JSFunction

From MDC

In JSAPI versions 1.8 and later, JSFunction is the type of Function objects. It can be thought of as a subclass of JSObject.

In earlier JSAPI versions, a JSFunction represented the code of a function, not the function object itself. The distinction is significant in the case of closures. Different closures generated from the same source code would sometimes share the same JSFunction. As a result, APIs (such as JS_CallFunction) that operate on JSFunctions often did not work properly on closures. This will be fixed in SpiderMonkey 1.8, when released.