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ES6 Coding Style

This document is intended to provide consistent and helpful coding convention for ES6 code. It should be usable either in Node/IO.js or in the browser.

The recommended transpiling tool for ES6 to ES5 is babel. The recommended bundler for the browser is webpack, but browserify is also acceptable.

The recommended text editor is Sublime Text using babel-sublime. Other editors are acceptable, as long as they have proper ES6 and optionally, JSX support.

Instructions denoted as must/must not are mandatory. Instructions denoted as should/should not are preferred.

Rationale for each instruction is italic. Some instructions are under the pure convention rationale: it means that when several, practically equivalent (besides bikeshedding) options are available, we pick one and stick to it to enforce consistency in the codebase.

Evil constructs

  • The following constructs must not be used, for both performance and critical security reasons.

    • eval and similar new Function, setTimeout(string), setInterval(string), setImmediate(string) (however setTimeout(function) etc. is perfectly fine).

    • with,

    • alert and confirm,

    • primitive type wrapper constructors (new Number, new String, new Boolean, new Array, new Object),

    • tabs (they are evil, don't use them).

eval and similar prevents runtime optimizations and leaks the execution context. See How evil is eval?.

with present the same kind of issues than eval. In addition, it is often ambiguous. See the MDN with docs.

alert and confirm are legacy of the past. They are synchronously blocking and highly user disruptive.

Primitive type wrappers are slower than their primitive counterparts. They also cause subtle bugs with instanceof.

Strict mode

  • Strict mode must be used. However, it should be inserted at build time, not in the original source.

Casing

  • Regular identifiers must be formatted likeThis.

  • Only functions with constructor semantics, classes, or namespaces must start with an Uppercase letter (PascalCase), LikeThis.

  • Macros (to be processed at build time) must be formatted __LIKE_THIS__.

  • Magic values (eg. numerical constants) must be formatted LIKE_THIS.

  • Private properties and methods must be formatted _likeThis.

const regularIdentifier;
function Constructor() { ... }
class Class { ... }
const Namespace = {};

if(__MACRO__) { ... }
const SOME_MAGIC_NUMBER = 1337;
const {
  _privateMethod() {
    ...
  }

  publicMethod() {
    ...
  }
}

Mostly pure convention. Magic values should always be assigned to named constants, not used directly in formulas, for easier debugging and maintenance. Enforcing lexical 'privacy' of methods and properties is possible using closures, but it comes with a performance cost and more coding efforts, when in the vast majority of cases you just want to enforce a conventional privacy: users can violate privacy, at their own risk.

Objects

  • Regular objects must be created using the Object syntax literal.
// bad
const obj = new Object();
const obj = Object.create(Object.prototype);

// good
const obj = {};
  • Object literals keys must be camelCase (this doesn't apply to programmatically generated keys).
// bad
const obj = {
  'not-camel-cased': ...
};

// good
const obj = {
  camelCased: ...
};
// acceptable
const myKey = 'something-from-somewhere';
const obj = {
  [myKey]: ...
};
  • An object declaration must have spaces after each { and before each }.

  • If the declaration is multiline, each line must end with a comma, including the last one.

  • If the declaration is not multiline, the last value must not end with a comma.

  • In an object declaration, there must not be space before each colon but at least one after each colon.

// bad
const a = {
  k1: v1,
  k2: v2
};
const b = { k1: v1, k2: v2, };
const c = {k1:v1};

// good
const a = {
  k1: v1,
  k2: v2,
};
const b = { k1: v1, k2: v2 };
const c = { k1: v2 };

Mostly pure convention. The trailing for multiline literals comma helps with adding/removing properties, and yield smaller and easier to read git diffs.

Arrays

  • Arrays must be created using the Array syntax literal.
// bad
const a = new Array();

// good
const a = [];

Strings

  • Static strings must be declared using single quotes or backticks.

  • Dynamic strings must be declared using ES6 template literals (backticks). Dynamic strings must not be declared using single- or double-quotes.

  • Never use double-quotes.

// bad
const a = "foobar";
const b = 'foo' + a + 'bar';

// acceptable
const c = `foobar`;

// good
const a  = 'foobar';
const b = `foo${a}bar`;
const c = 'foobar';

Mostly pure convention. Backticks are always preferred for dynamic strings as they are easier to read and easier to scan.

var, let, const

  • var must not be used.

  • const should be used by default. If you really need to use let, its acceptable, but its most often a sign that you should refactor.

  • const (or let) must be used exactly once per variable declaration.

// bad
const a = 1, b = 2, c = 3;

// good
const a = 1;
const b = 2;
const c = 3;

// actually best
const [a, b, c] = [1, 2, 3];

var is to be considered legacy; there are no practical use cases for var anymore. All values (rather than variables) should be const (single-assignment) by default; which means that unless otherwise specified, a value can not change behind your back. Mutability is therefore opt-in rather than opt-out. Note however that a const object can still be mutated; if you need deep immutability, consider using immutable-js.

Functions

  • Regular (named and hoisted) function declarations should be used instead of anonymous functions by default.

  • new must not be used with functions which are not ES6 classes.

// bad
function A() {

}
const a = new A();

// good
class A {

}
const a = new A();
  • When using anonymous functions, arrow function should be used instead of anonymous function.

  • When using 'anonymous' function, a name must be given.

const a = [1, 2, 3];
// bad
a.reduce(function(x, y) { return x + y; }, 0);

// better, but still bad
a.reduce(function sum(x, y) { return x + y; }, 0);

// acceptable but convoluted
function sum(x, y) {
  return x + y;
}
a.reduce(sum, 0);


// good
a.reduce((x, y) => x + y, 0);
  • Parens must be used for arrow functions parameters, even when there is only one parameter.
// bad
const triple = x => 3 * x;

// good
const triple = (x) => 3 * x;
  • Arrow functions should be used instead of Function.prototype.bind when applicable. self / _this / that trickery must not be used.
function method(...params) {
  ...
}

// terribly bad
const self = this;
const boundMethod = function(...params) {
  return method.apply(self, params);
}

// acceptable
const boundMethod = method.bind(this);

// best
const boundMethod = (...params) => method.apply(this, params);

Mostly pure conventions. Function#bind is relatively slow compared to lexical thisArg scoping, but lexical scoping is ugly and harder to read. Arrow functions desugar to lexical scoping and are a good compromise. In some cases the following snippet can be useful:

const scope = (fn, thisArg) => (...params) => fn.apply(thisArg, ...params);

Function parameters

  • A parameter must not be named arguments.

  • Parameters should BE explicitly typechecked in a __DEV__ block. They CAN be documented in addition.

  • ES6 default parameter value should be used when applicable.

  • Destructuring parameter syntax should be used when applicable.

  • Explicit variadic expansion should be used over ES6 rest syntax. If you really need true dynamic variadic functions, ES6 rest syntax must be used, not arguments inspection.

// bad
function sum() {
  let sum = 0;
  for(let k = 0; k < arguments.length; k++) {
    sum += arguments[k];
  }
  return sum;
}

// good
function sum(a = 0, b = 0, c = 0, d = 0, e = 0, f = 0, g = 0, h = 0, i = 0) {
  return a + b + c + d + e + f + g + h + i;
}

// acceptable
function sum(...rest) {
  return rest.reduce((x, y) => x + y, 0);
}
  • Object parameters (with default values) should be used for options parameters. Boolean parameters must not be used.
// bad
function divide(a, b, shouldCheckForZero = false ) {

}

// good
function divide(a, b, { shouldCheckForZero = false } = {}) {

}

ES6 destructuring assignment is a very powerful construct which considerably simplifies the use of object-as-hashes params. It also enables a kind of named parameters pattern. While named parameters are usually more readable than unnamed parameters, it really shines against the boolean trap antipattern.

Properties

  • Dot/subscript notation must be used consistently.

  • Dot notation should be used when possible.

// bad
t.x = 3;
t['y'] = 4;

// good
t['x'] = 3;
t['y'] = 4;

// better
t.x = 3;
t.y = 4;

Mostly pure convention. Dot notation is usually easier to read, but sometimes it's just not possible (for dynamic keys and special characters). In this case, consistency is best.

Operators

  • Short-hand operators +=-like and ++-like operators must not be used. Also, consider refactoring to get rid of the underlying let.
let n = 0;
// bad
n += 1;
n++;

// good
n = n + 1;
  • Non-strict comparison operators == or != must not be used. Strict comparison operators === or !== must be used instead.

  • Boolean force-casting !!expr should not be used. expr should BE used directly, unless you specifically care about leaking values in a function call or a return value.

// bad
if(!!expr) {
  ...
}

// good
if(expr) {
  ...
}

// acceptable to prevent leaking values
someUntrustedFunction(!!expr);

Pure convention for shorthand operators. Shorthand operators are marginally harder to read, but its really more of a convention. Boolean force-casting is useless and convoluted, unless you specifically care about the value of the expression leaking (besides its boolean value), but this doesn't matter in if-like constructs, only in function calls or return values.

Comments

  • /* ... */-style must be used for non-jsdoc multiline comments.

  • /** ... */-style must be used for jsdoc comments.

  • Be very careful when using trailing //. For single-line comments, comment on the previous line of whatever you're commenting.

Pure convention.

Whitespace

  • Tab characters must not be used. Ever. Tabs are evil. Don't use them.

  • Soft-spacing 2-characters must be used.

// bad
if(true) {
    return 42;
}

// good
if(true) {
  return 42;
}
  • 1 space must be used before the leading brace {.
// bad
if(true){
  ...
}

// good
if(true) {
  ...
}
  • 1 space must be used before the ending brace } in single-line object literals or function declaration.
// bad
function answer() { return 42;}
const a = { a: 42};

// good
function answer() { return 42; }
const a = { a: 42 };
  • Space must not be used before the leading bracket [ or the ending bracket ] in single-line array literals.
// bad
const a = [ 1, 2 ];

// good
const a = [1, 2];
  • Space must not be used before the leading paren (.
// bad
if (true) {
  ...
}

// good
if(true) {
  ...
}
  • Space must not be used between function and * when defining generator. Space must BE used between * and the function name when defining named generators.
// bad
function *g() {

}

// good
function* g() {

}
  • Space must not be used between * and the generator name when using the shorthand generator method syntax.
// bad
const a = {
  * g() {
    ...
  }
};

// good
const a = {
  *g() {
    ...
  }
};
  • Spaces must BE used between infix operators.
// bad
const a = b+c;
const a= b + c;

// good
const a = b + c;
  • Files must end with a single newline character.

  • Leading dot and proper indentation must be used for method chaining.

// bad
a.then(...).
then(...).
catch(...);

a.then(...)
.then(...)
.catch(...);

// good
a.then(...)
  .then(...)
  .then(...)
.catch(...);

Pure convention. Also, tabs are evil. Don't use them.

Blocks

  • Braces must BE used for blocks, even single-statement blocks. Only exception is 1-expression arrow functions.
// bad
if(true)
 return 42;
if(true) return 42;
const id = (x) => { return x; };

// good
if(true) { return 42; }
if(true) {
  return 42;
}
const id = (x) => x;
const id = (x) => {
  return x;
};
const id = (x) =>
  functionCall(x)
;

Always-braces is easier to maintain: one can add or remove statements/instructions without having to maintain the braces. Also easier to scan.

Commas

  • Trailing commas must be used for multiline literals, and must not be used for single-line literals.
// bad
const a = { a: 42, b: 43, };
const a = {
  a: 42,
  b: 42
};

// good
const a = { a: 42, b: 42 };
const a = {
  a: 42,
  b: 43,
};

Trailing commas are easier to maintain; one can add or remove properties without having to maintain the commas. Also produces cleaner git diffs.

Semicolons

  • Trailing semicolons must be used, even when they are not strictly required.
// bad
function id(x) { return x }

// good
function id(x) { return x; }

// actually better
const id = (x) => x;

Easier to maintain; one can add or remove statements without having to maintain the semicolon. Also produces cleaner git diffs. Avoid subtle bugs.

if/else construct

  • if/else if/else must be preceded by a newline.
// bad
if(...) {
  ...
} else if(...) {
  ...
} else {
  ...
}

// good
if(...) {
  ...
}
else if {
  ...
}
else {
  ...
}

Easier to read and refactor.

Ternary operator

  • Ternary operator (?:) should be avoided.

  • It must not be used if:

    • The whole expression cannot fit on one line;

    • The expression involve other ternary operators;

    • The members of the expression are too complex to keep it readable.

// bad
const foo = bar === baz ? aFunctionWithALongNameCalledIfTheFirstExpressionIsTrue()
: aFunctionWithALongNameCalledIfTheFirstExpressionIsFalse();

// bad
const foo = bar === baz
? aFunctionWithALongNameCalledIfTheFirstExpressionIsTrue()
: aFunctionWithALongNameCalledIfTheFirstExpressionIsFalse();

// bad
const foo = bar === baz ?
aFunctionWithALongNameCalledIfTheFirstExpressionIsTrue() :
aFunctionWithALongNameCalledIfTheFirstExpressionIsFalse();

// bad
const foo = fooBar(bar + qux) * qux ? fooBar(barFoo({ bar })[qux]).foo : fooBar(barFoo({ qux })[foo]).baz;

// bad
const foo = qux ? (bar ? qux : baz) : bar;

// acceptable
const foo = bar === baz ? bar + 1 : baz - 1;

// good
const foo = bar ? baz : qux;

switch construct

  • switch construct should not be used unless you are absolutely confident you are optimizing a super hot function which your JS engine can't optimize by itself. if/else if/else should be used instead.

When two constructs are equivalent, pick one and stick to it. Also, missing break bugs are very tedious.

for in, for of

  • for in and for of should not be used unless you are absolutely confident you are optimizing a super hot function which your JS engine can't optimize by itself. forEach/map should be used instead.

When two constructs are equivalent, pick one and stick to it. Also, missing hasOwnProperty check bugs are very tedious. for loops are very rarely needed in ES6 code where forEach/map and arrow functions can achieve almost every conceivable looping pattern and are easier to reason about. In edge cases where optimization is actually needed, it can be helpful though.

Method short notation

  • ES6 method short notation should be used when applicable.
// bad
const a = {
  f: function() {
    ...
  }
}

// good
const a =  {
  f() {

  }
}

Less clutter and easier to maintain.

ES6 modules

  • ES6 modules must be used, require-style module must not be used. Destructuring assignment should be used when applicable.
// bad
const moduleA = require('moduleA');
const func1 = moduleA.func1;
const func2 = moduleA.func2;

// bad
import * as moduleA from 'moduleA';
const func1 = moduleA.func1;
const func2 = moduleA.func2;

// good
import { func1, func2 } from 'moduleA';

More standards-friendly and easier to change the module implementation backend (eg. CommonJS/AMD) at the transpiler level.

undefined

  • undefined must not be used, void 0 must be used instead.
// bad
if(x === undefined) {
  ...
}

// good
if(x === void 0) {
  ...
}

Pure convention. Also void 0 looks more l33t.

Object extension and reshaping

  • Object reshaping should be avoided by declaring their shape early with null values, with the notable exceptions of Objects used as maps (but in this case, ES6 Map/Set/WeakMap/WeapSet is often more suited).

  • delete should not be used, with the notable exceptions of Objects used as maps (see above). Settings the value to null should be used to explicitly remove a reference for garbage collection.

  • In case reshaping is unavoidable, Object.assign must be used. In particular, Object.assign must be used to define prototype properties on ES6 classes.

  • In the particular case of a constructor function, an object should not be reshaped after the end of the constructor.

  • __proto__ should not be used. Object.setPrototypeOf should be used instead, but with great care.

// bad
const a = {};
a.x = 3;
delete a.x;

// if reshape unavoidable
const a = {};
Object.assign(a, { x: 3 });

// good
const a = { x: null };
a.x = 3;
a.x = null;


function F() { ... }

// bad
F.prototype.staticA = 43;
F.prototype.staticB = 44;
F.prototype.protoProp = null;
F.prototype.method = function() {
  ...
}

// good
Object.assign(F.prototype, {
  staticA: 43,
  staticB: 44,
  protoProp: null,
  method() {
    ...
  }
});

Object reshaping highly impacts performance on frequently created/used Objects. While negligible on less frequently used Objects (eg. singletons), it is preferable to be consistent and always avoid reshaping Objects. Objects used as hashes (maps) will often be reshaped anyway so this cannot be avoided, but in this case consider using ES6 Map/Set/WeakMap/WeakSet. Usage of Object.assign instead of multiple property assignment is marginally less efficient, but is easier to maintain and enables using the ES6 method short notation. Mutating the __proto__ has huge performance implications and should be used with extreme care.

Classes

  • Class names must be PascalCased.

  • When defined, constructor should be the first method of a class definition.

  • The order of class methods definitions should always be:

    • constructor

    • public get/set accessors, grouped by name,

    • public methods,

    • private get/set accessors (with a _ prefix), grouped by name,

    • private methods (with a _ prefix),

    • public static methods,

    • private static methods (with a _ prefix).

  • Prototype properties should be defined immediately after the class definition and use Object.assign and should not contain methods (set them in the class definition directly).

  • If your transpiler or environment support ES7 class properties, prototype properties of classes should not be used (use class properties instead).

class Foo extends Bar {
  // If your transpiler or environment support ES7 class properties:
  bax = null;
  baw = 1337;

  constructor() { // constructor
    ...
  }

  get baz() { // public getter
    ...
  }

  set baz(val) { // public setter
    ...
  }

  doFoo() { // public method
    ...
  }

  get _baz() { // private getter
    ...  
  }

  set _baz(val) { // private setter
    ...
  }

  _doFoo() { // private method
    ...  
  }

  // If your transpiler or environment support ES7 class properties:
  static theFoo = 'theBar';

  static doFoo() { // public static method
    ...
  }

  static _doFoo() { // private static method
    ...  
  }
}

// If your transpiler or environment doesn't support ES7 class properties:
Object.assign(Foo.prototype, { // prototype properties
  bax: null,
  baw: 1337,
});

Object.assign(Foo, { // static properties
  theFoo: 'theBar',
});

Mostly pure conventions. Public API are closer to the beginning of the declaration, making it easier to read. Also produces cleaner diffs since private methods are more susceptible to changes than public methods. The prototype properties must be defined outside of the class block due to ES6 limitations, but it should be very close to the class definition.

Proxies

  • Proxies must not be used at the moment.

There is no production-ready transpilation for ES6 Proxies. In addition, they may introduce questionable black magic.

Promises

  • Promise#then must not be used with 2 parameters. Promise#catch must be used instead.

  • Promise#try should be used for Promise-returning function, instead of return Promise.resolve/return Promise.reject.

// bad
p.then(() => {
  ...
}, () => {
  ...
});

function f() {
  if(...) {
    return Promise.resolve(...);
  }
  else {
    return Promise.reject(...);
  }
}

// good
p.then(() => {

})
.catch(() => {

});

function f() {
  return Promise.try(() => {
    if(...) {
      return ...;
    }
    else {
      throw ...;
    }
  });
}

Promise is a relatively low-level/interoperaribility spec. The excellent bluebird implements higher-level primitives that implement the hardest thing to do right with Promises: error propagation. In addition to its excellent performance (outperforming native Promises in most cases), it will most likely help you avoid common pitfalls with Promises.

Tail calls

  • Tail call optimization should not be assumed except for simple recursive functions (self-referencing functions). Tail call optimization must not be relied on for complex recursive function (eg. mutually recursive functions).
// bad
function odd(n) {
  if(n === 0) {
    return false;
  }
  if(n === 1) {
    return true;
  }
  return !even(n-1);
}

function even(n) {
  if(n === 0) {
    return true;
  }
  if(n === 1) {
    return false;
  }
  return !odd(n-1);
}

// good
function factorial(n) {
  function factorialLoop(n, acc) {
    if(n === 1) {
      return acc;
    }
    else {
      return factorialLoop(n - 1, n*acc);
    }
  }
  return factorialLoop(n, 1);
}

Tail calls are very hard to transpile efficiently. It is only partially supported by babel.

Generators

  • Generators must contain at least one yield statement. If not, consider using a Promise-returning function instead.

  • Generator short-hand definition must be used where applicable.

// bad
const a = {
  g: function*() {
    ...
  }
}

// good
const a = {
  *g() {
    ...
  }
}

Pure convention. Also, Promise and yield are interoperable, see Promise#coroutine.