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Obsolete since JSAPI 37
This feature is obsolete.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.

Determine if a specified numeric property exists.


JS_LookupElement(JSContext *cx, JS::HandleObject obj, uint32_t index,
                 JS::MutableHandleValue vp);
Name Type Description
cx JSContext * The context in which to look up the property. Requires request. In a JS_THREADSAFE build, the caller must be in a request on this JSContext.
obj JS::HandleObject The object to search.
index uint32_t The numeric id of the property to look up.
vp JS::MutableHandleValue Out parameter. On success, *vp receives the stored value of obj[index], or undefined if the element does not exist.


JS_LookupElement examines a specified JavaScript object, obj, for a numeric property numbered index.

Note: In the JavaScript language, numeric properties (also called "elements") are just ordinary properties whose names are numeric strings. For example, obj[6] is always exactly the same as obj["6"]. The ECMAScript standard also specifies this behavior: see ECMA 262-3 §11.2.1, step 6. So any call to JS_LookupElement is equivalent to a call to JS_LookupProperty passing a numeric string for the name.

If the property obj[index] exists, JS_LookupElement sets *vp to the property's stored value and returns true. If no such property exists, JS_LookupElement sets *vp to undefined and returns true (to indicate that no error occurred). On error or exception, JS_LookupElement returns false, and the value left in *vp is undefined.

JS_LookupElement does not distinguish between a property with a value of undefined and a property that does not exist.

See Also

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 Contributors to this page: arai, fscholz, Jorend, Dria, MMondor, Nickolay, Tservo
 Last updated by: arai,