Liste des changements liés à JavaScript par version de Firefox

No change.

• The ES2015 Symbol.toStringTag property has been implemented (bug 1114580).
• DateTimeFormat.prototype.formatToParts() got exposed to web content (bug 1289340).
• const and let are now fully ES2015-compliant (bug 950547).

• The ES2015 Symbol.hasInstance property has been implemented (bug 1054906).
• The ES2017 Object.getOwnPropertyDescriptors() method has been implemented (bug 1245024).
• The behavior of \W in RegExp with unicode and ignoreCase flags is changed to match recent draft spec. Now it doesn't match to K, S, k, s, and KELVIN SIGN (U+212A), and LATIN SMALL LETTER LONG S (U+017F) (bug 1281739).

• The ES2015 (ES6) getPrototypeOf() and setPrototypeOf() Proxy traps have been implemented (bug 888969).
• The ES2015 (ES6) RegExp.prototype[@@match](), RegExp.prototype[@@replace](), RegExp.prototype[@@search](), and RegExp.prototype[@@split]() methods, and RegExp[@@species] getter have been implemented (bug 887016).
• The deprecated, non-standard flags argument of String.prototype.match/search/replace has been removed (bug 1108382).
• The behavior of the Date.parse() method when parsing 2-digit years has been changed to be more interoperable with the Google Chrome browser (bug 1265136).

New APIs

• The ES2015 (ES6) Array.prototype.values() method has been implemented (bug 875433).
• The String.prototype.padStart() and String.prototype.padEnd() methods have been implemented (bug 1260509).
• The ES2015 (ES6) Symbol.unscopables and Array.prototype[@@unscopables] properties have been implemented (bug 1054759 and bug 1258163).
• The ES2015 (ES6) Symbol.isConcatSpreadable symbol has been implemented (bug 1041586).
• The ES2015 (ES6) Array[@@species] getter has been implemented (bug 1165052).
• The ES2015 (ES6) ArrayBuffer[@@species] getter and %TypedArray%[@@species] getter have been implemented (bug 1165053).
• The Intl.getCanonicalLocales() method of the ECMAScript Internationalization API draft has been implemented (bug 1263040).

Deprecations and removals

• The deprecated old Proxy API (Proxy.create and Proxy.createFunction()) has been removed. Use the standard Proxy object instead (bug 892903).
• The String.prototype.contains() method has been removed (it was deprecated since version 40). Use the String.prototype.includes() method instead (bug 1103588).
• The non-standard RegExp.multiline property (not RegExp.prototype.multiline) has been removed. Use the standard m flag instead (bug 1219757).
• The __defineGetter__ and __defineSetter__ methods can no longer be called at the global scope without any object. (bug 1253016).

• The new ES2017 / ES8 Object.values() and Object.entries() methods have been implemented (bug 1232639).
• The deprecated old Proxy API (Proxy.create and Proxy.createFunction) now presents a deprecation warning in the console and will be removed in a future version. Use the standard Proxy object instead (bug 892903).
• Support for the deprecated non-standard flags argument of String.prototype.match/search/replace has been dropped in non-release builds (bug 1245801).
• As per the new ES2016/ES7 specification, the Proxy enumerate trap for for...in statements has been removed (bug 1246318).
• The Array.prototype.indexOf() and Array.prototype.lastIndexOf() methods (and their TypedArray equivalents) have been updated to never return -0 as per the ECMAScript specification (bug 1242043).

• The ES2015 (ES6) RegExp unicode (u) flag has been implemented (bug 1135377).
• The ES2015 (ES6) block-level functions have been implemented (bug 1071646).
• The ES2015 (ES6) TypedArray.prototype.sort() method has been implemented (bug 1121937).
• The ES2015 (ES6) arguments[@@iterator] has been implemented (bug 1067049).
• The experimental EcmaScript Shared Memory API has been implemented. See the SharedArrayBuffer and Atomics objects. To use this experimental API set javascript.options.shared_memory to true in about:config.
• Redeclaration of let and const variables now throws a SyntaxError instead of a TypeError as per the ECMAScript specification (bug 1198833).
• In Strict mode, setting properties on primitive values will now throw a TypeError (bug 603201).
• The non-standard WeakMap.prototype.clear() and WeakSet.prototype.clear() methods have been removed (bug 1101817).
• The non-standard, static RegExp.multiline property is now deprecated (bug 1220457).
• Built-in accessor function names now have a "get" or "set" prefix (bug 1180290, bug 1235656).
• JS1.7/JS1.8 (legacy) array comprehensions and JS1.7/JS1.8 (legacy) generator comprehensions have been removed (bug 1220564).

• ES2015 (ES6) Classes are now enabled by default (bug 1197932).
• Expression closures are deprecated and will now present a warning in the console (bug 995610).
• String.prototype.replace does not restore RegExp static properties after executing function parameter anymore (bug 1226936).
• Math.random() has been updated to the better XorShift128+ algorithm (bug 322529).

Voir aussi :

New APIs

• The new ES2016 methods Array.prototype.includes() and TypedArray.prototype.includes() are now enabled by default (bug 1070767).

Changes regarding the arguments object

• To match the ES2015 specification, arrow functions no longer have their own arguments object. The arguments object is now lexically bound (inherited from the outer function). In most cases, rest parameters are a good alternative (...args) => args[i], see bug 889158.
• The arguments object is now allowed in conjunction with rest parameters (bug 1133298).
• From now on, a mapped arguments object in non-strict functions is only provided if the function does not contain any rest parameters, any default parameters or any destructured parameters (bug 1175394).

Other changes

• Generators and generator methods are no longer constructable as per ES2016/ES7 (bug 1191486).

• The Reflect object has been implemented (bug 987514).
• The implementation of the Proxy handler.ownKeys() trap has been updated to match the final ES2015 specification (bug 1049662).
• Calling Map, Set, or WeakMap without new, will now throw a TypeError (bug 1083752).

• Date.prototype is now an ordinary object, not a Date instance anymore (bug 861219).
• Date.prototype.toString is now a generic method (bug 861219).
• Symbol.species has been added (bug 1131043).
• Map[@@species] and Set[@@species] getters have been added (bug 1131043).
• Non-standard let expression support has been dropped (bug 1023609).
• Destructured parameters with default value assignment are now supported (bug 1018628).
• Per ES6, curly braces are required for method definitions. Syntax without them will fail from now on (bug 1150855).
• Method definitions (except for generator methods) are not constructable anymore (bug 1059908 and bug 1166950).
• As part of ES6 specification compliance, parenthesized destructuring patterns, like ([a, b]) = [1, 2] or ({a, b}) = { a: 1, b: 2 }, are now considered invalid and will throw a SyntaxError. See Jeff Walden's blog post for more details.
• The new.target syntax has been added (bug 1141865).

• Unreachable code after return statement (including unreachable expression after semicolon-less return statements) will now show a warning in the console (bug 1005110, bug 1151931).
• Symbol.match a été ajouté (bug 1054755).
• Passing an object which has a property named Symbol.match with a truthy value to String.prototype.startsWith, String.prototype.endsWith, and String.prototype.contains now throws a TypeError (bug 1054755).
• RegExp function returns pattern itself if called without new and pattern object has a property named Symbol.match with a truthy value, and the pattern object's constructor property equals to  RegExp function. (bug 1147817).
• Support for the non-standard JS1.7 destructuring for-in has been dropped (bug 1083498).
• Les initialiseurs d'expression non-standard dans les boucles for...in sont dorénavant ignorés et seront indiqués par un avertissement dans la console. (bug 748550 et bug 1164741).
• \u{xxxxxx} Unicode code point escapes have been added (bug 320500).
• String.prototype.contains has been replaced with String.prototype.includes, String.prototype.contains is kept as an alias (bug 1102219).
• If the DataView constructor is called as a function without the new operator, a TypeError is now thrown as per the ES6 specification.
• An issue regressed in Firefox 21, where proxyfied arrays without the get trap were not working properly, has been fixed. If the get trap in a Proxy was not defined, Array.length returned 0 and the set trap didn't get called. A workaround was to add the get trap even if was not necessary in your code. This issue has been fixed now (bug 895223).
• WeakMap.prototype and WeakSet.prototype have been updated to be just ordinary objects, per ES6 specification (bug 1055473).
• The RegExp.prototype.source property is now prototype accessor property rather than own data property of RegExp instances (bug 1120169, bug 1150297).

• The RegExp constructor no longer throws when the first argument is a RegExp and the second flags argument is present (bug 1108949).
• The Object.prototype.__noSuchMethod__ property is now deprecated and throws a console warning (see bug 1140428 and this platform announcement).
• The implementation of the Proxy object has been updated to comply more with the ES6 specification:
  • The defineProperty and set handlers now need to explicitly return true to be successful, otherwise a TypeError exception will be thrown in strict mode (bug 1132522).
  • If the window object is set as the target, those handlers will now throw a TypeError (bug 828137).
• When using arrow functions (=>), a line terminator (\n) is no longer allowed after arrow function arguments (() \n => {}) (bug 1141392).
• RegExp.prototype.toString is now a generic function (bug 1079919).
• The non-standard flags argument of String.prototype.match(), String.prototype.search(), and String.prototype.replace() is now deprecated and throws a console warning (bug 1142351).
• The try/catch behavior of Object.assign() has been removed to conform with the latest ES6 draft (bug 1103344).

• Generator.prototype.return() is implemented (bug 1115868).
• Setter with a rest parameter is now a SyntaxError (bug 1089632).
• The name property of functions is configurable now (bug 1084019).
• Several Array methods have been implemented for typed arrays as well:
  • The of() and from() methods (bug 896608).
  • The forEach() method (bug 1107645).
  • The filter() and map() methods (bug 1121936).
  • The slice() method (bug 1121935).
• Duplicated parameter name is no longer allowed when rest-parameter is present (bug 1096376).
• Duplicated parameter name is no longer allowed in arrow functions (bug 1096377).
• Duplicated parameter name is no longer allowed in concise method definitions (bug 1096378).
• Warning is shown when Map/Set/WeakMap constructor is called without new (bug 1108930).
• The get, has, and delete methods of WeakMap objects no longer throw when the key parameter is not an object (bug 1127827).
• The optional and non-standard second fallback parameter for WeakMap.prototype.get() has been removed (bug 1127827).
• When defining a generator method, "set" and "get" are no longer invalid names (bug 1073809).
• RegExp.prototype.source now returns "(?:)" instead of an empty string for empty regular expressions (bug 1130798).
• RegExp.prototype.source and RegExp.prototype.toString() now escape regular expression patterns properly (e.g. line terminators, "\n") (bug 1130860).
• The Regexp global, ignoreCase, multiline, and sticky properties are now prototype accessor properties rather than own data properties of RegExp instances (bug 1120169).
• The RegExp.prototype.source property is now prototype accessor property rather than own data property of RegExp instances (bug 1120169). Available only in non-Release version, due to bug 1150297.
• Function.prototype.toString() now throws for Proxy objects (bug 1100936).

• The Map, Set, WeakMap and WeakSet constructors now ignore null iterable (bug 1092538).
• The Map, Set, WeakMap and WeakSet constructors now supports monkey-patched prototype.set or prototype.add (bug 804279).
• The Non-standard String.prototype.quote() method has been removed (bug 1103181).
• The RegExp.prototype.flags property has been implemented (bug 1108467).
• Several Array methods have been implemented for typed arrays as well:
  • The every() and some() methods (bug 1116390).
  • The find() and findIndex() methods (bug 1078975).
  • The fill() method (bug 1113722).
  • The indexOf() and lastIndexOf() methods (bug 1107601).
  • The join() method (bug 1115817).
  • The reduce() and reduceRight() methods (bug 1117350).
  • The reverse() method (bug 1111516).
  • The keys(), values(), and entries() methods (bug 1119217).
• ES6 Proxy enumerate trap is implemented (bug 783829).
• The configurable attribute of the Function.length property is now true per the ES6 specification (bug 911142).
• The development of ParallelJS (PJS) has been discontinued due to the limited future prospects, little attention and code complexity. The experimental implementation that had been enabled only on the Nightly channel, including the Array.prototype.mapPar, filterPar and reducePar methods, has been completely removed.

• The ECMAScript 6 Symbol data type has been enabled by default (was available in the Nightly channel since version 33) (bug 1066322):
  • Symbol
  • Symbol.for()
  • Symbol.keyFor()
  • Object.getOwnPropertySymbols()
• The old placeholder string "@@iterator" has been replaced with the real ES6 well-known symbol Symbol.iterator for the iterable interface property key (bug 918828).
• The spec-internal abstract operation ToNumber(string) now supports binary (0b) and octal (0o) literals, this is a potentially breaking change from ES5 (bug 1079120).
  • Number("0b11") now returns 3, not NaN.
  • "0o11" == 9 now returns true, not false.
• The const declaration is now block-scoped and requires an initializer (bug 611388). It also can not be redeclared anymore (bug 1095439).
  • {const a=1}; a; now throws a ReferenceError and does not return 1 anymore due to block-scoping.
  • const a; now throws a SyntaxError ("missing = in const declaration"): An initializer is required.
  • const a = 1; a = 2; now also throws a SyntaxError ("invalid assignment to const a").
• The ES7 method Array.prototype.includes has been implemented, but for now, it is only enabled in Nightly builds (bug 1069063).
• The delete operator now triggers the "temporal dead zone" when using with let and const (bug 1074571).
• The non-standard let blocks and let expressions are deprecated and will now log a warning in the console. Do not use them anymore, they will be removed in the future.
• WeakMap constructor now handles optional iterable argument (bug 1092537).

• The "temporal dead zone" for let declarations has been implemented. In conformance with ES6 let semantics, the following situations
  now throw errors. See also this newsgroup announcement and bug 1001090.
  • Redeclaring existing variables or arguments using let within the same scope in function bodies is now a syntax error.
  • Using a variable declared using let in function bodies before the declaration is reached and evaluated is now a runtime error.
• ES6 Symbols (only available in the Nightly channel) have been updated to conform with recent specification changes:
  • String(Symbol("1")) now no longer throws a TypeError; instead a string ("Symbol(1)") gets returned (bug 1058396).
• The various TypedArray constructors now have as their [[Prototype]] a single function, denoted %TypedArray%  in ES6 (but otherwise not directly exposed).  Each typed array prototype now inherits from %TypedArray%.prototype.  (%TypedArray% and %TypedArray%.prototype inherit from Function.prototype and Object.prototype, respectively, so that typed array constructors and instances still have the properties found on those objects.)  Typed array function properties now reside on %TypedArray%.prototype and work on any typed array.  See TypedArray and bug 896116 for more information.
• ES6 semantics for prototype mutations using object literals have been implemented (bug 1061853).
  • Now only a single member notated as __proto__:value will mutate the [[Prototype]] in the object literal syntax.
  • Method members like __proto__() {} will not overwrite the [[Prototype]] anymore.

• The ES6 syntax for computed property names on object literals has been implemented (bug 924688).
  • This includes getter and setter method names as well (bug 1048384) and can also be used with destructuring.
• The ES6 shorthand syntax for defining methods on objects has been implemented (bug 924672).
• The ES6 Object method Object.assign() has been implemented (bug 937855).
• ES6 template strings and the String.raw() method are now supported (bug 1038259, bug 1039774).
• A new ES6 object WeakSet has been added (bug 792439).
• ES6 Symbols (only available in the Nightly channel) have been updated to conform with recent specification changes (bug 1042602):
  • When trying to convert a symbol to a number, a TypeError will be thrown now.
  • When using loose equality, Object(sym) == sym returns true now.
• The experimental TypedArray.prototype.move() method (only available in former Nightly and Aurora channels) has been replaced with the now implemented standard ES6 TypedArray.prototype.copyWithin() method (bug 1021379).
• In strict mode, setting a duplicate property name in object literals will no longer throw a SyntaxError as per ES6 specification (bug 1041128).
• In regular expressions (including String.replace), the matched text for a capturing group is now undefined instead of the empty string when that capturing group didn't get consulted because quantifiers prevented its exercise (see bug 369778 and this example code). Note that due to web compatibility, RegExp.$N will still return an empty string (bug 1053944).
• The ES6 spread operator is now supported in destructuring (bug 933276).
• Destructuring now uses iterator protocol instead of array-like protocol (bug 933276).
• Proxy.revocable() has been implemented (bug 978279).

• The non-standard method Number.toInteger() has been removed (bug 1022396).
• The Map.prototype.set(), WeakMap.prototype.set() and Set.prototype.add() methods are now chainable, return their equivalent objects and no longer undefined (bug 1031632).
• A default parameter is evaluated before function declarations inside the function body, so those functions cannot be referred from default parameter (bug 1022962).
• Shorthand properties are now allowed in object literals: if not explicitly defined, property keys are initialized by variables of the same name. E.g. function f(x, y) { return {x, y}; } is equivalent to function f(x, y) { return {x: x, y: y}; } (bug 875002).
• The parsing of yield and yield* has been updated to conform with the latest ES6 specification (bug 981599).
• The non-standard hasOwn trap has been removed (bug 980565).

• The following new ECMAScript 6 built-in methods got implemented:
  • Array.from() (bug 904723),
  • Array.prototype.copyWithin() (bug 934423),
  • Number.isSafeInteger() (bug 1003764).

New ECMAScript 6 features implemented:

• New Array built-in: Array.prototype.fill() (bug 911147)
• New Math function: Math.clz32() (bug 925123)
• New String built-in: String.prototype.normalize() is available in Firefox Desktop (bug 918987).
• New Object method Object.setPrototypeOf().
• New Number constants: Number.MAX_SAFE_INTEGER and Number.MIN_SAFE_INTEGER.
• The ES6 Proxy isExtensible trap have been implemented (bug 978235).

• New ES6-compatible array comprehensions [for (item of iterable) item] and generator comprehensions (for (item of iterable) item) have been implemented (bug 979865).
• Typed arrays are now extensible and support new named properties (bug 695438).
• The Error.prototype.stack property now contains column numbers (bug 762556) and has been improved when using Function() and eval() calls. This can help you to better debug minified or generated JavaScript code.
• The Promise.cast() method has been renamed to Promise.resolve() (bug 966348).

• New ECMAScript 6 String methods: String.prototype.codePointAt() and String.prototype.fromCodePoint() have been implemented (bug 918879).
• The ECMAScript Internationalization API (ECMA-402) has been implemented and is now enabled by default in Firefox Desktop (bug 853301):
  • New objects in the new Intl object namespace:
    • Intl.Collator
    • Intl.DateTimeFormat
    • Intl.NumberFormat
  • The following methods of String, Number and Date have been updated to include the locales and options arguments per ECMA-402:
    • String.prototype.localeCompare()
    • Number.prototype.toLocaleString()
    • Date.prototype.toLocaleString()
    • Date.prototype.toLocaleDateString()
    • Date.prototype.toLocaleTimeString()
• To match the updated ECMAScript6 draft specification, the Map and Set objects now treat -0 and +0 as the same when checking for key and value equality.
• Promise has been enabled by default (bug 918806).
• Completed generators now return an IteratorResult object instead of throwing (bug 958951).
• A malformed JSON string parsed by JSON.parse() now yields a more detailed error message containing the line and column number that caused the parsing error. This is useful when debugging large JSON data.
• The ArrayBuffer.isView() method has been added (bug 896105).

• ECMAScript 6 (Harmony) implementation continues:
  • New Array methods have been implemented: Array.prototype.entries() and Array.prototype.keys() (bug 894658).
• A bug causing that Object.getOwnPropertyNames() did not see unresolved properties of Error objects has been fixed (bug 724768).

EcmaScript 6 (Harmony) implementation continues!

• The spread operator is now supported in Function calls (bug 762363).
• The mathematical function Math.hypot() has been implemented (bug 896264).
• The yield* expression is now implemented (bug 666396).
• The MapIterator, SetIterator and ArrayIterator objects now match the specification (bug 881226).
• for..of loops now expect the ES6 standard iterator protocol moving away from SpiderMonkey old iterator protocol using StopIteration.
• String.match and String.replace now reset RegExp.lastIndex (bug 501739).

EcmaScript 6 (Harmony) implementation continues!

• The ECMAScript 6 compliant syntax for Generators (yield) has been implemented (bug 666399).
• Generator/Iterator results are now boxed like { value: foo, done: bool } (bug 907744).
• New mathematical methods have been implemented on Math: Math.fround() (bug 900125).
• The reserved words cannot be used for function names:such a usage now throws a SyntaxError (bug 907958).
• The default parameter syntax has been updated to allow parameters without defaults after default parameters, such as function f(x=1, y). See bug 777060.
• GeneratorFunction is implemented (bug 904701).

EcmaScript 6 (Harmony) implementation continues!

• The method Array.of() is now implemented on Array (bug 866849).
• Support for the methods Array.prototype.find() and Array.prototype.findIndex() has been added (bug 885553).
• The methods Number.parseInt() and Number.parseFloat() have been implemented (bug 886949)
• The methods Map.prototype.forEach() and Set.prototype.forEach() are now implemented (bug 866847).
• New mathematical methods have been implemented on Math: Math.log10(), Math.log2(), Math.log1p(), Math.expm1(), Math.cosh(), Math.sinh(), Math.tanh(), Math.acosh(), Math.asinh(), Math.atanh(), Math.trunc(), Math.sign() and Math.cbrt() (bug 717379).
• Support for binary and octal integer literals has been added: 0b10101010, 0B1010, 0o777, 0O237 are now valid (bug 894026).
• The machine epsilon constant, that is the smallest representible number that added to 1 will not be 1, is now available as Number.EPSILON (bug 885798).
• Typed arrays have been updated to no longer search in the prototype chain for indexed properties (bug 829896).

• Les fonctions fléchées ne sont plus automatiquement en mode strict sauf si c'est spécifié avec "use strict" (bug 852762).
• La méthode String.prototype.repeat a été implémentée (bug 815431).
• Les méthodes Set.prototype.values(), Set.prototype.keys() et Set.prototype.entries() de l'objet Set ont été implémentées (bug 869996).

• La méthode Object.defineProperty peut désormais être utilisée pour redéfinir la propriété length d'un objet Array.
• L'option pour désactiver JavaScript, incluant les options pour permettre de déplacer une fenêtre/remplacer le menu contextuel, a été retirée. Vous pouvez toujours désactiver JavaScript en double-cliquant sur l'option "javascript.enabled" dans about:config.

• Les optimizations Asm.js sont activées, rendant possible la compilation d'applications C / C++ vers un sous-ensemble Javascript pour de meilleures performances.
• La syntaxe ES6 Arrow Function a été implémentée (bug 846406).
• La nouvelle fonction Object.is a été ajoutée (bug 839979).

• E4X, une ancienne extension JavaScript, a été retirée. Implementée seulement par Gecko, elle n'a jamais reçu suffisament de soutien (bug 788293).
• parseInt ne traite plus les chaînes de caractères commençant par "0" comme un octal (bug 786135).

• Le support de la méthode Weakmap.prototype.clear(), récemment ajoutée au brouillon d'Harmony (EcmaScript 6), a été ajouté (bug 814562).
• Le support de la méthode Math.imul(), une fonction de multiplication 32 bits de type C. Bien qu'elle soit proposée pour Harmony (EcmaScript 6), elle n'a pas encore été acceptée et reste non-standard (bug 808148).
• Les applications Web utilisant le déplacement de texte grâce à Kinetic 3.x fonctionnent, même en utilisant le backend Cairo Canvas. (bug 835064)
• L'instruction for each...in a été dépréciée et ne doit plus être utilisée. Pensez à utiliser la nouvelle instruction for...of (bug 804834).

• La méthode size()des objets Map et Set devient la propriété size (bug 807001)
• Les objets Map et Set ont maintenant une méthode clear(). (bug 805003)

• Les Direct Proxies d'Harmony (ECMAScript 6) ont été lancés. (bug 703537)

• L'objet String offre à présent les méthodes d'Harmony startsWith, endsWith et contains. (bug 772733)
• Les méthodes de String link et anchor échappent désormais à " (guillemet). (bug 352437)
• Le support expérimental pour l'objet ParallelArray a été implémenté. (bug 778559)
• Support des itérateurs Map/Set. (bug 725909)
• E4X est désactivé par défaut pour le contenu Web. (bug 778851)
• __exposedProps__ doit désormais être défini pour les objets JavaScript Chrome exposés au contenu. Les tentatives d'accès à des objets de contenu sans Chrome __exposedProps__ échoueront ensembles silencieusement. (bug 553102)

• L'objet Number offre désormais les méthodes isFinite(), toInteger() et isInteger(). (bug 761480, bug 761495)
• L'opérateur de diffusion d'Harmony a été ajouté à l'objet Array. (bug 574130)

• Le support de l'interface DataView a été ajouté à partir de la spécification des tableaux typés. Cela fournit un accès de bas niveau aux données contenues dans un ArrayBuffer.
• Le support de Number.isNaN d'ECMAScript Harmony a été ajouté. (bug 749818, bug 761495, bug 761480)
• Le support du paramètre default d'ECMAScript Harmony a été ajouté. (bug 757676)
• Le support du paramètre rest d'ECMAScript Harmony a été ajouté. (bug 574132)

• Le support pour la construction for..of de l'ECMAScript 6 a été ajouté.
• Le support expérimental pour les objets ECMAScript 6 Map and Set a été ajouté.

• Le support des variables sharp (extension non-standard de Netscape) a été abandonné.

• La méthode WeakMap.set() retourne désormais undefined, au lieu de lui-même.
• Un bug a été introduit dans le traitement des expressions régulières dans Firefox 7, il a été corrigé. Voir le bug 683838.
• Vous ne pouvez plus utiliser E4X en mode strict de l'ECMAScript 5.

• RegExp.exec() et RegExp.test() appelés sans arguments correspondent maintenant à la chaîne "undefined".
• String.search() et String.match() appelés sans arguments ou undefined correspondent désormais à une chaîne vide et correspondent donc à chaque chaîne.
• Le support des listes de surveillance a été implémenté avec les nouvelles mtéhodes (non standards) watch() et unwatch().

• La propriété Function.arity a été enlevée, utilisez Function.length à la place.

• Avant, il était possible d'utiliser l'opérateur new sur plusieurs fonctions natives (eval, parseInt, Date.parse, etc) ce qui, conformément à la spécification, n'était pas autorisé. Désormais ce comportement n'est plus supporté. Cette façon d'utiliser l'opérateur new n'a jamais été officiellement supportée et était peu utilisée, donc il est peu probable que ce changement vous affecte.
• ECMAScript Harmony WeakMaps a été ajouté en tant que prototype.

• Les expressions régulières ne sont plus appelable comme si il s'agissait de fonctions, ce changement a été fait de concert avec l'équipe de WebKit afin d'assurer la compatibilité (voir WebKit bug 28285. Cette fonctionnalité existait depuis longtemps déjà mais n'a jamais été documentée (du moins, pas sur MDN).
• La méthode Function.prototype.isGenerator() est désormais supportée, ce qui vous permet de déterminer si une fonction est génératrice.
• Les mots suivants ont été réservés : class, enum, export, extends, import et super. Auparavant, ils étaient uniquement considérés comme réservés en mode strict.
• Les documents DOM crées dans le chrome ne peuvent plus être exposés à des scripts en sandbox.
• Le parser JSON a été ré-écrit pour améliorer la vitesse et la conformité. Ce qui inclut un correctif pour le bug 572279.

Pour les versions antérieures à Firefox 5, voir la page concernant l'historique JavaScript.