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Obsolete since JSAPI 17
This feature is obsolete.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.

This article covers features introduced in SpiderMonkey 1.8.5

Determine if a special this object was supplied to the constructor.


static JSBool
JS_IsConstructing_PossiblyWithGivenThisObject(JSContext *cx, const jsval *vp,
                                              JSObject **maybeThis);
Name Type Description
cx JSContext * The context. Requires request. In a JS_THREADSAFE build, the caller must be in a request on this JSContext.
vp const jsval *  
maybeThis JSObject **  


In the case of a constructor called from JS_ConstructObject and JS_InitClass where the class has the JSCLASS_CONSTRUCT_PROTOTYPE flag set, SpiderMonkey passes the constructor a non-standard this object. In such cases, the following example would provide the additional information of whether a special this was supplied.

JSBool foo_native(JSContext *cx, unsigned int argc, jsval *vp) {
  JSObject *maybeThis;
  if (JS_IsConstructing_PossiblyWithGivenThisObject(cx, vp, &maybeThis)) {
    // native called as a constructor
    if (maybeThis)
    // native called as a constructor with maybeThis as 'this'
  } else {
    // native called as function, maybeThis is still uninitialized
Note: A SpiderMonkey embedding does not need to use this query unless the embedding uses JS_ConstructObject(), JS_InitClass() and JSCLASS_CONSTRUCT_PROTOTYPE as described above.

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 Contributors to this page: teoli, arai, kscarfone, Sheppy, [email protected], markg
 Last updated by: teoli,