This article covers features introduced in SpiderMonkey 1.8
—9 March 2009
SpiderMonkey 1.8 has one known significant regression from 1.7.
JS_THREADSAFEbuilds, some objects are not safe to be shared among threads. This includes iterators and generators (bug 349263) and arrays (bug 417501). In short, applications that share objects among threads in such a way that two threads could access an array, iterator, or generator object at the same time should not use SpiderMonkey 1.8.
SyntaxError(bug 384758). This bug will be fixed in 1.8.1.
Migrating to SpiderMonkey 1.8
SpiderMonkey 1.8 is not binary-compatible with previous releases. It is source-compatible with SpiderMonkey 1.7 though, which means you can probably just recompile and relink your application. A handful of changes are not backwards-compatible:
x = .length = "7";would store the number
x. Now this statement stores the string
x. The new behavior conforms to the ECMAScript standard, ECMA 262-3 §11.13.1.
JSClass.resolvecallbacks repeatedly, as it did in previous versions. Specifically, if a property lookup first calls a resolve hook which does not define the property, then finds the property on a prototype, that result can be cached. Future lookups for the same property on the same sort of object may bypass the resolve hook.
JS_NewDoubleis now deprecated. Use
- The security APIs
JS_SetObjectPrincipalsFinderare still present but are deprecated in this release. In SpiderMonkey 1.8.1 they will be removed and replaced with a new set of APIs (
JS_GetSecurityCallbacksand friends, bug 451729).
js.mak, an old Windows-specific
nmakefile, has been deleted. It hasn't worked for a long time. It is fairly easy to create a Microsoft Visual Studio project file for SpiderMonkey 1.8 from scratch. (If you would like to contribute and maintain a project file, please feel free to contact the SpiderMonkey team via email, bugzilla, or IRC.) Alternatively, you can install MozillaBuild and run the command
make -f Makefile.refin the
SpiderMonkey 1.8 does not include the new TraceMonkey JIT or the
configure-based build system, both of which are (a) pretty darn awesome and (b) coming in SpiderMonkey 1.8.1.
On to the fun stuff.
New JSAPI features
It has been a couple years since SpiderMonkey 1.7 was released, and many improvements have been made to the JSAPI. However, one of the most important advances for JSAPI application developers is that the documentation is much improved. See the JSAPI User Guide and the JSAPI Reference.
- Native functions may implement the new callback signature
JSFastNativecan be significantly faster, but applications that use SpiderMonkey's fine-grained security features should heed the warning in its API documentation.
- New macros
JS_FS_ENDare recommended for populating
JS_NewObjectWithGivenProtois exactly like
JS_NewObjectexcept that it doesn't use a default prototype object if you pass
NULL. Instead the object is created with no prototype.
JS_AlreadyHasOwnPropertyand friends are new functions for examining an object without triggering resolve hooks.
- It was possible to configure SpiderMonkey 1.7 to treat C/C++
charstrings as UTF-8 by compiling with a non-default compiler option. In SpiderMonkey 1.8, it is no longer necessary to make a special build to get this behavior. Applications can turn on UTF-8 at run time by calling
JS_EncodeStringis a new variation on
JS_GetStringBytesthat returns a newly allocated, writable buffer which the application must
JS_ReportAllocationOverflowcan be used (instead of
JS_ReportOutOfMemory) to indicate that the script is trying to do something that would require more memory than the implementation is designed to handle. The main difference is that
JS_ReportAllocationOverflowthrows an exception which the application may catch.
JS_ThrowStopIterationthrows the appropriate
StopIterationexception object for the given context.
- Two new context options can be used with
JSOPTION_RELIMIT, which causes extremely long-running regular expression searches to fail with an error, and
JSOPTION_ANONFUNFIX, which bans anonymous functions from appearing anyplace where a statement could appear, such as in the argument to
- Functions that were
JS_THREADSAFE-only in SpiderMonkey 1.7 (
JS_BeginRequest, for example) are now present, but do nothing, in non-
JS_THREADSAFEbuilds. This is to help applications share and reuse code regardless of whether they use threads.
- There are two new
JSExtendedClass.wrappedObject. Both are documented but obscure.
Many new memory management features have been added as well.
- The new function
- Two new functions,
JS_DumpHeap, aim to help developers debug GC-related problems. These functions are only present in
JS_SetGCZealis especially helpful. When GC zeal is enabled, GC happens extremely frequently. This makes GC-related problems easier to reproduce, reveals GC problems that you may not have noticed before, and causes GC-safety bugs to surface much closer to their cause.
JS_DumpHeapdumps parts of the GC heap and can help detect leaks.
- A new GC parameter,
JSGC_STACKPOOL_LIFESPAN, controls how eagerly SpiderMonkey returns unused memory back to the system.
- The new function
JS_SetExtraGCRootsprovides another way to protect values from garbage collection.
- There is a new set of APIs for integrating SpiderMonkey's garbage collector with other memory management systems. These APIs are undocumented but fairly stable. The APIs include the new
stackQuota are related to memory management.
scatter, defined in
JS_THREADSAFE builds only, are for testing thread-safety.