ECMAScript 6 support in Mozilla

ECMAScript 2015 (6th Edition) is the current version of the ECMAScript Language Specification standard. Commonly referred to as "ES6", it defines the standard for the JavaScript implementation in SpiderMonkey, the engine used in Firefox and other Mozilla applications.

Code-named "ES.next" or "Harmony", the first working draft (based on ECMAScript 5.1) was published on July 12, 2011 as "ES.next". In August 2014, the ECMAScript 6 draft specification was feature frozen and went through a stabilization and bug fixing period. Finally, ECMA-262 Edition 6 got officially approved and published as a standard on June 17, 2015 by the ECMA General Assembly. It will also appear as the international industry standard ISO/IEC 16262:2016.

A PDF and HTML version of the standard can be freely downloaded on ecma-international.org.

A channel for feedback on ECMAScript standards is es-discuss.

Already supported by Firefox

The features listed here are already supported in Firefox; the version in which support was added is also indicated for each feature.

Standard library

Additions to the Array object

• Array iteration with for...of (Firefox 13)
• Array.from() (Firefox 32)
• Array.of() (Firefox 25)
• Array.prototype.fill() (Firefox 31)
• Array.prototype.find(), Array.prototype.findIndex() (Firefox 25)
• Array.prototype.entries(),
  Array.prototype.keys() (Firefox 28)
• Array.prototype.copyWithin() (Firefox 32)
• get Array[@@species] (Firefox 48)

New Map and Set objects, and their weak counterparts

• Map (Firefox 13)
  • Map iteration with for...of (Firefox 17)
  • Map.prototype.forEach() (Firefox 25)
  • Map.prototype.entries(),
    Map.prototype.keys(),
    Map.prototype.values() (Firefox 20)
  • Constructor argument: new Map(null) (Firefox 37)
  • Monkey-patched set() in Constructor (Firefox 37)
  • get Map[@@species] (Firefox 41)
• Set (Firefox 13)
  • Set iteration with for...of (Firefox 17)
  • Set.prototype.forEach() (Firefox 25)
  • Set.prototype.entries(),
    Set.prototype.keys(),
    Set.prototype.values() (Firefox 24)
  • Constructor argument: new Set(null) (Firefox 37)
  • Monkey-patched add() in Constructor (Firefox 37)
  • get Set[@@species] (Firefox 41)
• WeakMap (Firefox 6)
  • WeakMap.clear() (Firefox 20)
  • optional iterable argument in WeakMap constructor (Firefox 36)
  • Constructor argument: new WeakMap(null) (Firefox 37)
  • Monkey-patched set() in Constructor (Firefox 37)
• WeakSet (Firefox 34)
  • Constructor argument: new WeakSet(null) (Firefox 37)
  • Monkey-patched add() in Constructor (Firefox 37)

New Math functions

• Math.imul() (Firefox 20)
• Math.clz32() (Firefox 31)
• Math.fround() (Firefox 26)
• Math.log10(), Math.log2(), Math.log1p(), Math.expm1(), Math.cosh(), Math.sinh(), Math.tanh(), Math.acosh(), Math.asinh(), Math.atanh(), Math.hypot(), Math.trunc(), Math.sign(), Math.cbrt() (Firefox 25)

Additions to the Number object

• Number.isNaN() (Firefox 16)
• Number.isFinite() (Firefox 16)
• Number.isInteger() (Firefox 16)
• Number.parseInt() (Firefox 25)
• Number.parseFloat() (Firefox 25)
• Number.EPSILON (Firefox 25)
• Number.MAX_SAFE_INTEGER, Number.MIN_SAFE_INTEGER (Firefox 31)
• Number.isSafeInteger() (Firefox 32)

Additions to the Object object

• Object.prototype.__proto__ has been standardized
• Object.is() (Firefox 22)
• Object.setPrototypeOf() (Firefox 31)
• Object.assign() (Firefox 34)
• Object.getOwnPropertySymbols() (Firefox 33)

Additions to the Date object

• Date.prototype is an ordinary object (Firefox 41)
• generic Date.prototype.toString (Firefox 41)
• Date.prototype[@@toPrimitive] (Firefox 44)

New Promise object

• Promise (Firefox 24, enabled by default in Firefox 29)

New Proxy object

• Proxy (Firefox 18)
• preventExtensions() trap (Firefox 22)
• isExtensible() trap (Firefox 31)
• getPrototypeOf() and setPrototypeOf() traps (Firefox 49)

New Reflect object

• Reflect (Firefox 42)

Additions to the RegExp object

• RegExp sticky (y) flag (Firefox 38)
• RegExp unicode (u) flag (Firefox 46)
• generic RegExp.prototype.toString (Firefox 39)
• RegExp.prototype[@@match]() (Firefox 49)
• RegExp.prototype[@@replace]() (Firefox 49)
• RegExp.prototype[@@search]() (Firefox 49)
• RegExp.prototype[@@split]() (Firefox 49)
• get RegExp[@@species] (Firefox 49)

Additions to the String object

• String.fromCodePoint() (Firefox 29)
• String.prototype.codePointAt() (Firefox 29)
• String.prototype.startsWith(), String.prototype.endsWith() (Firefox 17)
• String.prototype.includes() (Firefox 40) (formerly String.prototype.contains() (Firefox 17))
• String.prototype.repeat() (Firefox 24)
• String.prototype.normalize() (Firefox 31)
• String.raw() (Firefox 34)
• \u{XXXXXX} Unicode code point escapes (Firefox 40)

New Symbol object

• Symbol (Firefox 36)
• Symbol.iterator (Firefox 36)
• Symbol.for() - global Symbol registry (Firefox 36)
• Symbol.match (Firefox 40)
• Symbol.species (Firefox 41)
• Symbol.toPrimitive (Firefox 44)
• Symbol.prototype[@@toPrimitive] (Firefox 44)
• Symbol.replace (Firefox 49)
• Symbol.search (Firefox 49)
• Symbol.split (Firefox 49)
• Symbol.hasInstance (Firefox 50)

Typed Arrays

Typed arrays are specified as part of ECMAScript 6 and no longer in their own specification.

• ArrayBuffer
• get ArrayBuffer[@@species] (Firefox 48)
• DataView
• Int8Array
• Uint8Array
• Uint8ClampedArray
• Int16Array
• Uint16Array
• Int32Array
• Uint32Array
• Float32Array
• Float64Array
• get %TypedArray%[@@species] (Firefox 48)

Expressions and operators

• new.target (Firefox 41)
• Spread operator for arrays (Firefox 16)
  • use Symbol.iterator property (Firefox 36)
• Spread operator for function calls (Firefox 27)
  • use Symbol.iterator property (Firefox 36)

Statements

• for...of (Firefox 13)
  • works in terms of .iterator() and .next() (Firefox 17)
  • use "@@iterator" property (Firefox 27)
  • use Symbol.iterator property (Firefox 36)

Functions

• Rest parameters (Firefox 15)
• Default parameters (Firefox 15)
  • Parameters without defaults after default parameters (Firefox 26)
  • Destructured parameters with default value assignment (Firefox 41)
• Arrow functions (Firefox 22)
• Generator function (Firefox 26)
  • yield (Firefox 26)
  • yield* (Firefox 27)
• arguments[@@iterator] (Firefox 46)

Other features

• Binary and octal numeric literals (Firefox 25)
• Template strings (Firefox 34)
• Object initializer: shorthand property names (Firefox 33)
• Object initializer: computed property names (Firefox 34)
• Object initializer: shorthand method names (Firefox 34)

Features not compliant with the ES6 specification

The following features are (partially) implemented in Firefox, but are not compliant with the specification. There are a couple of possible reasons for this: Firefox's implementation is based on an older draft of the ES6 specification, or Firefox's current implementation is an experimental design added before a similar feature was added to the ES6 specification.

• Legacy iterators and generators (JS 1.7, Firefox 2)
• Destructuring assignment (JS 1.7, Firefox 2) (ES6 compliance bug 1055984)
• const (JS 1.5, Firefox 1.0) (ES6 compliance bug 950547 implemented in Firefox 51)
• let (JS 1.7, Firefox 2) (ES6 compliance bug 950547 implemented in Firefox 51)

See also

• ECMAScript 7 support in Mozilla
• ECMAScript 6 specification drafts
• Mozilla ES6 tracking bug
• ECMAScript 6 support across browsers