ECMAScript Features in Chromium

This doc extends the style guide by specifying which new features of ES2015 and beyond are allowed in Chromium.

You can propose changing the status of a feature by sending an email to chromium-dev@chromium.org. Include a short blurb on what the feature is and why you think it should or should not be allowed, along with links to any relevant previous discussion. If the list arrives at some consensus, send a codereview to change this file accordingly, linking to your discussion thread.

[TOC]

ES2015 Support In Chromium

This is a list of ECMAScript 6 a.k.a. ES2015 features allowed in Chromium code.

This is not a status list of V8's support for language features.

Some descriptions and usage examples were taken from kangax and http://es6-features.org/

Allowed Features

The following features are allowed in Chromium development.

=> (Arrow Functions)

Arrow functions provide a concise syntax to create a function, and fix a number of difficulties with this (e.g. eliminating the need to write const self = this). Particularly useful for nested functions or callbacks.

Prefer arrow functions over .bind(this).

Arrow functions have an implicit return when used without a body block.

Usage Example:

// General usage, eliminating need for .bind(this).
setTimeout(() => {
  this.doSomething();
}, 1000);  // no need for .bind(this) or const self = this.

// Another example...
window.addEventListener('scroll', (event) => {
  this.doSomething(event);
});  // no need for .bind(this) or const self = this.

// Implicit return: returns the value if expression not inside a body block.
() => 1  // returns 1.
() => {1}  // returns undefined - body block does not implicitly return.
() => {return 1;}  // returns 1.

Documentation: link

Discussion Notes / Link to Thread: link

Promise

The Promise object is used for asynchronous computations. A Promise represents a value which may be available now, or in the future, or never.

Usage Example:

/** @type {!Promise} */
const fullyLoaded = new Promise(function(resolve) {
  function isLoaded() { return document.readyState == 'complete'; }

  if (isLoaded()) {
    resolve();
  } else {
    document.onreadystatechange = function() {
      if (isLoaded()) resolve();
    };
  }
});

// ... some time later ...
fullyLoaded.then(startTheApp).then(maybeShowFirstRun);

Documentation: link link

Discussion Notes: Feature already extensively used prior to creation of this document.

Proxy

Hooking into runtime-level object meta-operations.

Usage Example:

const keyTracker = new Proxy({}, {
  keysCreated: 0,

  get (receiver, key) {
    if (key in receiver) {
      console.log('key already exists');
    } else {
      ++this.keysCreated;
      console.log(this.keysCreated + ' keys created!');
      receiver[key] = true;
    }
  },
});

keyTracker.key1;  // '1 keys created!'
keyTracker.key1;  // 'key already exists'
keyTracker.key2;  // '2 keys created!'

Documentation: link

Discussion Notes / Link to Thread: link

Classes

OOP-style and boilerplate-free class syntax, including inheritance, super(), static members, and getters and setters.

Usage Example:

class Shape {
  constructor(x, y) {
    this.x = x;
    this.y = y;
  }
}
// Note: const Shape = class {...}; is also valid.

class Rectangle extends Shape {
  constructor(x, y, width, height) {
    super(id, x, y);
    this.width  = width;
    this.height = height;
  }

  static goldenRectangle() {
    const PHI = (1 + Math.sqrt(5)) / 2;
    return new Rectangle(0, 0, PHI, 1);
  }
}

Documentation: link

Discussion Notes / Link to Thread: https://groups.google.com/a/chromium.org/d/msg/chromium-dev/S1h-0m2ohOw/jyaiMGDlCwAJ

Map

A simple key/value map in which any value (both objects and primitive values) may be used as either a key or a value.

Usage Example:

const map = new Map();
map.size === 0;  // true
map.get('foo');  // undefined

const key = 54;
map.set(key, 123);
map.size === 1;  // true
map.has(key);  // true
map.get(key);  // 123

map.delete(key);
map.has(key);  // false
map.size === 0;  // true

Documentation: link link

Discussion Notes: Feature already extensively used prior to creation of this document.

Set

An object that lets you store unique values of any type, whether primitive values or object references.

Usage Example:

const set = new Set();

set.add(123);
set.size();  // 1
set.has(123);  // true

set.add(123);
set.size();  // 1

Documentation: link link

Discussion Notes: Feature already extensively used prior to creation of this document.

const (Block-Scoped Constants)

Constants (also known as "immutable variables") are variables which cannot be re-assigned new content. Note that if the value is an object, the object itself is still mutable.

const is block-scoped, just like let.

Usage Example:

const gravity = 9.81;
gravity = 0;  // TypeError: Assignment to constant variable.
gravity === 9.81;  // true

const frobber = {isFrobbing: true};
frobber = {isFrobbing: false};  // TypeError: Assignment to constant variable.
frobber.isFrobbing = false;  // Works.

Documentation: link

See also: Object.freeze()

Discussion Notes / Link to Thread: link

let (Block-Scoped Variables)

let declares a variable within the scope of a block, like const. This differs from var, which uses function-level scope.

Usage Example:

function varTest() {
  var x = 1;
  if (true) {
    var x = 2;  // Same variable!
    console.log(x);  // 2
  }
  console.log(x);  // 2
}

function letTest() {
  let x = 1;
  if (true) {
    let x = 2;  // Different variable.
    console.log(x);  // 2
  }
  console.log(x);  // 1
}

// Redeclaration throws.
function f() {
  var a = 'hello';
  var a = 'hola';  // No error!

  let b = 'world';
  let b = 'mundo;  // TypeError Identifier 'b' has already been declared.
}

Documentation: link

Discussion Notes / Link to Thread: link

Array Static & Prototype Methods

Usage Example:

// Static methods
const a1 = Array.from(document.querySelectorAll('div'));
const a2 = Array.of(7);

// Prototype methods
['a', 'b', 'c', 'd'].copyWithin(2, 0);  // Returns ['a', 'b', 'a', 'b']
[2, 4, 6, 8].find(i => i == 6);  // Returns 6
[2, 4, 6, 8].findIndex(i => i == 6); // Returns 2
[2, 4, 6, 8].fill(1);  // Returns [1, 1, 1, 1]

[2, 4, 6, 8].keys();  // Returns an Array iterator
[2, 4, 6, 8].entries();  // Returns an Array iterator

Documentation: link

Discussion Notes / Link to Thread: link

Number Properties

Usage Example:

// Number.isFinite
// Number.isInteger
// Number.isSafeInteger
// Number.isNaN
// Number.EPSILON
// Number.MIN_SAFE_INTEGER
// Number.MAX_SAFE_INTEGER

Documentation: link

Discussion Notes / Link to Thread: link

Object Static Methods

Usage Example:

// Object.assign
var o = Object.assign({a:true}, {b:true}, {c:true});  // {a: true, b: true, c: true}
'a' in o && 'b' in o && 'c' in o;  // true

// Object.setPrototypeOf
Object.setPrototypeOf({}, Array.prototype) instanceof Array;  // true

// Object.is
Object.is(null, null)  // true
Object.is(NaN, NaN)  // true
Object.is(-0, +0)  // false, btw: -0 === +0 is true

Documentation: link

Discussion Notes / Link to Thread: link

for...of Loops

Convenient operator to iterate over all values in an iterable collection. This differs from for...in, which iterates over all enumerable properties of an object.

Usage Example:

// Given an iterable collection of Fibonacci numbers...
for (const n of fibonacci) {
  console.log(n);  // 1, 1, 2, 3, ...
}

Documentation: link1 link2

Discussion Notes / Link to Thread: link

Template Literals

Expression interpolation for Strings, with the ability to access raw template pieces.

Usage Example:

// Simple example
const greeting = 'hello';
const myName = {first: 'Foo', last: 'Bar'};
const from = 1900;
const to = 2000;

var message = `${greeting}, I am ${myName.first}${myName.last},
and I am ${to - from} years old`;
// message == 'hello,\nI am FooBar,\nand I am 100 years old'

Documentation: link

Discussion Notes / Link to Thread:

Spread Operators

Spreading the elements from an iterable collection into individual literals as function parameters.

This only applies to arrays and not objects.

Usage Example:

// Spreading an Array
var params = ['hello', true, 7];
var other = [1, 2, ...params];  // [1, 2, 'hello', true, 7]

// Spreading a String
var str = 'foo';
var chars = [...str];  // ['f', 'o', 'o']

Documentation: link

Discussion Notes / Link to Thread: link

Rest Parameters

Aggregation of function arguments into one Array variable.

This only applies to arrays and function parameters, and not objects.

Usage Example:

function usesRestParams(a, b, ...theRest) {
  console.log(a);  // 'a'
  console.log(b);  // 'b'
  console.log(theRest);  // [1, 2, 3]
}

usesRestParams('a', 'b', 1, 2, 3);

Documentation: link

Discussion Notes / Link to Thread: link

Destructuring Assignment

Flexible destructuring of collections or parameters.

Usage Example:

// Array
const [a, , b] = [1, 2, 3];  // a = 1, b = 3

// Object
const {width, height} = document.body.getBoundingClientRect();
// width = rect.width, height = rect.height

// Parameters
function f([name, val]) {
  console.log(name, val);  // 'bar', 42
}
f(['bar', 42, 'extra 1', 'extra 2']);  // 'extra 1' and 'extra 2' are ignored.

function g({name: n, val: v}) {
  console.log(n, v);  // 'foo', 7
}
g({name: 'foo', val:  7});

function h({name, val}) {
  console.log(name, val);  // 'bar', 42
}
h({name: 'bar', val: 42});

Mixing with Rest Parameters

Using rest parameters while destructuring objects is not supported by iOS 10 and requires setting the closure arg language_in to ECMASCRIPT_2018.

const {one, ...rest} = {one: 1, two: 2, three: 3};

Using rest parameters while destructuring arrays, on the other hand, is supported by iOS 10 and ECMASCRIPT_2017.

const [one, ...rest] = [1, 2, 3];

Documentation: link

Discussion Notes / Link to Thread: link

Modules

Support for exporting/importing values from/to modules without global namespace pollution.

Usage Example:

// lib/rect.js
export function getArea() {...};
export {width, height, unimportant};

// app.js
import {getArea, width, height} from './lib/rect.js';

Documentation: link

Discussion Notes / Link to Thread: Dynamic Import link are not allowed yet, see separate entry in the Features To Be Discussed section.

Object Literal Extensions

Convenient new ways for object property definition.

Usage Example:

// Computed property name
const prop = 'foo';
const o = {
  [prop]: 'hey',
  ['b' + 'ar']: 'there',
};
console.log(o);  // {foo: 'hey', bar: 'there'}

// Shorthand property
const foo = 1;
const bar = 2;
const o = {foo, bar};
console.log(o);  // {foo: 1, bar: 2}

// Method property
const clearSky = {
  // Basically the same as clouds: function() { return 0; }.
  clouds() { return 0; },
  color() { return 'blue'; },
};
console.log(clearSky.color());  // 'blue'
console.log(clearSky.clouds());  // 0

Documentation: link link

Discussion Notes / Link to Thread: https://groups.google.com/a/chromium.org/d/msg/chromium-dev/RqOdTlxuGVg/M7I0CTryDQAJ

Note: clang-format has some issues formatting complex computed property names.

Banned Features

The following features are banned for Chromium development.

Features To Be Discussed

The following features are currently disallowed. See the top of this page on how to propose moving a feature from this list into the allowed or banned sections.

Block Scope Functions

Usage Example:

{
  function foo() {
    return 1;
  }
  // foo() === 1
  {
    function foo() {
      return 2;
    }
    // foo() === 2
  }
  // foo() === 1
}

Documentation: link

Discussion Notes / Link to Thread:

Default Function Parameters

Initialize parameters with default values if no value or undefined is passed.

Usage Example:

/**
 * @param {!Element} element An element to hide.
 * @param {boolean=} animate Whether to animatedly hide |element|.
 */
function hide(element, animate=true) {
  function setHidden() { element.hidden = true; }

  if (animate)
    element.animate({...}).then(setHidden);
  else
    setHidden();
}

hide(document.body);  // Animated, animate=true by default.
hide(document.body, false);  // Not animated.

Documentation: link link

Discussion Notes / Link to Thread:

Binary & Octal Literals

Usage Example:

0b111110111 === 503;
0o767 === 503;

Documentation: link

Discussion Notes / Link to Thread:

`/u` Unicode Regex Literal

Usage Example:

'𠮷'.match(/./u)[0].length === 2;

Documentation: link

Discussion Notes / Link to Thread:

`\u{}` Unicode String

Usage Example:

'\u{1d306}' == '\ud834\udf06';  // true

Documentation: link

Discussion Notes / Link to Thread:

`/y` Regex Sticky Matching

Keep the matching position sticky between matches and this way support efficient parsing of arbitrarily long input strings, even with an arbitrary number of distinct regular expressions.

Usage Example:

var re = new RegExp('yy', 'y');
re.lastIndex = 3;
var result = re.exec('xxxyyxx')[0];
result === 'yy' && re.lastIndex === 5;  // true

Documentation: link

Discussion Notes / Link to Thread:

Dynamic Import

Dynamic import() introduces a new function-like form of import that returns a promise for the module namespace object of the requested module.

Usage Example:

// lib/rect.js
export function getArea() {...};
export {width, height, unimportant};

// app.js
if (calculateArea) {
  import('./lib/rect.js').then(rect => {
    rect.getArea(...);
  });
}

Documentation: link

Discussion Notes / Link to Thread:

Symbol Type

Unique and immutable data type to be used as an identifier for object properties.

Usage Example:

const foo = Symbol();
const bar = Symbol();
typeof foo === 'symbol';  // true
typeof bar === 'symbol';  // true
const obj = {};
obj[foo] = 'foo';
obj[bar] = 'bar';
JSON.stringify(obj);  // {}
Object.keys(obj);  // []
Object.getOwnPropertyNames(obj);  // []
Object.getOwnPropertySymbols(obj);  // [foo, bar]

Documentation: link

Discussion Notes / Link to Thread:

String Static & Prototype methods

Usage Example:

// String.raw
// String.fromCodePoint

// String.prototype.codePointAt
// String.prototype.normalize
// String.prototype.repeat
// String.prototype.startsWith
// String.prototype.endsWith
// String.prototype.includes

Documentation: link

Discussion Notes / Link to Thread:

Iterators

Usage Example:

const fibonacci = {
  [Symbol.iterator]() {
    let pre = 0, cur = 1;
    return {
      next () {
        [pre, cur] = [cur, pre + cur];
        return {done: false, value: cur};
      }
    };
  }
};

Documentation: link

Discussion Notes / Link to Thread:

Generators

Special iterators with specified pausing points.

Usage Example:

function* range(start, end, step) {
  while (start < end) {
    yield start;
    start += step;
  }
}

for (const i of range(0, 10, 2)) {
  console.log(i);  // 0, 2, 4, 6, 8
}

Documentation: link

Discussion Notes / Link to Thread:

WeakMap

WeakMap does not prevent garbage collection if nothing else refers to an object within the collection.

Usage Example:

const key = {};
const weakmap = new WeakMap();

weakmap.set(key, 123);

weakmap.has(key) && weakmap.get(key) === 123;  // true

Documentation: link

Discussion Notes / Link to Thread:

WeakSet

WeakSet does not prevent garbage collection if nothing else refers to an object within the collection.

Usage Example:

const obj1 = {};
const weakset = new WeakSet();

weakset.add(obj1);
weakset.add(obj1);

weakset.has(obj1);  // true

Documentation: link

Discussion Notes / Link to Thread:

Typed Arrays

A lot of new typed Arrays...

Usage Example:

new Int8Array();
new UInt8Array();
new UInt8ClampedArray();
// ... You get the idea. Click on the Documentation link below to see all.

Documentation: link

Discussion Notes / Link to Thread:

Reflection

Make calls corresponding to the object meta-operations.

Usage Example:

const obj = {a: 1};
Object.defineProperty(obj, 'b', {value: 2});
obj[Symbol('c')] = 3;
Reflect.ownKeys(obj);  // ['a', 'b', Symbol(c)]

Documentation: link

Discussion Notes / Link to Thread:

Math Methods

A lot of new Math methods.

Usage Example:

// See Doc

Documentation: link

Discussion Notes / Link to Thread:

ES2016 Support In Chromium

This is a list of ECMAScript 7 a.k.a. ES2016 features allowed in Chromium code.

Allowed Features

The following features are allowed in Chromium development.

Banned Features

The following features are banned for Chromium development.

Features To Be Discussed

The following features are currently disallowed. See the top of this page on how to propose moving a feature from this list into the allowed or banned sections.

Array.prototype.includes()

The includes() method determines whether an array includes a certain element, returning true or false as appropriate.

Usage Example:

var array1 = [1, 2, 3];

console.log(array1.includes(2));
// expected output: true

var pets = ['cat', 'dog', 'bat'];

console.log(pets.includes('cat'));
// expected output: true

console.log(pets.includes('at'));
// expected output: false

Documentation: link

Discussion Notes / Link to Thread:

ES2017 Support In Chromium

This is a list of ECMAScript 8 a.k.a. ES2017 features allowed in Chromium code.

Allowed Features

The following features are allowed in Chromium development.

Async Functions (aka async/await)

The await expression causes async function execution to pause until a Promise is resolved, that is fulfilled or rejected, and to resume execution of the async function after fulfillment. When resumed, the value of the await expression is that of the fulfilled Promise.

Usage Example:

function resolveAfter2Seconds(x) {
  return new Promise(resolve => {
    setTimeout(() => {
      resolve(x);
    }, 2000);
  });
}

async function f1() {
  var x = await resolveAfter2Seconds(10);
  console.log(x); // 10
}
f1();

Documentation: link

Discussion Notes / Link to Thread: link

Banned Features

The following features are banned for Chromium development.

Features To Be Discussed

The following features are currently disallowed. See the top of this page on how to propose moving a feature from this list into the allowed or banned sections.

String.prototype.padEnd()/padStart()

The padStart() and padEnd() methods pad the current string with another string (multiple times, if needed) until the resulting string reaches the given length.

Usage Example:

const str1 = '5';

console.log(str1.padStart(2, '0'));
// expected output: "05"

const fullNumber = '2034399002125581';
const last4Digits = fullNumber.slice(-4);
const maskedNumber = last4Digits.padStart(fullNumber.length, '*');

console.log(maskedNumber);
// expected output: "************5581"

Documentation: link

Discussion Notes / Link to Thread:

Object.values() and Object.entries()

The Object.values() method returns an array of a given object's own enumerable property values, and Object.entries() method returns an array of a given object's own enumerable property [key, value] pairs.

Usage Example:

const object1 = {
  a: 'somestring',
  b: 42,
  c: false
};

console.log(Object.values(object1));
// expected output: Array ["somestring", 42, false]

const object2 = { foo: 'bar', baz: 42 };
console.log(Object.entries(object2)[1]);
// expected output: Array ["baz", 42]

Documentation: link link

Discussion Notes / Link to Thread:

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ECMAScript Features in Chromium

ES2015 Support In Chromium

Allowed Features

=> (Arrow Functions)

Promise

Proxy

Classes

Map

Set

const (Block-Scoped Constants)

let (Block-Scoped Variables)

Array Static & Prototype Methods

Number Properties

Object Static Methods

for...of Loops

Template Literals

Spread Operators

Rest Parameters

Destructuring Assignment

Modules

Object Literal Extensions

Banned Features

Features To Be Discussed

Block Scope Functions

Default Function Parameters

Binary & Octal Literals

/u Unicode Regex Literal

\u{} Unicode String

/y Regex Sticky Matching

Dynamic Import

Symbol Type

String Static & Prototype methods

Iterators

Generators

WeakMap

WeakSet

Typed Arrays

Reflection

Math Methods

ES2016 Support In Chromium

Allowed Features

Banned Features

Features To Be Discussed

Array.prototype.includes()

ES2017 Support In Chromium

Allowed Features

Async Functions (aka async/await)

Banned Features

Features To Be Discussed

String.prototype.padEnd()/padStart()

Object.values() and Object.entries()

`/u` Unicode Regex Literal

`\u{}` Unicode String

`/y` Regex Sticky Matching