baz(() => this);

function Foo() {
    // OK, this is in a legacy style constructor.
    this.a = 0;
    baz(() => this);
}

class Foo {
    constructor() {
        // OK, this is in a constructor.
        this.a = 0;
        baz(() => this);
    }
}

var obj = {
    foo: function foo() {
        // OK, this is in a method (this function is on object literal).
        this.a = 0;
    }
};

var obj = {
    foo() {
        // OK, this is in a method (this function is on object literal).
        this.a = 0;
    }
};

var obj = {
    get foo() {
        // OK, this is in a method (this function is on object literal).
        return this.a;
    }
};

var obj = Object.create(null, {
    foo: {value: function foo() {
        // OK, this is in a method (this function is on object literal).
        this.a = 0;
    }}
});

Object.defineProperty(obj, "foo", {
    value: function foo() {
        // OK, this is in a method (this function is on object literal).
        this.a = 0;
    }
});

Object.defineProperties(obj, {
    foo: {value: function foo() {
        // OK, this is in a method (this function is on object literal).
        this.a = 0;
    }}
});

function Foo() {
    this.foo = function foo() {
        // OK, this is in a method (this function assigns to a property).
        this.a = 0;
        baz(() => this);
    };
}

obj.foo = function foo() {
    // OK, this is in a method (this function assigns to a property).
    this.a = 0;
};

Foo.prototype.foo = function foo() {
    // OK, this is in a method (this function assigns to a property).
    this.a = 0;
};

class Foo {

    // OK, this is in a class field initializer.
    a = this.b;

    // OK, static initializers also have valid this.
    static a = this.b;

    foo() {
        // OK, this is in a method.
        this.a = 0;
        baz(() => this);
    }

    static foo() {
        // OK, this is in a method (static methods also have valid this).
        this.a = 0;
        baz(() => this);
    }

    static {
        // OK, static blocks also have valid this.
        this.a = 0;
        baz(() => this);
    }
}

var foo = (function foo() {
    // OK, the `bind` method of this function is called directly.
    this.a = 0;
}).bind(obj);

foo.forEach(function() {
    // OK, `thisArg` of `.forEach()` is given.
    this.a = 0;
    baz(() => this);
}, thisArg);

/** @this Foo */
function foo() {
    // OK, this function has a `@this` tag in its JSDoc comment.
    this.a = 0;
}

👎 Examples of incorrect code

"use strict";

(function() {
    this.a = 0;
    baz(() => this);
})();

function foo() {
    this.a = 0;
    baz(() => this);
}

var foo = function() {
    this.a = 0;
    baz(() => this);
};

foo(function() {
    this.a = 0;
    baz(() => this);
});

var obj = {
    aaa: function() {
        return function foo() {
            // There is in a method `aaa`, but `foo` is not a method.
            this.a = 0;
            baz(() => this);
        };
    }
};

foo.forEach(function() {
    this.a = 0;
    baz(() => this);
});

No This Before Super

In the constructor of derived classes, if this/super are used before super() calls, it raises a reference error.

https://eslint.org/docs/latest/rules/no-this-before-super

👍 Examples of correct code

class A {
    constructor() {
        this.a = 0; // OK, this class doesn't have an `extends` clause.
    }
}

class A extends B {
    constructor() {
        super();
        this.a = 0; // OK, this is after `super()`.
    }
}

class A extends B {
    foo() {
        this.a = 0; // OK. this is not in a constructor.
    }
}

👎 Examples of incorrect code

class A extends B {
    constructor() {
        this.a = 0;
        super();
    }
}

class A extends B {
    constructor() {
        this.foo();
        super();
    }
}

class A extends B {
    constructor() {
        super.foo();
        super();
    }
}

class A extends B {
    constructor() {
        super(this.foo());
    }
}

No Obj Calls

Disallow calling global object properties as functions

https://eslint.org/docs/latest/rules/no-obj-calls

👍 Examples of correct code

function area(r) {
    return Math.PI * r * r;
}

var object = JSON.parse("{}");

var value = Reflect.get({ x: 1, y: 2 }, "x");

var first = Atomics.load(foo, 0);

var segmenterFr = new Intl.Segmenter("fr", { granularity: "word" });

👎 Examples of incorrect code

var math = Math();

var newMath = new Math();

var json = JSON();

var newJSON = new JSON();

var reflect = Reflect();

var newReflect = new Reflect();

var atomics = Atomics();

var newAtomics = new Atomics();

var intl = Intl();

var newIntl = new Intl();

No Empty Pattern

Disallow empty destructuring patterns

https://eslint.org/docs/latest/rules/no-empty-pattern

👍 Examples of correct code

👎 Examples of incorrect code

var {} = foo;
var [] = foo;
var {a: {}} = foo;
var {a: []} = foo;
function foo({}) {}
function foo([]) {}
function foo({a: {}}) {}
function foo({a: []}) {}

No Useless Computed Key

Disallow unnecessary computed property keys in objects and classes

https://eslint.org/docs/latest/rules/no-useless-computed-key

👍 Examples of correct code

var c = { 'a': 0 };
var c = { 0: 0 };
var a = { x() {} };
var c = { a: 0 };
var c = { '0+1,234': 0 };

👎 Examples of incorrect code

var a = { ['0']: 0 };
var a = { ['0+1,234']: 0 };
var a = { [0]: 0 };
var a = { ['x']: 0 };
var a = { ['x']() {} };

Useless Call Code

Disallow useless code

https://eslint.org/docs/rules/no-useless-call#no-useless-call

👍 Examples of correct code

foo.call(obj, 1, 2, 3);
foo.apply(obj, [1, 2, 3]);
obj.foo.call(null, 1, 2, 3);
obj.foo.apply(null, [1, 2, 3]);
obj.foo.call(otherObj, 1, 2, 3);
obj.foo.apply(otherObj, [1, 2, 3]);

// The argument list is variadic.
// Those are warned by the `prefer-spread` rule.
foo.apply(undefined, args);
foo.apply(null, args);
obj.foo.apply(obj, args);

a[++i].foo.call(a[i], 1, 2, 3);

👎 Examples of incorrect code

foo.call(undefined, 1, 2, 3);
foo.apply(undefined, [1, 2, 3]);
foo.call(null, 1, 2, 3);
foo.apply(null, [1, 2, 3]);

// These are same as `obj.foo(1, 2, 3);`
obj.foo.call(obj, 1, 2, 3);
obj.foo.apply(obj, [1, 2, 3]);

a[i++].foo.call(a[i++], 1, 2, 3);

Useless Catch Code

Disallow useless code

https://eslint.org/docs/rules/no-useless-catch#no-useless-catch

👍 Examples of correct code

try {
  doSomethingThatMightThrow();
} catch (e) {
  doSomethingBeforeRethrow();
  throw e;
}

try {
  doSomethingThatMightThrow();
} catch (e) {
  handleError(e);
}

try {
  doSomethingThatMightThrow();
} finally {
  cleanUp();
}

👎 Examples of incorrect code

try {
  doSomethingThatMightThrow();
} catch (e) {
  throw e;
}

try {
  doSomethingThatMightThrow();
} catch (e) {
  throw e;
} finally {
  cleanUp();
}

Useless Expression Code

Disallow useless code

https://eslint.org/docs/rules/no-unused-expressions

👍 Examples of correct code

{} // In this context, this is a block statement, not an object literal

{myLabel: someVar} // In this context, this is a block statement with a label and expression, not an object literal

function namedFunctionDeclaration () {}

(function aGenuineIIFE () {}());

f()

a = 0

new C

delete a.b

void a

a ? b() : c();
a ? (b = c) : d();

👎 Examples of incorrect code

0

if(0) 0

{0}

f(0), {}

a && b()

a, b()

c = a, b;

a() && function namedFunctionInExpressionContext () {f();}

(function anIncompleteIIFE () {});

injectGlobal`body{ color: red; }`

function foo() {
    "bar" + 1;
}

class Foo {
    static {
        "use strict"; // class static blocks do not have directive prologues
    }
}

Useless Return Code

Disallow useless code

https://eslint.org/docs/rules/no-useless-return#no-useless-return

👍 Examples of correct code

function foo() { return 5; }

function foo() {
  return doSomething();
}

function foo() {
  if (condition) {
    bar();
    return;
  } else {
    baz();
  }
  qux();
}

function foo() {
  switch (bar) {
    case 1:
      doSomething();
      return;
    default:
      doSomethingElse();
  }
}

function foo() {
  for (const foo of bar) {
    return;
  }
}

👎 Examples of incorrect code

function foo() { return; }

function foo() {
  doSomething();
  return;
}

function foo() {
  if (condition) {
    bar();
    return;
  } else {
    baz();
  }
}

function foo() {
  switch (bar) {
    case 1:
      doSomething();
    default:
      doSomethingElse();
      return;
  }
}

Useless Construct Code

Disallow useless code

https://eslint.org/docs/rules/no-useless-constructor#options

👍 Examples of correct code

class A { }

class A {
    constructor () {
        doSomething();
    }
}

class B extends A {
    constructor() {
        super('foo');
    }
}

class B extends A {
    constructor() {
        super();
        doSomething();
    }
}

👎 Examples of incorrect code

class A {
    constructor () {
    }
}

class B extends A {
    constructor (...args) {
      super(...args);
    }
}

No Use Before Define

In JavaScript, prior to ES6, variable and function declarations are hoisted to the top of a scope, so it’s possible to use identifiers before their formal declarations in code.

https://eslint.org/docs/latest/rules/no-use-before-define

👍 Examples of correct code

var a;
a = 10;
alert(a);

function f() {}
f(1);

var b = 1;
function g() {
    return b;
}

{
    let c;
    c++;
}

{
    class C {
        static x = C;
    }
}

{
    const C = class C {
        static x = C;
    }
}

{
    const C = class {
        x = C;
    }
}

{
    const C = class C {
        static {
            C.x = "foo";
        }
    }
}

const foo = 1;
export { foo };

👎 Examples of incorrect code

alert(a);
var a = 10;

f();
function f() {}

function g() {
    return b;
}
var b = 1;

{
    alert(c);
    let c = 1;
}

{
    class C extends C {}
}

{
    class C {
        static x = "foo";
        [C.x]() {}
    }
}

{
    const C = class {
        static x = C;
    }
}

{
    const C = class {
        static {
            C.x = "foo";
        }
    }
}

export { foo };
const foo = 1;

No Base To String

JavaScript will call toString() on an object when it is converted to a string, such as when + adding to a string or in ${} template literals.

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/no-base-to-string.md

👍 Examples of correct code

// These types all have useful .toString()s
'Text' + true;
`Value: ${123}`;
`Arrays too: ${[1, 2, 3]}`;
(() => {}).toString();

// Defining a custom .toString class is considered acceptable
class CustomToString {
  toString() {
    return 'Hello, world!';
  }
}
`Value: ${new CustomToString()}`;

const literalWithToString = {
  toString: () => 'Hello, world!',
};

`Value: ${literalWithToString}`;

👎 Examples of incorrect code

// Passing an object or class instance to string concatenation:
'' + {};

class MyClass {}
const value = new MyClass();
value + '';

// Interpolation and manual .toString() calls too:
`Value: ${value}`;
({}.toString());

Not IN Primitive type

"in" should not be used with primitive types

https://sonarsource.github.io/rspec/#/rspec/S3785/javascript

👍 Examples of correct code

const x = "Foo";
if(typeof x === "string")
    x.length

👎 Examples of incorrect code

var x = "Foo";
"length" in x; // Noncompliant: TypeError
0 in x;        // Noncompliant: TypeError

Useless String Operation

Results of operations on strings should not be ignored

https://sonarsource.github.io/rspec/#/rspec/S1154/javascript

👍 Examples of correct code

var str = "..."
str = str.toUpperCase();

👎 Examples of incorrect code

var str = "..."
str.toUpperCase(); // Noncompliant

Super Invocation

There are situations where super() must be invoked and situations where super() cannot be invoked. The basic rule is: a constructor in a non-derived class cannot invoke super(); a constructor in a derived class must invoke super().

Furthermore: super() must be invoked before the this and super keywords can be used. super() must be invoked with the same number of arguments as the base class' constructor. super() can only be invoked in a constructor - not in any other method. super() cannot be invoked multiple times in the same constructor.

https://sonarsource.github.io/rspec/#/rspec/S3854/javascript

👍 Examples of correct code

class Dog extends Animal {
  constructor(name) {
    super();
    this.name = name;
    super.doSomething();
  }
}

👎 Examples of incorrect code

class Dog extends Animal {
  constructor(name) {
    super();
    this.name = name;
    super();         // Noncompliant
    super.doSomething();
  }
}

No Useless Intersection

Types without members, 'any' and 'never' should not be used in type intersections

https://sonarsource.github.io/rspec/#/rspec/S4335/javascript

👍 Examples of correct code

function foo(p: MyType | null) {
 // ...
}
// or
function foo(p: MyType & AnotherType) {
 // ...
}

function bar(p: any) {
 // ...
}

👎 Examples of incorrect code

function foo(p: MyType & null) { // Noncompliant
 // ...
}

function bar(p: MyType & any) { // Noncompliant
 // ...
}

Stateful Regex

Regular expressions with the global flag turned on can be a source of tricky bugs for uninformed users, and should therefore be used with caution. Such regular expressions are stateful, that is, they maintain an internal state through the lastIndex property, which is updated and used as starting point on every call to RegExp.prototype.test() and RegExp.prototype.exec(), even when testing a different string. The lastIndex property is eventually reset when these functions return false and null respectively.

https://sonarsource.github.io/rspec/#/rspec/S6351/javascript

👍 Examples of correct code

const datePattern = /\d{4}-\d{2}-\d{2}/;
datePattern.test('2020-08-06');
datePattern.test('2019-10-10'); // Compliant

const reg = /foo*/g;
const str = 'foodie fooled football';
while ((result = reg.exec(str)) !== null) { // Compliant
  /* ... */
}

const stickyPattern = /abc/y; // Compliant
stickyPattern.test(/* ... */);

👎 Examples of incorrect code

const datePattern = /\d{4}-\d{2}-\d{2}/g;
datePattern.test('2020-08-06');
datePattern.test('2019-10-10'); // Noncompliant: the regex will return "false" despite the date being well-formed

const str = 'foodie fooled football';
// Noncompliant: a regex is defined at each iteration causing an infinite loop
while ((result = /foo*/g.exec(str)) !== null) {
  /* ... */
}

const stickyPattern = /abc/gy; // Noncompliant: a regex defined as both sticky and global ignores the global flag
stickyPattern.test(/* ... */);

Possible Errors

For Direction

Enforces for loop update clause moving the counter in the right direction.

https://eslint.org/docs/latest/rules/for-direction https://sonarsource.github.io/rspec/#/rspec/S888/javascript

👍 Examples of correct code

for (var i = 0; i < 10; i++) {
}

👎 Examples of incorrect code

for (var i = 0; i < 10; i--) {
}

for (var i = 10; i >= 0; i++) {
}

for (var i = 0; i > 10; i++) {
}

No Extra Bind

Disallow unnecessary calls to .bind()

https://eslint.org/docs/latest/rules/no-extra-bind

👍 Examples of correct code

var x = function () {
    this.foo();
}.bind(bar);

var x = function (a) {
    return a + 1;
}.bind(foo, bar);

👎 Examples of incorrect code

var x = function () {
    foo();
}.bind(bar);

var x = (() => {
    foo();
}).bind(bar);

var x = (() => {
    this.foo();
}).bind(bar);

var x = function () {
    (function () {
      this.foo();
    }());
}.bind(bar);

var x = function () {
    function foo() {
      this.bar();
    }
}.bind(baz);

No Template Curly In String

Disallow template literal placeholder syntax in regular strings

https://eslint.org/docs/latest/rules/no-template-curly-in-string

👍 Examples of correct code

`Hello ${name}!`;
`Time: ${12 * 60 * 60 * 1000}`;

templateFunction`Hello ${name}`;

👎 Examples of incorrect code

"Hello ${name}!";
'Hello ${name}!';
"Time: ${12 * 60 * 60 * 1000}";

No Dupe Else If

Disallow duplicate conditions in if-else-if chains

https://eslint.org/docs/latest/rules/no-dupe-else-if

👍 Examples of correct code

if (isSomething(x)) {
    foo();
} else if (isSomethingElse(x)) {
    bar();
}

if (a) {
    foo();
} else if (b) {
    bar();
} else if (c && d) {
    baz();
} else if (c && e) {
    quux();
} else {
    quuux();
}

if (n === 1) {
    foo();
} else if (n === 2) {
    bar();
} else if (n === 3) {
    baz();
} else if (n === 4) {
    quux();
} else if (n === 5) {
    quuux();
}

👎 Examples of incorrect code

if (isSomething(x)) {
    foo();
} else if (isSomething(x)) {
    bar();
}

if (a) {
    foo();
} else if (b) {
    bar();
} else if (c && d) {
    baz();
} else if (c && d) {
    quux();
} else {
    quuux();
}

if (n === 1) {
    foo();
} else if (n === 2) {
    bar();
} else if (n === 3) {
    baz();
} else if (n === 2) {
    quux();
} else if (n === 5) {
    quuux();
}

No Unused Private Class Members

Disallow unused private class members

https://eslint.org/docs/latest/rules/no-unused-private-class-members

👍 Examples of correct code

class Foo {
    #usedMember = 42;
    method() {
        return this.#usedMember;
    }
}

class Foo {
    #usedMethod() {
        return 42;
    }
    anotherMethod() {
        return this.#usedMethod();
    }
}

class Foo {
    get #usedAccessor() {}
    set #usedAccessor(value) {}

    method() {
        this.#usedAccessor = 42;
    }
}

👎 Examples of incorrect code

class Foo {
    #unusedMember = 5;
}

class Foo {
    #usedOnlyInWrite = 5;
    method() {
        this.#usedOnlyInWrite = 42;
    }
}

class Foo {
    #usedOnlyToUpdateItself = 5;
    method() {
        this.#usedOnlyToUpdateItself++;
    }
}

class Foo {
    #unusedMethod() {}
}

class Foo {
    get #unusedAccessor() {}
    set #unusedAccessor(value) {}
}

No Constant Binary Expression

Disallow expressions where the operation doesn't affect the value

https://eslint.org/docs/latest/rules/no-constant-binary-expression

👍 Examples of correct code

const value1 = x == null;

const value2 = (condition ? x : {}) || DEFAULT;

const value3 = !(foo == null);

const value4 = Boolean(foo) === true;

const objIsEmpty = Object.keys(someObj).length === 0;

const arrIsEmpty = someArr.length === 0;

👎 Examples of incorrect code

const value1 = +x == null;

const value2 = condition ? x : {} || DEFAULT;

const value3 = !foo == null;

const value4 = new Boolean(foo) === true;

const objIsEmpty = someObj === {};

const arrIsEmpty = someArr === [];

No Unneeded Ternary

Disallow ternary operators when simpler alternatives exist

https://eslint.org/docs/latest/rules/no-unneeded-ternary

👍 Examples of correct code

var a = x === 2 ? "Yes" : "No";

var a = x !== false;

var a = x ? "Yes" : "No";

var a = x ? y : x;

f(x || 1);

👎 Examples of incorrect code

var a = x === 2 ? true : false;

var a = x ? true : false;

f(x ? x : 1);

No Unsafe Negation

Disallow negating the left operand of relational operators

https://eslint.org/docs/latest/rules/no-unsafe-negation

👍 Examples of correct code

if (!(key in object)) {
    // key is not in object
}

if (!(obj instanceof Ctor)) {
    // obj is not an instance of Ctor
}

👎 Examples of incorrect code

if (!key in object) {
    // operator precedence makes it equivalent to (!key) in object
    // and type conversion makes it equivalent to (key ? "false" : "true") in object
}

if (!obj instanceof Ctor) {
    // operator precedence makes it equivalent to (!obj) instanceof Ctor
    // and it equivalent to always false since boolean values are not objects.
}

Text Encoding Identifier Case

Enforce consistent case for text encoding identifiers

https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/text-encoding-identifier-case.md

👍 Examples of correct code

await fs.readFile(file, 'utf8');
await fs.readFile(file, 'ascii');
const string = buffer.toString('utf8');

👎 Examples of incorrect code

await fs.readFile(file, 'UTF-8');
await fs.readFile(file, 'ASCII');
const string = buffer.toString('utf-8');

Arguments Order

Parameters should be passed in the correct order

https://sonarsource.github.io/rspec/#/rspec/S2234/javascript

👍 Examples of correct code

function divide(divisor, dividend) {
  return divisor/dividend;
}

function doTheThing() {
  var divisor = 15;
  var dividend = 5;

  var result = divide(divisor, dividend);
  //...
}

👎 Examples of incorrect code

function divide(divisor, dividend) {
  return divisor/dividend;
}

function doTheThing() {
  var divisor = 15;
  var dividend = 5;

  var result = divide(dividend, divisor);  // Noncompliant; operation succeeds, but result is unexpected
  //...
}

Assertions In Tests

A test case without assertions ensures only that no exceptions are thrown. Beyond basic runnability, it ensures nothing about the behavior of the code under test.

https://sonarsource.github.io/rspec/#/rspec/S2699/javascript

👍 Examples of correct code

const expect = require('chai').expect;

describe("Has assertions", function() {
    it("tests a string", function() {
        const str = "";
        expect(str).to.be.a('string');
    });
});

👎 Examples of incorrect code

const expect = require('chai').expect;

describe("No assertion", function() {
    it("doesn't test anything", function() { // Noncompliant
        const str = "";
    });
});

Test Multiple Possible Assert

A test case without assertions ensures only that no exceptions are thrown. Beyond basic runnability, it ensures nothing about the behavior of the code under test.

https://sonarsource.github.io/rspec/#/rspec/S2699/javascript

👍 Examples of correct code

const expect = require('chai').expect;

describe("uncertain assertions", function() {
    const throwsTypeError = () => { throw new TypeError() }

    it("uses chai 'expect'", function() {
        expect(throwsTypeError).to.throw(TypeError)
        expect({a: 42}).to.not.have.any.keys('b', 'c');
        expect({a: 21}).to.not.have.property('b');
        expect({a: 21}).to.not.have.ownPropertyDescriptor('b');
        expect([21, 42]).to.not.include(1).and.not.include(2);

        var myObj = { value: 1 }
        const incThree = () => { myObj.value += 3; };
        const decThree = () => { myObj.value -= 3; };
        const doNothing = () => {};

        expect(incThree).to.increase(myObj, 'value').by(3);
        expect(decThree).to.decrease(myObj, 'value').by(3);

        expect(decThree).to.decrease(myObj, 'value').by(3);
        expect(incThree).to.increase(myObj, 'value').by(3);

        expect(doNothing).to.not.change(myObj, 'value');

        expect(incThree).to.increase(myObj, 'value').by(3);

        let toCheck;
        // Either of the following is valid
        expect(toCheck).to.be.a('string');
        expect(toCheck).to.be.NaN;
        expect(toCheck).to.equal(Infinity);
        expect(toCheck).to.equal(-Infinity);
    });
});

👎 Examples of incorrect code

const expect = require('chai').expect;

describe("uncertain assertions", function() {
    const throwsTypeError = () => { throw new TypeError() }

    it("uses chai 'expect'", function() {
        expect(throwsTypeError).to.not.throw(ReferenceError) // Noncompliant
        expect({a: 42}).to.not.include({b: 10, c: 20});  // Noncompliant
        expect({a: 21}).to.not.have.property('b', 42); // Noncompliant
        expect({a: 21}).to.not.have.ownPropertyDescriptor('b', {   // Noncompliant
            configurable: true,
            enumerable: true,
            writable: true,
            value: 42,
        });
        expect([21, 42]).to.not.have.members([1, 2]); // Noncompliant

        var myObj = { value: 1 }
        const incThree = () => { myObj.value += 3; };
        const decThree = () => { myObj.value -= 3; };
        const doNothing = () => {};

        expect(incThree).to.change(myObj, 'value').by(3); // Noncompliant
        expect(decThree).to.change(myObj, 'value').by(3); // Noncompliant

        expect(decThree).to.not.increase(myObj, 'value'); // Noncompliant
        expect(incThree).to.not.decrease(myObj, 'value'); // Noncompliant

        expect(doNothing).to.not.increase(myObj, 'value'); // Noncompliant
        expect(doNothing).to.not.decrease(myObj, 'value'); // Noncompliant

        expect(incThree).to.increase(myObj, 'value').but.not.by(1); // Noncompliant

        let toCheck;
        expect(toCheck).to.not.be.finite; // Noncompliant
    });
});

No Disable Timeout

Timeout should be disabled by setting it to "0".

https://sonarsource.github.io/rspec/#/rspec/S6080/javascript

👍 Examples of correct code

describe("testing this.timeout", function() {
  it("doesn't disable the timeout", function(done) {
    this.timeout(1000);
  });
});

👎 Examples of incorrect code

describe("testing this.timeout", function() {
  it("unexpectedly disables the timeout", function(done) {
    this.timeout(2147483648); // Noncompliant
  });
});

describe("testing this.timeout", function() {
  it("disables the timeout as expected", function(done) {
    this.timeout(0);
  });
});

No Empty Static Block

Empty static blocks, while not technically errors, usually occur due to refactoring that wasn’t completed. They can cause confusion when reading code.

https://eslint.org/docs/latest/rules/no-empty-static-block

👍 Examples of correct code

class Foo {
    static {
        bar();
    }
}

class Foo {
    static {
        // comment
    }
}

👎 Examples of incorrect code

class Foo {
    static {}
}

No Fallthrough

The switch statement in JavaScript is one of the more error-prone constructs of the language thanks in part to the ability to “fall through” from one case to the next. For example:

Use // falls through comments to run multiple cases

https://eslint.org/docs/latest/rules/no-fallthrough

👍 Examples of correct code

switch(foo) {
    case 1:
        doSomething();
        break;

    case 2:
        doSomething();
}

function bar(foo) {
    switch(foo) {
        case 1:
            doSomething();
            return;

        case 2:
            doSomething();
    }
}

switch(foo) {
    case 1:
        doSomething();
        throw new Error("Boo!");

    case 2:
        doSomething();
}

switch(foo) {
    case 1:
    case 2:
        doSomething();
}

switch(foo) {
    case 1:
        doSomething();
        // falls through

    case 2:
        doSomething();
}

switch(foo) {
    case 1: {
        doSomething();
        // falls through
    }

    case 2: {
        doSomethingElse();
    }
}

👎 Examples of incorrect code

switch(foo) {
    case 1:
        doSomething();

    case 2:
        doSomething();
}

No Octal

Octal literals are numerals that begin with a leading zero, such as:

https://eslint.org/docs/latest/rules/no-octal

👍 Examples of correct code

var test = 71 + 5; // 76

👎 Examples of incorrect code

var num = 071; // 57
var result = 5 + 07;

var test = 071 + 5; // 62

Octal Scape

As of the ECMAScript 5 specification, octal escape sequences in string literals are deprecated and should not be used. Unicode escape sequences should be used instead.

https://eslint.org/docs/latest/rules/no-octal-escape

👍 Examples of correct code

var foo = "Copyright \u00A9";   // unicode

var foo = "Copyright \xA9";     // hexadecimal

👎 Examples of incorrect code

var foo = "Copyright \251";

No Global Assign

JavaScript environments contain a number of built-in global variables, such as window in browsers and process in Node.js. In almost all cases, you don’t want to assign a value to these global variables as doing so could result in losing access to important functionality. For example, you probably don’t want to do this in browser code:

https://eslint.org/docs/latest/rules/no-global-assign

👍 Examples of correct code

a = 1
const b = 1
b = 2

👎 Examples of incorrect code

Object = null
undefined = 1
window = {}
length = 1
top = 1

No Case Declarations

This rule disallows lexical declarations (let, const, function and class) in case/default clauses. The reason is that the lexical declaration is visible in the entire switch block but it only gets initialized when it is assigned, which will only happen if the case where it is defined is reached.

https://eslint.org/docs/latest/rules/no-case-declarations

👍 Examples of correct code

// Declarations outside switch-statements are valid
const a = 0;

switch (foo) {
    // The following case clauses are wrapped into blocks using brackets
    case 1: {
        let x = 1;
        break;
    }
    case 2: {
        const y = 2;
        break;
    }
    case 3: {
        function f() {}
        break;
    }
    case 4:
        // Declarations using var without brackets are valid due to function-scope hoisting
        var z = 4;
        break;
    default: {
        class C {}
    }
}

👎 Examples of incorrect code

switch (foo) {
    case 1:
        let x = 1;
        break;
    case 2:
        const y = 2;
        break;
    case 3:
        function f() {}
        break;
    default:
        class C {}
}

No Confusing Void Expression

void in TypeScript refers to a function return that is meant to be ignored. Attempting to use a void-typed value, such as storing the result of a called function in a variable, is often a sign of a programmer error.

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/no-confusing-void-expression.md

👍 Examples of correct code

// just a regular void function in a statement position
alert('Hello, world!');

// this function returns a boolean value so it's ok
const response = confirm('Are you sure?');
console.log(confirm('Are you sure?'));

// now it's obvious that `postMessage` doesn't return any response
promise.then(value => {
  window.postMessage(value);
});

// now it's explicit that we want to log the error and return early
function doSomething() {
  if (!somethingToDo) {
    console.error('Nothing to do!');
    return;
  }

  console.log('Doing a thing...');
}

// using logical expressions for their side effects is fine
cond && console.log('true');
cond || console.error('false');
cond ? console.log('true') : console.error('false');

👎 Examples of incorrect code

// somebody forgot that `alert` doesn't return anything
const response = alert('Are you sure?');
console.log(alert('Are you sure?'));

// it's not obvious whether the chained promise will contain the response (fixable)
promise.then(value => window.postMessage(value));

// it looks like we are returning the result of `console.error` (fixable)
function doSomething() {
  if (!somethingToDo) {
    return console.error('Nothing to do!');
  }

  console.log('Doing a thing...');
}

No Duplicate Enum Values

Although TypeScript supports duplicate enum member values, people usually expect members to have unique values within the same enum. Duplicate values can lead to bugs that are hard to track down.

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/no-duplicate-enum-values.md

👍 Examples of correct code

enum E {
  A = 0,
  B = 1,
}
enum E {
  A = 'A',
  B = 'B',
}

👎 Examples of incorrect code

enum E {
  A = 0,
  B = 0,
}
enum E {
  A = 'A',
  B = 'A',
}

No Floating Promises

A "floating" Promise is one that is created without any code set up to handle any errors it might throw. Floating Promises can cause several issues, such as improperly sequenced operations, ignored Promise rejections, and more.

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/no-floating-promises.md

👍 Examples of correct code

const promise = new Promise((resolve, reject) => resolve('value'));
await promise;

async function returnsPromise() {
  return 'value';
}
returnsPromise()
    .then(() => {})
    .catch(() => {});

Promise.reject('value').catch(() => {});

Promise.reject('value').finally(() => {});

👎 Examples of incorrect code

const promise = new Promise((resolve, reject) => resolve('value'));
promise;

async function returnsPromise() {
  return 'value';
}

returnsPromise().then(() => {});

Promise.reject('value').catch();

Promise.reject('value').finally();

No Invalid Void Type

void in TypeScript refers to a function return that is meant to be ignored. Attempting to use a void type outside of a return type or generic type argument is often a sign of programmer error.

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/no-invalid-void-type.md

👍 Examples of correct code

type NoOp = () => void;

function noop(): void {}

let trulyUndefined = void 0;

async function promiseMeSomething(): Promise<void> {}

type stillVoid = void | never;

👎 Examples of incorrect code

type PossibleValues = string | number | void;
type MorePossibleValues = string | ((number & any) | (string | void));

function logSomething(thing: void) {}
function printArg<T = void>(arg: T) {}

logAndReturn<void>(undefined);

interface Interface {
  lambda: () => void;
  prop: void;
}

class MyClass {
  private readonly propName: void;
}

No Non Null Asserted Nullish Coalescing

The ?? nullish coalescing runtime operator allows providing a default value when dealing with null or undefined. Using a ! non-null assertion type operator in the left operand of a nullish coalescing operator is redundant, and likely a sign of programmer error or confusion over the two operators.

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/no-non-null-asserted-nullish-coalescing.md

👍 Examples of correct code

foo ?? bar;
foo ?? bar!;
foo!.bazz ?? bar;
foo!.bazz ?? bar!;
foo() ?? bar;

// This is considered correct code because there's no way for the user to satisfy it.
let x: string;
x! ?? '';

👎 Examples of incorrect code

foo! ?? bar;
foo.bazz! ?? bar;
foo!.bazz! ?? bar;
foo()! ?? bar;

let x!: string;
x! ?? '';

let x: string;
x = foo();
x! ?? '';

No Non Null Asserted Optional Chain

?. optional chain expressions provide undefined if an object is null or undefined. Using a ! non-null assertion to assert the result of an ?. optional chain expression is non-nullable is likely wrong.

Most of the time, either the object was not nullable and did not need the ?. for its property lookup, or the ! is incorrect and introducing a type safety hole.

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/no-non-null-asserted-optional-chain.md

👍 Examples of correct code

foo?.bar;
foo?.bar();

foo.bar!;

👎 Examples of incorrect code

foo?.bar!;
foo?.bar()!;

No Unsafe Declaration Merging

TypeScript's "declaration merging" supports merging separate declarations with the same name.

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/no-unsafe-declaration-merging.md

👍 Examples of correct code

interface Foo {}
class Bar implements Foo {}

namespace Baz {}
namespace Baz {}
enum Baz {}

namespace Qux {}
function Qux() {}

👎 Examples of incorrect code

interface Foo {}

class Foo {}

No Useless Empty Export

An empty export {} statement is sometimes useful in TypeScript code to turn a file that would otherwise be a script file into a module file.

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/no-useless-empty-export.md

👍 Examples of correct code

export const value = 'Hello, world!';
export {};
// or
import 'some-other-module';
export {};

👎 Examples of incorrect code

export const value = 'Hello, world!';
// or
import 'some-other-module';

Restrict Template Expressions

JavaScript will call toString() on an object when it is converted to a string, such as when + adding to a string or in ${} template literals. The default Object .toString() returns "[object Object]", which is often not what was intended. This rule reports on values used in a template literal string that aren't strings. primitives and don't define a more useful .toString() method.

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/restrict-template-expressions.md

👍 Examples of correct code

const arg = 'foo';
const msg1 = `arg = ${arg}`;
const msg2 = `arg = ${arg || 'default'}`;

const stringWithKindProp: string & { _kind?: 'MyString' } = 'foo';
const msg3 = `stringWithKindProp = ${stringWithKindProp}`;

const arg = 123;
const msg1 = `arg = ${arg}`;
const msg2 = `arg = ${arg || 'zero'}`;

const arg = true;
const msg1 = `arg = ${arg}`;
const msg2 = `arg = ${arg || 'not truthy'}`;

👎 Examples of incorrect code

const arg1 = [1, 2];
const msg1 = `arg1 = ${arg1}`;

const arg2 = { name: 'Foo' };
const msg2 = `arg2 = ${arg2 || null}`;

Return Await Try Catch

Returning an awaited promise can make sense for better stack trace information as well as for consistent error handling (returned promises will not be caught in an async function try/catch).

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/return-await.md

👍 Examples of correct code

async function validInTryCatch1() {
  try {
    return await Promise.resolve('try');
  } catch (e) {}
}

async function validInTryCatch2() {
  try {
    throw new Error('error');
  } catch (e) {
    return Promise.resolve('catch');
  }
}

async function validInTryCatch3() {
  try {
    throw new Error('error');
  } catch (e) {
    return await Promise.resolve('catch');
  } finally {
    console.log('cleanup');
  }
}

async function validInTryCatch4() {
  try {
    throw new Error('error');
  } catch (e) {
    throw new Error('error2');
  } finally {
    return Promise.resolve('finally');
  }
}

async function validInTryCatch5() {
  return Promise.resolve('try');
}

async function validInTryCatch6() {
  return 'value';
}

👎 Examples of incorrect code

async function invalidInTryCatch1() {
  try {
    return Promise.resolve('try');
  } catch (e) {}
}

async function invalidInTryCatch2() {
  try {
    throw new Error('error');
  } catch (e) {
    return await Promise.resolve('catch');
  }
}

async function invalidInTryCatch3() {
  try {
    throw new Error('error');
  } catch (e) {
    return Promise.resolve('catch');
  } finally {
    console.log('cleanup');
  }
}

async function invalidInTryCatch4() {
  try {
    throw new Error('error');
  } catch (e) {
    throw new Error('error2');
  } finally {
    return await Promise.resolve('finally');
  }
}

async function invalidInTryCatch5() {
  return await Promise.resolve('try');
}

async function invalidInTryCatch6() {
  return await 'value';
}

Switch Exhaustiveness Check

When working with union types in TypeScript, it's common to want to write a switch statement intended to contain a case for each constituent (possible type in the union). However, if the union type changes, it's easy to forget to modify the cases to account for any new types.

https://github.com/typescript-eslint/typescript-eslint/blob/main/packages/eslint-plugin/docs/rules/switch-exhaustiveness-check.md

👍 Examples of correct code

type Day =
  | 'Monday'
  | 'Tuesday'
  | 'Wednesday'
  | 'Thursday'
  | 'Friday'
  | 'Saturday'
  | 'Sunday';

const day = 'Monday' as Day;
let result = 0;

switch (day) {
  case 'Monday':
    result = 1;
    break;
  case 'Tuesday':
    result = 2;
    break;
  case 'Wednesday':
    result = 3;
    break;
  case 'Thursday':
    result = 4;
    break;
  case 'Friday':
    result = 5;
    break;
  case 'Saturday':
    result = 6;
    break;
  case 'Sunday':
    result = 7;
    break;
}

// OR default

switch (day) {
  case 'Monday':
    result = 1;
    break;
  default:
    result = 42;
}

👎 Examples of incorrect code

type Day =
  | 'Monday'
  | 'Tuesday'
  | 'Wednesday'
  | 'Thursday'
  | 'Friday'
  | 'Saturday'
  | 'Sunday';

const day = 'Monday' as Day;
let result = 0;

switch (day) {
  case 'Monday':
    result = 1;
    break;
}

Index Of Compare To Positive Number

Most checks against an indexOf call against an array compare it with -1 because 0 is a valid index. Any checks which look for values >0 ignore the first element, which is likely a bug. If you’re merely checking the presence of the element, consider using includes instead. Before using includes method make sure that your browser version is supporting it.

https://sonarsource.github.io/rspec/#/rspec/S2692/javascript

👍 Examples of correct code

var color = "blue";
var name = "ishmael";

var arr = [color, name];

if (arr.indexOf("blue") >= 0) {
  // ...
}
if (arr.includes("blue")) {
  // ...
}

👎 Examples of incorrect code

var color = "blue";
var name = "ishmael";

var arr = [color, name];

if (arr.indexOf("blue") > 0) { // Noncompliant
  // ...
}

No Invariant Returns

Function returns should not be invariant

https://sonarsource.github.io/rspec/#/rspec/S3516/javascript

👍 Examples of correct code

function foo(a) {  // Noncompliant
  let b = 12;
  if (a) {
    return b + a; // Not equals
  }
  return b;
}

👎 Examples of incorrect code

function foo(a) {  // Noncompliant
  let b = 12;
  if (a) {
    return b;
  }
  return b;
}

Inconsistent Function Call

Constructor functions, which create new object instances, must only be called with new. Non-constructor functions must not. Mixing these two usages could lead to unexpected results at runtime.

https://sonarsource.github.io/rspec/#/rspec/S3686/javascript

👍 Examples of correct code

function getNum() {
  return 5;
}

function Num(numeric, alphabetic) {
  this.numeric = numeric;
  this.alphabetic = alphabetic;
}

var myFirstNum = getNum();

var myNumObj1 = new Num(1, "A");

👎 Examples of incorrect code

function getNum() {
  return 5;
}

function Num(numeric, alphabetic) {
  this.numeric = numeric;
  this.alphabetic = alphabetic;
}

var my2ndNum = new getNum();  // Noncompliant. An empty object is returned, NOT 5

var myNumObj2 = Num();  // Noncompliant. undefined is returned, NOT an object

Duplicate Conditions

Having all branches in a switch or if chain with the same implementation is an error. Either a copy-paste error was made and something different should be executed

https://github.com/SonarSource/eslint-plugin-sonarjs/blob/HEAD/docs/rules/no-all-duplicated-branches.md

👍 Examples of correct code

if (b == 0) { // Noncompliant
  doOneMoreThing();
} else {
  doOneMoreThing();
}

let a = b === 0 ? getValue() : getDefaultValue(); // Noncompliant

switch (i) { // Noncompliant
  case 1:
    doSomethingOne();
    break;
  case 2:
    doSomethingTwo();
    break;
  case 3:
  default:
    doSomething();
}

👎 Examples of incorrect code

if (b == 0) { // Noncompliant
  doOneMoreThing();
} else {
  doSomethingElse();
}

let a = b === 0 ? getValue() : getValue(); // Noncompliant

switch (i) { // Noncompliant
  case 1:
    doSomething();
    break;
  case 2:
    doSomething();
    break;
  case 3:
    doSomething();
    break;
  default:
    doSomething();
}

Redundant Type Aliases

Redundant type aliases should not be used

https://sonarsource.github.io/rspec/#/rspec/S6564/javascript

👍 Examples of correct code

function isPalindrom(s: string): boolean {
    return s === s.split('').reverse().join('');
}

👎 Examples of incorrect code

type MyString  = string;
type MyBoolean = boolean;

function isPalindrom(s: MyString): MyBoolean {
    return s === s.split('').reverse().join('');
}

No Element Overwrite

It is highly suspicious when a value is saved for a key or index and then unconditionally overwritten. Such replacements are likely in error.

https://github.com/SonarSource/eslint-plugin-sonarjs/blob/HEAD/docs/rules/no-element-overwrite.md

👍 Examples of correct code

fruits[1] = "banana";

myMap.set("key", 1);

mySet.add(1);

👎 Examples of incorrect code

fruits[1] = "banana";
fruits[1] = "apple";  // Noncompliant - value on index 1 is overwritten

myMap.set("key", 1);
myMap.set("key", 2); // Noncompliant - value for key "key" is replaced

mySet.add(1);
mySet.add(1); // Noncompliant - element is already in the set

No Empty Collection

When a collection is empty it makes no sense to access or iterate it. Doing so anyway is surely an error; either population was accidentally omitted or the developer doesn’t understand the situation.

https://github.com/SonarSource/eslint-plugin-sonarjs/blob/HEAD/docs/rules/no-empty-collection.md

👍 Examples of correct code

let strings = [ ...anotherArray ];

if (strings.includes("foo")) {}

for (str of strings) {}

strings.forEach(str => doSomething(str));

👎 Examples of incorrect code

let strings = [];

if (strings.includes("foo")) {}  // Noncompliant

for (str of strings) {}  // Noncompliant

strings.forEach(str => doSomething(str)); // Noncompliant

No Extra Arguments

You can easily call a JavaScript function with more arguments than the function needs, but the extra arguments will be just ignored by function execution.

https://github.com/SonarSource/eslint-plugin-sonarjs/blob/HEAD/docs/rules/no-extra-arguments.md

👍 Examples of correct code

function doSomething(a, b) {
  compute(arguments);
}

doSomething(1, 2, 3);

// Or

function say(a, b) {
  print(a + " " + b);
}

say("hello", "world");

👎 Examples of incorrect code

function say(a, b) {
  print(a + " " + b);
}

say("hello", "world", "!"); // Noncompliant; last argument is not used

No Identical Expressions

Using the same value on either side of a binary operator is almost always a mistake. In the case of logical operators, it is either a copy/paste error and therefore a bug, or it is simply wasted code, and should be simplified. In the case of bitwise operators and most binary mathematical operators, having the same value on both sides of an operator yields predictable results, and should be simplified.

https://github.com/SonarSource/eslint-plugin-sonarjs/blob/HEAD/docs/rules/no-identical-expressions.md

👍 Examples of correct code

if (a == b) {
  doX();
}
if (a > b) {
  doW();
}

var j = 1; //always 1
var k = 0; //always 0

👎 Examples of incorrect code

if (a == b && a == b) { // if the first one is true, the second one is too
  doX();
}
if (a > a) { // always false
  doW();
}

var j = 5 / 5; //always 1
var k = 5 - 5; //always 0

No Ignored Return

When the call to a function doesn’t have any side effects, what is the point of making the call if the results are ignored? In such case, either the function call is useless and should be dropped or the source code doesn’t behave as expected.

https://github.com/SonarSource/eslint-plugin-sonarjs/blob/HEAD/docs/rules/no-ignored-return.md

👍 Examples of correct code

let char = 'hello'.lastIndexOf('e');

👎 Examples of incorrect code

'hello'.lastIndexOf('e'); // Noncompliant

No Use Of Empty Return Value

If a function does not return anything, it makes no sense to use its output. Specifically, passing it to another function, or assigning its "result" to a variable is probably a bug because such functions return undefined, which is probably not what was intended.

https://github.com/SonarSource/eslint-plugin-sonarjs/blob/HEAD/docs/rules/no-use-of-empty-return-value.md

👍 Examples of correct code

function foo() {
  console.log("Hello, World!");
}

foo();

👎 Examples of incorrect code

function foo() {
  console.log("Hello, World!");
}

a = foo();

No Collection Size Mischeck

The size of a collection and the length of an array are always greater than or equal to zero. So testing that a size or length is greater than or equal to zero doesn't make sense, since the result is always true. Similarly testing that it is less than zero will always return false. Perhaps the intent was to check the non-emptiness of the collection or array instead.

https://github.com/SonarSource/eslint-plugin-sonarjs/blob/HEAD/docs/rules/no-collection-size-mischeck.md

👍 Examples of correct code

if (someSet.size > 0) {...}

if (someMap.size == 0) {...}

const result = someArray.length > 0;

👎 Examples of incorrect code

if (someSet.size >= 0) {...} // Noncompliant

if (someMap.size < 0) {...} // Noncompliant

const result = someArray.length >= 0;  // Noncompliant

No Gratuitous Expressions

If a boolean expression doesn’t change the evaluation of the condition, then it is entirely unnecessary, and can be removed. If it is gratuitous because it does not match the programmer’s intent, then it’s a bug and the expression should be fixed.

https://github.com/SonarSource/eslint-plugin-sonarjs/blob/HEAD/docs/rules/no-gratuitous-expressions.md

👍 Examples of correct code

if (a) {
  if (b) {
    doSomething();
  }
}

// or
if (a) {
  doSomething();
}

👎 Examples of incorrect code

if (a) {
  if (a) { // Noncompliant
    doSomething();
  }
}

// Or

if (a) {
  console.log("anything");

  if (a) { // Noncompliant
    doSomething();
  }
}

No Unused Collection

When a collection is populated but its contents are never used, then it is surely some kind of mistake. Either refactoring has rendered the collection moot, or an access is missing.

https://github.com/SonarSource/eslint-plugin-sonarjs/blob/HEAD/docs/rules/no-unused-collection.md

👍 Examples of correct code

function getLength(a, b, c) {
  return a.length + b.length + c.length;
}

👎 Examples of incorrect code

function getLength(a, b, c) {
  const strings = [];  // Noncompliant
  strings.push(a);
  strings.push(b);
  strings.push(c);

  return a.length + b.length + c.length;
}

No Inconsistent Docblock

Check a valid typescript and docblock

https://github.com/ODGodinho/ODGEslintPlugin#no-inconsistent-docblock

👍 Examples of correct code

/**
 * Valid Param
 *
 * @param {string} param
 */
function name(param: string) {

}

/**
 * Valid return
 *
 * @returns {string}
 */
function name2(): string {

}

👎 Examples of incorrect code

/**
 * Valid Param
 *
 * @param {number} param
 */
function name(param: string) {

}

/**
 * Valid return
 *
 * @returns {number}
 */
function name2(): string {

}

Yaml Json

add suport yaml and json files

ODGodinho/ODG-Linter-Js

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