Obsolete since JavaScript 1.8.5
This feature is obsolete. Although it may still work in some browsers, its use is discouraged since it could be removed at any time. Try to avoid using it.
Warning: JSObjectOps is not a supported API. Details of the API may change from one release to the next. This documentation should be considered SpiderMonkey internals documentation, not API documentation. See bug 408416 for details.
The JSObjectOps.dropProperty callback releases a JSProperty.
Syntax
typedef void (*JSPropertyRefOp)(JSContext *cx, JSObject *obj, JSProperty *prop);
| Name | Type | Description |
|---|---|---|
cx |
JSContext * |
A context that was the cx argument to an earlier call to JSObjectOps.lookupProperty that found a property. |
obj |
JSObject * |
The object of which prop is an own property. That is, the value that the JSObjectOps.lookupProperty hook stored in the *objp out parameter. |
prop |
JSProperty * |
The property to release. That is, the value that JSObjectOps.lookupProperty hook stored in the *propp out parameter. |
Description
The following contract governs JSObjectOps callers and implementations:
- Whenever
JSObjectOps.lookupPropertyreturns aJSPropertypointer, the property is locked. - While the property is locked, the caller may access it using other
JSObjectOpscallbacks. It may not otherwise call into the JSAPI in ways that might trigger other property accesses. (In aJS_THREADSAFEbuild, that would risk deadlock.) - When finished with a locked property, the caller must release it by calling the
dropPropertycallback. - The lifetime of the
JSProperty *extends from the successfuldefinePropertyorlookupPropertycall to thedropPropertycall.
As a SpiderMonkey implementation detail, what is actually locked here (under the native implementation of JSObjectOps) is not the property but some collection of objects including the object on which the property is defined. The granularity of the locking is up to the JSObjectOps implementation; deadlock does not happen because each thread accesses only one property at a time.
A single, built-in JSObjectOps implementation is used for most SpiderMonkey objects that are exposed to scripts. Custom JSObjectOps implementations can either retain the SpiderMonkey property storage and locking scheme (by copying all or most of the built-in JSObjectOps) or replace it entirely.
Threads. In a JS_THREADSAFE build, any consistency observed by multiple threads operating on the same data is provided solely by the property locking scheme described above. In SpiderMonkey 1.8 and earlier, the main built-in JSObjectOps implementation serialized all accesses to a given object's properties. (That is, for each object, all property accesses happened in some order, and what each thread observed was consistent with that order: no stale reads, for example.) However, SpiderMonkey does not guarantee this high degree of serialization.