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Node.js, JavaScript Style Guide

A mostly reasonable approach to JavaScript

Table of Contents

  1. Standard Library
  2. References
  3. Objects
  4. Arrays
  5. Destructuring
  6. Strings
  7. Functions
  8. Arrow Functions
  9. Classes & Constructors
  10. Properties
  11. Variables
  12. Comparison Operators & Equality
  13. Blocks
  14. Control Statements
  15. Comments
  16. Whitespace
  17. Commas
  18. Semicolons
  19. Type Casting & Coercion
  20. Naming Conventions
  21. Accessors
  22. Events
  23. Testing
  24. Performance
  25. License
  26. Amendments

Standard Library

The Standard Library contains utilities that are functionally broken but remain for legacy reasons.

  • 1.1 Use Number.isNaN instead of global isNaN. eslint: no-restricted-globals

    Why? The global isNaN coerces non-numbers to numbers, returning true for anything that coerces to NaN. If this behavior is desired, make it explicit.

    // bad
    isNaN('1.2'); // false
    isNaN('1.2.3'); // true
    
    // good
    Number.isNaN('1.2.3'); // false
    Number.isNaN(Number('1.2.3')); // true

  • 1.2 Use Number.isFinite instead of global isFinite. eslint: no-restricted-globals

    Why? The global isFinite coerces non-numbers to numbers, returning true for anything that coerces to a finite number. If this behavior is desired, make it explicit.

    // bad
    isFinite('2e3'); // true
    
    // good
    Number.isFinite('2e3'); // false
    Number.isFinite(parseInt('2e3', 10)); // true

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References

  • 2.1 Use const for all of your references; avoid using var. eslint: prefer-const)

    Why? This ensures that you can’t reassign your references, which can lead to bugs and difficult to comprehend code.

    // bad
    var a = 1;
    var b = 2;
    
    // good
    const a = 1;
    const b = 2;

  • 2.2 If you must reassign references, use let instead of var)

    Why? let is block-scoped rather than function-scoped like var.

    // bad
    var count = 1;
    if (true) {
      count += 1;
    }
    
    // good, use the let.
    let count = 1;
    if (true) {
      count += 1;
    }

  • 2.3 Note that both let and const are block-scoped.

    // const and let only exist in the blocks they are defined in.
    {
      let a = 1;
      const b = 1;
    }
    console.log(a); // ReferenceError
    console.log(b); // ReferenceError

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Objects

  • 3.1 Use object method shorthand. eslint: object-shorthand

    // bad
    const atom = {
      value: 1,
    
      addValue: function (value) {
        return atom.value + value;
      },
    };
    
    // good
    const atom = {
      value: 1,
    
      addValue(value) {
        return atom.value + value;
      },
    };

  • 3.2 Use property value shorthand. eslint: object-shorthand

    Why? It is shorter and descriptive.

    const lukeSkywalker = 'Luke Skywalker';
    
    // bad
    const obj = {
      lukeSkywalker: lukeSkywalker,
    };
    
    // good
    const obj = {
      lukeSkywalker,
    };

  • 3.3 Group your shorthand properties at the beginning of your object declaration.

    Why? It’s easier to tell which properties are using the shorthand.

    const anakinSkywalker = 'Anakin Skywalker';
    const lukeSkywalker = 'Luke Skywalker';
    
    // bad
    const obj = {
      episodeOne: 1,
      twoJediWalkIntoACantina: 2,
      lukeSkywalker,
      episodeThree: 3,
      mayTheFourth: 4,
      anakinSkywalker,
    };
    
    // good
    const obj = {
      lukeSkywalker,
      anakinSkywalker,
      episodeOne: 1,
      twoJediWalkIntoACantina: 2,
      episodeThree: 3,
      mayTheFourth: 4,
    };

  • 3.4 Only quote properties that are invalid identifiers. eslint: quote-props

    Why? In general we consider it subjectively easier to read. It improves syntax highlighting, and is also more easily optimized by many JS engines.

    // bad
    const bad = {
      'foo': 3,
      'bar': 4,
      'data-blah': 5,
    };
    
    // good
    const good = {
      foo: 3,
      bar: 4,
      'data-blah': 5,
    };

  • 3.5 Prefer the object spread operator over Object.assign to shallow-copy objects. Use the object rest operator to get a new object with certain properties omitted.

    // very bad
    const original = { a: 1, b: 2 };
    const copy = Object.assign(original, { c: 3 }); // this mutates `original` ಠ_ಠ
    delete copy.a; // so does this
    
    // bad
    const original = { a: 1, b: 2 };
    const copy = Object.assign({}, original, { c: 3 }); // copy => { a: 1, b: 2, c: 3 }
    
    // good
    const original = { a: 1, b: 2 };
    const copy = { ...original, c: 3 }; // copy => { a: 1, b: 2, c: 3 }
    
    const { a, ...noA } = copy; // noA => { b: 2, c: 3 }

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Arrays

  • 4.1 Use array spreads ... to copy arrays.

    // bad
    const len = items.length;
    const itemsCopy = [];
    let i;
    
    for (i = 0; i < len; i += 1) {
      itemsCopy[i] = items[i];
    }
    
    // good
    const itemsCopy = [...items];

  • 4.2 Use Array.from for converting an array-like object to an array.

    const arrLike = { 0: 'foo', 1: 'bar', 2: 'baz', length: 3 };
    
    // bad
    const arr = Array.prototype.slice.call(arrLike);
    
    // good
    const arr = Array.from(arrLike);

  • 4.3 Use return statements in array method callbacks. It’s ok to omit the return if the function body consists of a single statement returning an expression without side effects, following 8.2. eslint: array-callback-return

    // good
    [1, 2, 3].map((x) => {
      const y = x + 1;
      return x * y;
    });
    
    // good
    [1, 2, 3].map((x) => x + 1);
    
    // bad - no returned value means `acc` becomes undefined after the first iteration
    [[0, 1], [2, 3], [4, 5]].reduce((acc, item, index) => {
      const flatten = acc.concat(item);
    });
    
    // good
    [[0, 1], [2, 3], [4, 5]].reduce((acc, item, index) => {
      const flatten = acc.concat(item);
      return flatten;
    });
    
    // bad
    inbox.filter((msg) => {
      const { subject, author } = msg;
      if (subject === 'Mockingbird') {
        return author === 'Harper Lee';
      } else {
        return false;
      }
    });
    
    // good
    inbox.filter((msg) => {
      const { subject, author } = msg;
      if (subject === 'Mockingbird') {
        return author === 'Harper Lee';
      }
    
      return false;
    });

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Destructuring

  • 5.1 Use object destructuring when accessing and using multiple properties of an object. eslint: prefer-destructuring

    Why? Destructuring saves you from creating temporary references for those properties.

    // bad
    function getFullName(user) {
      const firstName = user.firstName;
      const lastName = user.lastName;
    
      return `${firstName} ${lastName}`;
    }
    
    // good
    function getFullName(user) {
      const { firstName, lastName } = user;
      return `${firstName} ${lastName}`;
    }
    
    // best
    function getFullName({ firstName, lastName }) {
      return `${firstName} ${lastName}`;
    }

  • 5.2 Use array destructuring. eslint: prefer-destructuring

    const arr = [1, 2, 3, 4];
    
    // bad
    const first = arr[0];
    const second = arr[1];
    
    // good
    const [first, second] = arr;

  • 5.3 Use object destructuring for multiple return values, not array destructuring.

    Why? You can add new properties over time or change the order of things without breaking call sites.

    // bad
    function processInput(input) {
      // then a miracle occurs
      return [left, right, top, bottom];
    }
    
    // the caller needs to think about the order of return data
    const [left, __, top] = processInput(input);
    
    // good
    function processInput(input) {
      // then a miracle occurs
      return { left, right, top, bottom };
    }
    
    // the caller selects only the data they need
    const { left, top } = processInput(input);

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Strings

  • 6.1 Use single quotes '' for strings. eslint: quotes

    // bad
    const name = "Capt. Janeway";
    
    // bad - template literals should contain interpolation or newlines
    const name = `Capt. Janeway`;
    
    // good
    const name = 'Capt. Janeway';

  • 6.2 When programmatically building up strings, use template strings instead of concatenation. eslint: prefer-template template-curly-spacing

    Why? Template strings give you a readable, concise syntax with proper newlines and string interpolation features.

    // bad
    function sayHi(name) {
      return 'How are you, ' + name + '?';
    }
    
    // bad
    function sayHi(name) {
      return ['How are you, ', name, '?'].join();
    }
    
    // bad
    function sayHi(name) {
      return `How are you, ${ name }?`;
    }
    
    // good
    function sayHi(name) {
      return `How are you, ${name}?`;
    }

  • 6.3 Do not unnecessarily escape characters in strings. eslint: no-useless-escape

    Why? Backslashes harm readability, thus they should only be present when necessary.

    // bad
    const foo = '\'this\' \i\s \"quoted\"';
    
    // good
    const foo = '\'this\' is "quoted"';
    const foo = `my name is '${name}'`;

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Functions

  • 7.1 Use named function expressions instead of function declarations. eslint: func-style

    Why? Function declarations are hoisted, which means that it’s easy - too easy - to reference the function before it is defined in the file. This harms readability and maintainability. If you find that a function’s definition is large or complex enough that it is interfering with understanding the rest of the file, then perhaps it’s time to extract it to its own module!

    // bad
    function foo() {
      // ...
    }
    
    // good
    const foo = function () {
      // ...
    };
    
    // good
    const foo = () => {
      // ...
    };

  • 7.2 Never declare a function in a non-function block (if, while, etc). Assign the function to a variable instead. Browsers will allow you to do it, but they all interpret it differently, which is bad news bears. eslint: no-loop-func

  • 7.3 Note: ECMA-262 defines a block as a list of statements. A function declaration is not a statement.

    // bad
    if (currentUser) {
      function test() {
        console.log('Nope.');
      }
    }
    
    // good
    let test;
    if (currentUser) {
      test = () => {
        console.log('Yup.');
      };
    }

  • 7.4 Never use arguments, opt to use rest syntax ... instead. eslint: prefer-rest-params

    Why? ... is explicit about which arguments you want pulled. Plus, rest arguments are a real Array, and not merely Array-like like arguments.

    // bad
    function concatenateAll() {
      const args = Array.prototype.slice.call(arguments);
      return args.join('');
    }
    
    // good
    function concatenateAll(...args) {
      return args.join('');
    }

  • 7.5 Use default parameter syntax rather than mutating function arguments.

    // really bad
    function handleThings(opts) {
      // No! We shouldn’t mutate function arguments.
      // Double bad: if opts is falsy it'll be set to an object which may
      // be what you want but it can introduce subtle bugs.
      opts = opts || {};
      // ...
    }
    
    // still bad
    function handleThings(opts) {
      if (opts === void 0) {
        opts = {};
      }
      // ...
    }
    
    // good
    function handleThings(opts = {}) {
      // ...
    }

  • 7.6 Always put default parameters last.

    // bad
    function handleThings(opts = {}, name) {
      // ...
    }
    
    // good
    function handleThings(name, opts = {}) {
      // ...
    }

  • 7.7 Spacing in a function signature. eslint: space-before-function-paren space-before-blocks

    Why? Consistency is good, and you shouldn’t have to add or remove a space when adding or removing a name.

    // bad
    const f = function(){};
    const g = function (){};
    const h = function() {};
    
    // good
    const x = function () {};
    const y = function a() {};

  • 7.8 Never mutate parameters. eslint: no-param-reassign

    Why? Manipulating objects passed in as parameters can cause unwanted variable side effects in the original caller.

    // bad
    function f1(obj) {
      obj.key = 1;
    }
    
    // good
    function f2(obj) {
      const key = Object.prototype.hasOwnProperty.call(obj, 'key') ? obj.key : 1;
    }

  • 7.9 Never reassign parameters. eslint: no-param-reassign

    Why? Reassigning parameters can lead to unexpected behavior, especially when accessing the arguments object. It can also cause optimization issues, especially in V8.

    // bad
    function f1(a) {
      a = 1;
      // ...
    }
    
    function f2(a) {
      if (!a) { a = 1; }
      // ...
    }
    
    // good
    function f3(a) {
      const b = a || 1;
      // ...
    }
    
    function f4(a = 1) {
      // ...
    }

  • 7.10 Functions with multiline signatures, or invocations, should be indented just like every other multiline list in this guide: with each item on a line by itself, with a trailing comma on the last item. eslint: function-paren-newline

    // bad
    function foo(bar,
                 baz,
                 quux) {
      // ...
    }
    
    // good
    function foo(
      bar,
      baz,
      quux,
    ) {
      // ...
    }
    
    // bad
    console.log(foo,
      bar,
      baz);
    
    // good
    console.log(
      foo,
      bar,
      baz,
    );

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Arrow Functions

  • 8.1 When you must use an anonymous function (as when passing an inline callback), use arrow function notation. eslint: prefer-arrow-callback, arrow-spacing

    Why? It creates a version of the function that executes in the context of this, which is usually what you want, and is a more concise syntax.

    Why not? If you have a fairly complicated function, you might move that logic out into its own named function expression.

    // bad
    [1, 2, 3].map(function (x) {
      const y = x + 1;
      return x * y;
    });
    
    // good
    [1, 2, 3].map((x) => {
      const y = x + 1;
      return x * y;
    });

  • 8.2 If the function body consists of a single statement returning an expression without side effects, omit the braces and use the implicit return. Otherwise, keep the braces and use a return statement. eslint: arrow-parens, arrow-body-style

    Why? Syntactic sugar. It reads well when multiple functions are chained together.

    // bad
    [1, 2, 3].map((number) => {
      const nextNumber = number + 1;
      `A string containing the ${nextNumber}.`;
    });
    
    // good
    [1, 2, 3].map((number) => `A string containing the ${number + 1}.`);
    
    // good
    [1, 2, 3].map((number) => {
      const nextNumber = number + 1;
      return `A string containing the ${nextNumber}.`;
    });
    
    // good
    [1, 2, 3].map((number, index) => ({
      [index]: number,
    }));
    
    // No implicit return with side effects
    function foo(callback) {
      const val = callback();
      if (val === true) {
        // Do something if callback returns true
      }
    }
    
    let bool = false;
    
    // bad
    foo(() => bool = true);
    
    // good
    foo(() => {
      bool = true;
    });

  • 8.3 Include parentheses around arguments when it's necessary. eslint: arrow-parens

    Why? Syntactic sugar.

    // bad
    [1, 2, 3].map((x) => x * x);
    
    // good
    [1, 2, 3].map(x => x * x);

  • 8.4 Enforce the location of arrow function bodies with implicit returns. eslint: implicit-arrow-linebreak

    // bad
    (foo) =>
      bar;
    
    (foo) =>
      (bar);
    
    // good
    (foo) => bar;
    (foo) => (bar);
    (foo) => (
       bar
    )

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Classes & Constructors

  • 9.1 Always use class. Avoid manipulating prototype directly.

    Why? class syntax is more concise and easier to reason about.

    // bad
    function Queue(contents = []) {
      this.queue = [...contents];
    }
    Queue.prototype.pop = function () {
      const value = this.queue[0];
      this.queue.splice(0, 1);
      return value;
    };
    
    // good
    class Queue {
      constructor(contents = []) {
        this.queue = [...contents];
      }
      pop() {
        const value = this.queue[0];
        this.queue.splice(0, 1);
        return value;
      }
    }

  • 9.2 Use extends for inheritance.

    Why? It is a built-in way to inherit prototype functionality without breaking instanceof.

    // bad
    const inherits = require('inherits');
    function PeekableQueue(contents) {
      Queue.apply(this, contents);
    }
    inherits(PeekableQueue, Queue);
    PeekableQueue.prototype.peek = function () {
      return this.queue[0];
    };
    
    // good
    class PeekableQueue extends Queue {
      peek() {
        return this.queue[0];
      }
    }

  • 9.3 Avoid duplicate class members. eslint: no-dupe-class-members

    Why? Duplicate class member declarations will silently prefer the last one - having duplicates is almost certainly a bug.

    // bad
    class Foo {
      bar() { return 1; }
      bar() { return 2; }
    }
    
    // good
    class Foo {
      bar() { return 1; }
    }

  • 9.4 Class methods should use this or be made into a static method unless an external library or framework requires to use specific non-static methods. Being an instance method should indicate that it behaves differently based on properties of the receiver. eslint: class-methods-use-this

    // bad
    class Foo {
      bar() {
        console.log('bar');
      }
    }
    
    // good - this is used
    class Foo {
      bar() {
        console.log(this.bar);
      }
    }
    
    // good - constructor is exempt
    class Foo {
      constructor() {
        // ...
      }
    }
    
    // good - static methods aren't expected to use this
    class Foo {
      static bar() {
        console.log('bar');
      }
    }

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Properties

  • 10.1 Use dot notation when accessing properties. eslint: dot-notation

    const luke = {
      jedi: true,
      age: 28,
    };
    
    // bad
    const isJedi = luke['jedi'];
    
    // good
    const isJedi = luke.jedi;

  • 10.2 Use bracket notation [] when accessing properties with a variable.

    const luke = {
      jedi: true,
      age: 28,
    };
    
    function getProp(prop) {
      return luke[prop];
    }
    
    const isJedi = getProp('jedi');

  • 10.3 Use exponentiation operator ** when calculating exponentiations. eslint: no-restricted-properties.

    // bad
    const binary = Math.pow(2, 10);
    
    // good
    const binary = 2 ** 10;

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Variables

  • 11.1 Always use const or let to declare variables. Not doing so will result in global variables. We want to avoid polluting the global namespace. Captain Planet warned us of that. eslint: no-undef prefer-const

    Tip: Don't use var. var declarations get hoisted to the top of their closest enclosing function scope. Function declarations hoist their name and the function body.

    // bad
    superPower = new SuperPower();
    
    // good
    const superPower = new SuperPower();

  • 11.2 Use one const or let declaration per variable or assignment. eslint: one-var

    Why? It’s easier to add new variable declarations this way, and you never have to worry about swapping out a ; for a , or introducing punctuation-only diffs. You can also step through each declaration with the debugger, instead of jumping through all of them at once.

    // bad
    const items = getItems(),
        goSportsTeam = true,
        dragonball = 'z';
    
    // bad
    // (compare to above, and try to spot the mistake)
    const items = getItems(),
        goSportsTeam = true;
        dragonball = 'z';
    
    // good
    const items = getItems();
    const goSportsTeam = true;
    const dragonball = 'z';

  • 11.3 Group all your consts and then group all your lets.

    Why? This is helpful when later on you might need to assign a variable depending on one of the previous assigned variables.

    // bad
    let i, len, dragonball,
        items = getItems(),
        goSportsTeam = true;
    
    // bad
    let i;
    const items = getItems();
    let dragonball;
    const goSportsTeam = true;
    let len;
    
    // good
    const goSportsTeam = true;
    const items = getItems();
    let dragonball;
    let i;
    let length;

  • 11.4 Assign variables where you need them, but place them in a reasonable place.

    Why? let and const are block scoped and not function scoped.

    // bad - unnecessary function call
    function checkName(hasName) {
      const name = getName();
    
      if (hasName === 'test') {
        return false;
      }
    
      if (name === 'test') {
        this.setName('');
        return false;
      }
    
      return name;
    }
    
    // good
    function checkName(hasName) {
      if (hasName === 'test') {
        return false;
      }
    
      const name = getName();
    
      if (name === 'test') {
        this.setName('');
        return false;
      }
    
      return name;
    }

  • 11.5 Don’t chain variable assignments. eslint: no-multi-assign

    Why? Chaining variable assignments creates implicit global variables.

    // bad
    (function example() {
      // JavaScript interprets this as
      // let a = ( b = ( c = 1 ) );
      // The let keyword only applies to variable a; variables b and c become
      // global variables.
      let a = b = c = 1;
    }());
    
    console.log(a); // throws ReferenceError
    console.log(b); // 1
    console.log(c); // 1
    
    // good
    (function example() {
      let a = 1;
      let b = a;
      let c = a;
    }());
    
    console.log(a); // throws ReferenceError
    console.log(b); // throws ReferenceError
    console.log(c); // throws ReferenceError
    
    // the same applies for `const`

  • 11.6 Avoid using unary increments and decrements (++, --). eslint no-plusplus

    Why? Per the eslint documentation, unary increment and decrement statements are subject to automatic semicolon insertion and can cause silent errors with incrementing or decrementing values within an application. It is also more expressive to mutate your values with statements like num += 1 instead of num++ or num ++. Disallowing unary increment and decrement statements also prevents you from pre-incrementing/pre-decrementing values unintentionally which can also cause unexpected behavior in your programs.

    // bad
    
    const array = [1, 2, 3];
    let num = 1;
    num++;
    --num;
    
    let sum = 0;
    let truthyCount = 0;
    for (let i = 0; i < array.length; i++) {
      let value = array[i];
      sum += value;
      if (value) {
        truthyCount++;
      }
    }
    
    // good
    
    const array = [1, 2, 3];
    let num = 1;
    num += 1;
    num -= 1;
    
    const sum = array.reduce((a, b) => a + b, 0);
    const truthyCount = array.filter(Boolean).length;

  • 11.7 Avoid linebreaks before or after = in an assignment. If your assignment violates max-len, surround the value in parens. eslint operator-linebreak.

    Why? Linebreaks surrounding = can obfuscate the value of an assignment.

    // bad
    const foo =
      superLongLongLongLongLongLongLongLongFunctionName();
    
    // bad
    const foo
      = 'superLongLongLongLongLongLongLongLongString';
    
    // good
    const foo = (
      superLongLongLongLongLongLongLongLongFunctionName()
    );
    
    // good
    const foo = 'superLongLongLongLongLongLongLongLongString';

  • 11.8 Disallow unused variables. eslint: no-unused-vars

    Why? 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.

    // bad
    
    var some_unused_var = 42;
    
    // 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;
    
    // Unused function arguments.
    function getX(x, y) {
        return x;
    }
    
    // good
    
    function getXPlusY(x, y) {
      return x + y;
    }
    
    var x = 1;
    var y = a + 2;
    
    alert(getXPlusY(x, y));
    
    // 'type' is ignored even if unused because it has a rest property sibling.
    // This is a form of extracting an object that omits the specified keys.
    var { type, ...coords } = data;
    // 'coords' is now the 'data' object without its 'type' property.

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Comparison Operators & Equality

  • 12.1 Use === and !== over == and !=. eslint: eqeqeq

  • 12.2 Use shortcuts for booleans, but explicit comparisons for strings and numbers.

    // bad
    if (isValid === true) {
      // ...
    }
    
    // good
    if (isValid) {
      // ...
    }
    
    // bad
    if (name) {
      // ...
    }
    
    // good
    if (name !== '') {
      // ...
    }
    
    // bad
    if (collection.length) {
      // ...
    }
    
    // good
    if (collection.length > 0) {
      // ...
    }

  • 12.3 Ternaries should not be nested and generally be single line expressions. eslint: no-nested-ternary

    // bad
    const foo = maybe1 > maybe2
      ? "bar"
      : value1 > value2 ? "baz" : null;
    
    // split into 2 separated ternary expressions
    const maybeNull = value1 > value2 ? 'baz' : null;
    
    // better
    const foo = maybe1 > maybe2
      ? 'bar'
      : maybeNull;
    
    // best
    const foo = maybe1 > maybe2 ? 'bar' : maybeNull;

  • 12.4 Avoid unneeded ternary statements. eslint: no-unneeded-ternary

    // bad
    const foo = a ? a : b;
    const bar = c ? true : false;
    const baz = c ? false : true;
    
    // good
    const foo = a || b;
    const bar = !!c;
    const baz = !c;

  • 12.5 When mixing operators, enclose them in parentheses. The only exception is the standard arithmetic operators: +, -, and ** since their precedence is broadly understood. We recommend enclosing / and * in parentheses because their precedence can be ambiguous when they are mixed. eslint: no-mixed-operators

    Why? This improves readability and clarifies the developer’s intention.

    // bad
    const foo = a && b < 0 || c > 0 || d + 1 === 0;
    
    // bad
    const bar = a ** b - 5 % d;
    
    // bad
    // one may be confused into thinking (a || b) && c
    if (a || b && c) {
      return d;
    }
    
    // bad
    const bar = a + b / c * d;
    
    // good
    const foo = (a && b < 0) || c > 0 || (d + 1 === 0);
    
    // good
    const bar = a ** b - (5 % d);
    
    // good
    if (a || (b && c)) {
      return d;
    }
    
    // good
    const bar = a + (b / c) * d;

⬆ back to top

Blocks

  • 13.1 If you’re using multiline blocks with if and else, put else on the same line as your if block’s closing brace. eslint: brace-style

    // bad
    if (test) {
      thing1();
      thing2();
    }
    else {
      thing3();
    }
    
    // good
    if (test) {
      thing1();
      thing2();
    } else {
      thing3();
    }

  • 13.2 If an if block always executes a return statement, the subsequent else block is unnecessary. A return in an else if block following an if block that contains a return can be separated into multiple if blocks. eslint: no-else-return

    // bad
    function foo() {
      if (x) {
        return x;
      } else {
        return y;
      }
    }
    
    // bad
    function cats() {
      if (x) {
        return x;
      } else if (y) {
        return y;
      }
    }
    
    // bad
    function dogs() {
      if (x) {
        return x;
      } else {
        if (y) {
          return y;
        }
      }
    }
    
    // good
    function foo() {
      if (x) {
        return x;
      }
    
      return y;
    }
    
    // good
    function cats() {
      if (x) {
        return x;
      }
    
      if (y) {
        return y;
      }
    }
    
    // good
    function dogs(x) {
      if (x) {
        if (z) {
          return y;
        }
      } else {
        return z;
      }
    }

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Control Statements

  • 14.1 In case your control statement (if, while etc.) gets too long or exceeds the maximum line length, each (grouped) condition could be put into a new line. The logical operator should begin the line.

    Why? Requiring operators at the beginning of the line keeps the operators aligned and follows a pattern similar to method chaining. This also improves readability by making it easier to visually follow complex logic.

    // bad
    if ((foo === 123 || bar === 'abc') && doesItLookGoodWhenItBecomesThatLong() && isThisReallyHappening()) {
      thing1();
    }
    
    // bad
    if (foo === 123 &&
      bar === 'abc') {
      thing1();
    }
    
    // bad
    if (foo === 123
      && bar === 'abc') {
      thing1();
    }
    
    // bad
    if (
      foo === 123 &&
      bar === 'abc'
    ) {
      thing1();
    }
    
    // good
    if (
      foo === 123
      && bar === 'abc'
    ) {
      thing1();
    }
    
    // good
    if (
      (foo === 123 || bar === 'abc')
      && doesItLookGoodWhenItBecomesThatLong()
      && isThisReallyHappening()
    ) {
      thing1();
    }
    
    // good
    if (foo === 123 && bar === 'abc') {
      thing1();
    }

  • 14.2 Don't use selection operators in place of control statements.

    // bad
    !isRunning && startRunning();
    
    // good
    if (!isRunning) {
      startRunning();
    }

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Comments

  • 15.1 Use /** ... */ for multiline comments.

    // bad
    // make() returns a new element
    // based on the passed in tag name
    //
    // @param {String} tag
    // @return {Element} element
    function make(tag) {
    
      // ...
    
      return element;
    }
    
    // good
    /**
     * make() returns a new element
     * based on the passed-in tag name
     */
    function make(tag) {
    
      // ...
    
      return element;
    }

  • 15.2 Use // for single line comments. Place single line comments on a newline above the subject of the comment. Put an empty line before the comment unless it’s on the first line of a block.

    // bad
    const active = true;  // is current tab
    
    // good
    // is current tab
    const active = true;
    
    // bad
    function getType() {
      console.log('fetching type...');
      // set the default type to 'no type'
      const type = this.type || 'no type';
    
      return type;
    }
    
    // good
    function getType() {
      console.log('fetching type...');
    
      // set the default type to 'no type'
      const type = this.type || 'no type';
    
      return type;
    }
    
    // also good
    function getType() {
      // set the default type to 'no type'
      const type = this.type || 'no type';
    
      return type;
    }

  • 15.3 Start all comments with a space to make it easier to read. eslint: spaced-comment

    // bad
    //is current tab
    const active = true;
    
    // good
    // is current tab
    const active = true;
    
    // bad
    /**
     *make() returns a new element
     *based on the passed-in tag name
     */
    function make(tag) {
    
      // ...
    
      return element;
    }
    
    // good
    /**
     * make() returns a new element
     * based on the passed-in tag name
     */
    function make(tag) {
    
      // ...
    
      return element;
    }

  • 15.4 Prefixing your comments with FIXME or TODO helps other developers quickly understand if you’re pointing out a problem that needs to be revisited, or if you’re suggesting a solution to the problem that needs to be implemented. These are different than regular comments because they are actionable. The actions are FIXME: -- need to figure this out or TODO: -- need to implement.

  • 15.5 Use // FIXME: to annotate problems.

    class Calculator extends Abacus {
      constructor() {
        super();
    
        // FIXME: shouldn’t use a global here
        total = 0;
      }
    }

  • 15.6 Use // TODO: to annotate solutions to problems.

    class Calculator extends Abacus {
      constructor() {
        super();
    
        // TODO: total should be configurable by an options param
        this.total = 0;
      }
    }

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Whitespace

  • 16.1 Use soft tabs (space character) set to 4 spaces. eslint: indent

    // bad
    function foo() {
    ∙∙let name;
    }
    
    // bad
    function bar() {
    ∙let name;
    }
    
    // good
    function baz() {
    ∙∙∙∙let name;
    }

  • 16.2 Place 1 space before the leading brace. eslint: space-before-blocks

    // bad
    function test(){
      console.log('test');
    }
    
    // good
    function test() {
      console.log('test');
    }
    
    // bad
    dog.set('attr',{
      age: '1 year',
      breed: 'Bernese Mountain Dog',
    });
    
    // good
    dog.set('attr', {
      age: '1 year',
      breed: 'Bernese Mountain Dog',
    });

  • 16.3 Place 1 space before the opening parenthesis in control statements (if, while etc.). Place no space between the argument list and the function name in function calls and declarations. eslint: keyword-spacing

    // bad
    if(isJedi) {
      fight ();
    }
    
    // good
    if (isJedi) {
      fight();
    }
    
    // bad
    function fight () {
      console.log ('Swooosh!');
    }
    
    // good
    function fight() {
      console.log('Swooosh!');
    }

  • 16.4 Set off operators with spaces. eslint: space-infix-ops

    // bad
    const x=y+5;
    
    // good
    const x = y + 5;

  • 16.5 End files with a single newline character. eslint: eol-last

    // bad
    import { es6 } from './AirbnbStyleGuide';
      // ...
    export default es6;
    // bad
    import { es6 } from './AirbnbStyleGuide';
      // ...
    export default es6;
    
    // good
    import { es6 } from './AirbnbStyleGuide';
      // ...
    export default es6;

  • 16.6 Use indentation when making long method chains (more than 2 method chains). Use a leading dot, which emphasizes that the line is a method call, not a new statement. eslint: newline-per-chained-call no-whitespace-before-property

    // bad
    $('#items').find('.selected').highlight().end().find('.open').updateCount();
    
    // bad
    $('#items').
      find('.selected').
        highlight().
        end().
      find('.open').
        updateCount();
    
    // good
    $('#items')
      .find('.selected')
        .highlight()
        .end()
      .find('.open')
        .updateCount();
    
    // bad
    const leds = stage.selectAll('.led').data(data).enter().append('svg:svg').classed('led', true)
        .attr('width', (radius + margin) * 2).append('svg:g')
        .attr('transform', `translate(${radius + margin},${radius + margin})`)
        .call(tron.led);
    
    // good
    const leds = stage.selectAll('.led')
        .data(data)
      .enter().append('svg:svg')
        .classed('led', true)
        .attr('width', (radius + margin) * 2)
      .append('svg:g')
        .attr('transform', `translate(${radius + margin},${radius + margin})`)
        .call(tron.led);
    
    // good
    const leds = stage.selectAll('.led').data(data);

  • 16.7 Leave a blank line after blocks and before the next statement.

    // bad
    if (foo) {
      return bar;
    }
    return baz;
    
    // good
    if (foo) {
      return bar;
    }
    
    return baz;
    
    // bad
    const obj = {
      foo() {
      },
      bar() {
      },
    };
    return obj;
    
    // good
    const obj = {
      foo() {
      },
    
      bar() {
      },
    };
    
    return obj;
    
    // bad
    const arr = [
      function foo() {
      },
      function bar() {
      },
    ];
    return arr;
    
    // good
    const arr = [
      function foo() {
      },
    
      function bar() {
      },
    ];
    
    return arr;

  • 16.8 Do not pad your blocks with blank lines. eslint: padded-blocks

    // bad
    function bar() {
    
      console.log(foo);
    
    }
    
    // bad
    if (baz) {
    
      console.log(qux);
    } else {
      console.log(foo);
    
    }
    
    // bad
    class Foo {
    
      constructor(bar) {
        this.bar = bar;
      }
    }
    
    // good
    function bar() {
      console.log(foo);
    }
    
    // good
    if (baz) {
      console.log(qux);
    } else {
      console.log(foo);
    }

  • 16.9 Do not use multiple blank lines to pad your code. eslint: no-multiple-empty-lines

    // bad
    class Person {
      constructor(fullName, email, birthday) {
        this.fullName = fullName;
    
    
        this.email = email;
    
    
        this.setAge(birthday);
      }
    
    
      setAge(birthday) {
        const today = new Date();
    
    
        const age = this.getAge(today, birthday);
    
    
        this.age = age;
      }
    
    
      getAge(today, birthday) {
        // ..
      }
    }
    
    // good
    class Person {
      constructor(fullName, email, birthday) {
        this.fullName = fullName;
        this.email = email;
        this.setAge(birthday);
      }
    
      setAge(birthday) {
        const today = new Date();
        const age = getAge(today, birthday);
        this.age = age;
      }
    
      getAge(today, birthday) {
        // ..
      }
    }

  • 16.10 Do not add spaces inside parentheses. eslint: space-in-parens

    // bad
    function bar( foo ) {
      return foo;
    }
    
    // good
    function bar(foo) {
      return foo;
    }
    
    // bad
    if ( foo ) {
      console.log(foo);
    }
    
    // good
    if (foo) {
      console.log(foo);
    }

  • 16.11 Do not add spaces inside brackets. eslint: array-bracket-spacing

    // bad
    const foo = [ 1, 2, 3 ];
    console.log(foo[ 0 ]);
    
    // good
    const foo = [1, 2, 3];
    console.log(foo[0]);

  • 16.12 Add spaces inside curly braces. eslint: object-curly-spacing

    // bad
    const foo = {clark: 'kent'};
    
    // good
    const foo = { clark: 'kent' };

  • 16.13 Avoid having lines of code that are longer than 100 characters (including whitespace). Note: per above, long strings are exempt from this rule, and should not be broken up. eslint: max-len

    Why? This ensures readability and maintainability.

    // bad
    const foo = jsonData && jsonData.foo && jsonData.foo.bar && jsonData.foo.bar.baz && jsonData.foo.bar.baz.quux && jsonData.foo.bar.baz.quux.xyzzy;
    
    // bad
    $.ajax({ method: 'POST', url: 'https://airbnb.com/', data: { name: 'John' } }).done(() => console.log('Congratulations!')).fail(() => console.log('You have failed this city.'));
    
    // good
    const foo = jsonData
      && jsonData.foo
      && jsonData.foo.bar
      && jsonData.foo.bar.baz
      && jsonData.foo.bar.baz.quux
      && jsonData.foo.bar.baz.quux.xyzzy;
    
    // good
    $.ajax({
      method: 'POST',
      url: 'https://airbnb.com/',
      data: { name: 'John' },
    })
      .done(() => console.log('Congratulations!'))
      .fail(() => console.log('You have failed this city.'));

  • 16.14 Require consistent spacing inside an open block token and the next token on the same line. This rule also enforces consistent spacing inside a close block token and previous token on the same line. eslint: block-spacing

    // bad
    function foo() {return true;}
    if (foo) { bar = 0;}
    
    // good
    function foo() { return true; }
    if (foo) { bar = 0; }

  • 16.15 Avoid spaces before commas and require a space after commas. eslint: comma-spacing

    // bad
    var foo = 1,bar = 2;
    var arr = [1 , 2];
    
    // good
    var foo = 1, bar = 2;
    var arr = [1, 2];

  • 16.16 Enforce spacing inside of computed property brackets. eslint: computed-property-spacing

    // bad
    obj[foo ]
    obj[ 'foo']
    var x = {[ b ]: a}
    obj[foo[ bar ]]
    
    // good
    obj[foo]
    obj['foo']
    var x = { [b]: a }
    obj[foo[bar]]

  • 16.17 Avoid spaces between functions and their invocations. eslint: func-call-spacing

    // bad
    func ();
    
    func
    ();
    
    // good
    func();

  • 16.18 Enforce spacing between keys and values in object literal properties. eslint: key-spacing

    // bad
    var obj = { "foo" : 42 };
    var obj2 = { "foo":42 };
    
    // good
    var obj = { "foo": 42 };

  • 16.20 Avoid multiple empty lines, only allow one newline at the end of files, and avoid a newline at the beginning of files. eslint: no-multiple-empty-lines

    // bad - multiple empty lines
    var x = 1;
    
    
    var y = 2;
    
    // bad - 2+ newlines at end of file
    var x = 1;
    var y = 2;
    
    
    // bad - 1+ newline(s) at beginning of file
    
    var x = 1;
    var y = 2;
    
    // good
    var x = 1;
    var y = 2;

⬆ back to top

Commas

  • 17.1 Leading commas: Nope. eslint: comma-style

    // bad
    const story = [
        once
      , upon
      , aTime
    ];
    
    // good
    const story = [
      once,
      upon,
      aTime,
    ];
    
    // bad
    const hero = {
        firstName: 'Ada'
      , lastName: 'Lovelace'
      , birthYear: 1815
      , superPower: 'computers'
    };
    
    // good
    const hero = {
      firstName: 'Ada',
      lastName: 'Lovelace',
      birthYear: 1815,
      superPower: 'computers',
    };

  • 17.2 Additional trailing comma: Yup. eslint: comma-dangle

    Why? This leads to cleaner git diffs. Also, transpilers like Babel will remove the additional trailing comma in the transpiled code which means you don’t have to worry about the trailing comma problem in legacy browsers.

    // bad - git diff without trailing comma
    const hero = {
         firstName: 'Florence',
    -    lastName: 'Nightingale'
    +    lastName: 'Nightingale',
    +    inventorOf: ['coxcomb chart', 'modern nursing']
    };
    
    // good - git diff with trailing comma
    const hero = {
         firstName: 'Florence',
         lastName: 'Nightingale',
    +    inventorOf: ['coxcomb chart', 'modern nursing'],
    };
    // bad
    const hero = {
      firstName: 'Dana',
      lastName: 'Scully'
    };
    
    const heroes = [
      'Batman',
      'Superman'
    ];
    
    // good
    const hero = {
      firstName: 'Dana',
      lastName: 'Scully',
    };
    
    const heroes = [
      'Batman',
      'Superman',
    ];
    
    // bad
    function createHero(
      firstName,
      lastName,
      inventorOf
    ) {
      // does nothing
    }
    
    // good
    function createHero(
      firstName,
      lastName,
      inventorOf,
    ) {
      // does nothing
    }
    
    // good (note that a comma must not appear after a "rest" element)
    function createHero(
      firstName,
      lastName,
      inventorOf,
      ...heroArgs
    ) {
      // does nothing
    }
    
    // bad
    createHero(
      firstName,
      lastName,
      inventorOf
    );
    
    // good
    createHero(
      firstName,
      lastName,
      inventorOf,
    );
    
    // good (note that a comma must not appear after a "rest" element)
    createHero(
      firstName,
      lastName,
      inventorOf,
      ...heroArgs
    );

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Semicolons

  • 18.1 Yup. eslint: semi

    Why? When JavaScript encounters a line break without a semicolon, it uses a set of rules called Automatic Semicolon Insertion to determine whether or not it should regard that line break as the end of a statement, and (as the name implies) place a semicolon into your code before the line break if it thinks so. ASI contains a few eccentric behaviors, though, and your code will break if JavaScript misinterprets your line break. These rules will become more complicated as new features become a part of JavaScript. Explicitly terminating your statements and configuring your linter to catch missing semicolons will help prevent you from encountering issues.

    // bad - raises exception
    const luke = {}
    const leia = {}
    [luke, leia].forEach((jedi) => jedi.father = 'vader')
    
    // bad - raises exception
    const reaction = "No! That’s impossible!"
    (async function meanwhileOnTheFalcon() {
      // handle `leia`, `lando`, `chewie`, `r2`, `c3p0`
      // ...
    }())
    
    // bad - returns `undefined` instead of the value on the next line - always happens when `return` is on a line by itself because of ASI!
    function foo() {
      return
        'search your feelings, you know it to be foo'
    }
    
    // good
    const luke = {};
    const leia = {};
    [luke, leia].forEach((jedi) => {
      jedi.father = 'vader';
    });
    
    // good
    const reaction = "No! That’s impossible!";
    (async function meanwhileOnTheFalcon() {
      // handle `leia`, `lando`, `chewie`, `r2`, `c3p0`
      // ...
    }());
    
    // good
    function foo() {
      return 'search your feelings, you know it to be foo';
    }

    Read more.

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Type Casting & Coercion

  • 19.1 Perform type coercion at the beginning of the statement.

  • 19.2 Numbers: Use Number for type casting and parseInt always with a radix for parsing strings. eslint: radix

    const inputValue = '4';
    
    // bad
    const val = parseInt(inputValue);
    
    // bad
    const val = Number(inputValue);
    
    // good
    const val = +inputValue;
    
    // good
    const val = parseInt(inputValue, 10);

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Naming Conventions

  • 20.1 Avoid single letter names. Be descriptive with your naming. eslint: id-length

    // bad
    function q() {
      // ...
    }
    
    // good
    function query() {
      // ...
    }

  • 20.2 Use PascalCase only when naming constructors or classes. eslint: new-cap

    // bad
    function user(options) {
      this.name = options.name;
    }
    
    const bad = new user({
      name: 'nope',
    });
    
    // good
    class User {
      constructor(options) {
        this.name = options.name;
      }
    }
    
    const good = new User({
      name: 'yup',
    });

  • 20.3 Don’t save references to this. Use arrow functions or Function#bind.

    // bad
    function foo() {
      const self = this;
      return function () {
        console.log(self);
      };
    }
    
    // bad
    function foo() {
      const that = this;
      return function () {
        console.log(that);
      };
    }
    
    // good
    function foo() {
      return () => {
        console.log(this);
      };
    }

  • 20.4 A base filename should exactly match the name of its default export.

    // file 1 contents
    class CheckBox {
      // ...
    }
    export default CheckBox;
    
    // file 2 contents
    export default function fortyTwo() { return 42; }
    
    // file 3 contents
    export default function insideDirectory() {}
    
    // in some other file
    // bad
    import CheckBox from './checkBox'; // PascalCase import/export, camelCase filename
    import FortyTwo from './FortyTwo'; // PascalCase import/filename, camelCase export
    import InsideDirectory from './InsideDirectory'; // PascalCase import/filename, camelCase export
    
    // bad
    import CheckBox from './check_box'; // PascalCase import/export, snake_case filename
    import forty_two from './forty_two'; // snake_case import/filename, camelCase export
    import inside_directory from './inside_directory'; // snake_case import, camelCase export
    import index from './inside_directory/index'; // requiring the index file explicitly
    import insideDirectory from './insideDirectory/index'; // requiring the index file explicitly
    
    // good
    import CheckBox from './CheckBox'; // PascalCase export/import/filename
    import fortyTwo from './fortyTwo'; // camelCase export/import/filename
    import insideDirectory from './insideDirectory'; // camelCase export/import/directory name/implicit "index"
    // ^ supports both insideDirectory.js and insideDirectory/index.js

  • 20.5 Acronyms and initialisms should always be all uppercased, or all lowercased.

    Why? Names are for readability, not to appease a computer algorithm.

    // bad
    import SmsContainer from './containers/SmsContainer';
    
    // bad
    const HttpRequests = [
      // ...
    ];
    
    // good
    import SMSContainer from './containers/SMSContainer';
    
    // good
    const HTTPRequests = [
      // ...
    ];
    
    // also good
    const httpRequests = [
      // ...
    ];
    
    // best
    import TextMessageContainer from './containers/TextMessageContainer';
    
    // best
    const requests = [
      // ...
    ];

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Accessors

  • 21.1 Accessor functions for properties are not required.

  • 21.2 Do not use JavaScript getters/setters as they cause unexpected side effects and are harder to test, maintain, and reason about. Instead, if you do make accessor functions, use getVal() and setVal('hello').

    // bad
    class Dragon {
      get age() {
        // ...
      }
    
      set age(value) {
        // ...
      }
    }
    
    // good
    class Dragon {
      getAge() {
        // ...
      }
    
      setAge(value) {
        // ...
      }
    }

  • 21.3 If the property/method is a boolean, use isVal() or hasVal().

    // bad
    if (!dragon.age()) {
      return false;
    }
    
    // good
    if (!dragon.hasAge()) {
      return false;
    }

  • 21.4 It’s okay to create get() and set() functions, but be consistent.

    class Jedi {
      constructor(options = {}) {
        const lightsaber = options.lightsaber || 'blue';
        this.set('lightsaber', lightsaber);
      }
    
      set(key, val) {
        this[key] = val;
      }
    
      get(key) {
        return this[key];
      }
    }

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Events

  • 22.1 When attaching data payloads to events (whether DOM events or something more proprietary like Backbone events), pass an object literal (also known as a "hash") instead of a raw value. This allows a subsequent contributor to add more data to the event payload without finding and updating every handler for the event. For example, instead of:

    // bad
    $(this).trigger('listingUpdated', listing.id);
    
    // ...
    
    $(this).on('listingUpdated', (e, listingID) => {
      // do something with listingID
    });

    prefer:

    // good
    $(this).trigger('listingUpdated', { listingID: listing.id });
    
    // ...
    
    $(this).on('listingUpdated', (e, data) => {
      // do something with data.listingID
    });

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Testing

  • 23.1 Yup.

    function foo() {
      return true;
    }

  • 23.2 No, but seriously:
    • Whichever testing framework you use, you should be writing tests!
    • Strive to write many small pure functions, and minimize where mutations occur.
    • Be cautious about stubs and mocks - they can make your tests more brittle.
    • We primarily use mocha at our company.
    • 100% test coverage is a good goal to strive for, even if it’s not always practical to reach it.
    • Whenever you fix a bug, write a regression test. A bug fixed without a regression test is almost certainly going to break again in the future.

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Performance

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License

(The MIT License)

Copyright (c) 2012 Airbnb

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the 'Software'), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NON INFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

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Amendments

We encourage you to fork this guide and change the rules to fit your team’s style guide. Below, you may list some amendments to the style guide. This allows you to periodically update your style guide without having to deal with merge conflicts.