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The following is a changelog for JavaScript features in Firefox releases.

Firefox 52

New features

Changes and removals

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Firefox 51

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Firefox 50

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Firefox 49

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Firefox 48

New APIs

Deprecations and removals

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Firefox 47

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Firefox 46

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Firefox 45

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Firefox 44

New APIs

Changes

Removals

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Firefox 43

New APIs

Changes regarding the arguments object

Other changes

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Firefox 42

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Firefox 41

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Firefox 40

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Firefox 39

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Firefox 38

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Firefox 37

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Firefox 36

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Firefox 35

  • The "temporal dead zone" for let declarations has been implemented. In conformance with ES2015 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.
  • ES2015 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 ES2015 (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.
  • ES2015 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.

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Firefox 34

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Firefox 33

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Firefox 32

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Firefox 31

New ECMAScript 2015 features implemented:

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Firefox 30

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Firefox 29

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Firefox 28

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Firefox 27

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Firefox 26

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Firefox 25

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Firefox 24

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Firefox 23

  • The Object.defineProperty method can now be used to redefine the length property of an Array object.
  • The option to disable JavaScript, including the options to allow moving windows/replace context menu, have been removed. You may still disable JavaScript by double clicking the "javascript.enabled" option in about:config.

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Firefox 22

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Firefox 21

  • E4X, an ancient JavaScript extension, has been removed. Implemented only in Gecko, it never got significant traction (bug 788293).
  • parseInt no longer treats strings with leading "0" as octal (bug 786135).

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Firefox 20

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Firefox 19

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Firefox 18

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Firefox 17

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Firefox 16

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Firefox 15

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Firefox 14

No change.

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Firefox 13

  • Support for the ECMAScript 2015 for..of construct has been added.
  • Experimental support for ECMAScript 2015 Map and Set objects has been implemented.

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Firefox 12

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Firefox 11

No change.

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Firefox 10

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Firefox 9

No change.

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Firefox 8

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Firefox 7

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Firefox 6

  • In the past, it was possible to use the new operator on several built-in functions (eval, parseInt, Date.parse...) that should not have allowed it, according to the specification. This behavior is no longer supported. Using the new operator in this way was never officially supported and was not widely done, so it's unlikely that this change affects you.
  • ECMAScript 2015 WeakMaps have been added as a prototype implementation.

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Firefox 5

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Older versions

For versions prior to Firefox 5, see the old JavaScript versions.

 

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