These release notes are incomplete.

The Mozilla JavaScript team is pleased to announce the release of SpiderMonkey 31.

SpiderMonkey 31 is the JavaScript engine that shipped in Firefox 31. It continues to improve performance over previous SpiderMonkey releases, with a significantly improved garbage collector and other features.  It also contains new language and API features described in detail below.

Please let us know about your experiences with this release by posting in the newsgroup. Or file bugs at under Product: Core, Component: JavaScript engine.

The download url is outdated and SpiderMonkey not RELEASE alone! Get it here mozilla-esr31

You will find it in "Firefox Extended Support Release 31" package on hg release

Platform support

SpiderMonkey 31 is supported on all the platforms where Firefox 31 runs.  Compiling it requires a C++ compiler, and the JSAPI can only be used from C++ code.  If you are compiling with Microsoft's Visual Studio, note that the minimum supported version is MSVC10/2010: MSVC8/9 support has been dropped.

SpiderMonkey 31 includes a just-in-time compiler (JIT) that compiles JavaScript to machine code, for a significant speed increase. It is supported on x86, x86_64, and ARM architectures. On some other platforms (SPARC, MIPS), the JIT is provided but not supported.  On all other platforms the JIT is simply disabled; JavaScript code runs in an interpreter, as in previous versions. It's the same language, just not as fast.

Migrating to SpiderMonkey 31

The first change most embedders will notice is that SpiderMonkey must now be initialized before it can be used, using the newly-repurposed JS_Init method.  After this method is called, normal JSAPI operations are permitted.  When all JSAPI operation has completed, the corresponding JS_ShutDown method (currently non-mandatory, but highly recommended as it may become mandatory in the future) uninitializes SpiderMonkey, cleaning up memory and allocations performed by JS_Init.

A major change to SpiderMonkey in 31 is that its APIs support a moving garbage collector.  This entailed changing the vast majority of the JSAPI from raw types, such as JS::Value or JS::Value*, to JS::Handle and JS::MutableHandle template types that encapsulate access to the provided value/string/object or its location.  The JS::Handle<JS::Value> and JS::MutableHandle<JS::Value> classes have been specialized to implement the same interface as JS::Value, for simplicity and to ease migration pain.  Changes to introduce handles to the JSAPI are not individually documented, because of the breadth of the changes involved.

The following features in earlier versions of SpiderMonkey have been dropped.

SpiderMonkey 31 is not binary-compatible with previous releases, nor is it source-code compatible. Many JSAPI types, functions, and callback signatures have changed, though most functions that have changed still have the same names and implement essentially unchanged functionality. Applications will need significant changes, but most of those changes will be detected by the C/C++ compiler, so they are easy to detect and updating the code is a fairly straightforward job. Here is a list of the most significant changes:

These and other changes are explained in detail below.

New JavaScript language features

JavaScript 31 includes significant updates to language features, yo.

New C++ APIs

JSAPI is now a C++-only API. Please note that SpiderMonkey reserves the JS:: namespace for itself (and the js:: namespace for internal use).

Obsolete APIs

Deleted APIs

API changes

SpiderMonkey must now be initialized before it can be used.  This is performed by calling the new JS_Init method before creating any runtimes or contexts and before compiling, evaluating, or executing any JS script.  Once this method has been successfully called, it's okay to call JS_NewRuntime and all the other existing SpiderMonkey APIs as usual.  Once all JSAPI operation has completed, the corresponding JS_ShutDown method uninitializes SpiderMonkey, cleaning up memory and allocations performed by JS_Init.  It is not required to call JS_ShutDown at present, but it is strongly recommended: calling it may be required in a future SpiderMonkey release.

JavaScript shell changes

Detail added/removed methods here...

On POSIX platforms, building a threadsafe shell no longer requires NSPR. Embedders still need to build with NSPR, as the new wrappers require using SpiderMonkey internal functions which are not exposed to the public API.

Known Issues

Detail any known issues here...

Future Direction

...insert details on future plans...

SpiderMonkey embedders should be aware that

Release Notes Errata

This is a list of changes which need to be made to the release notes ASAP. Feel free to fix any problems you spot -- this is a Wiki!