JSObjectOps.defaultValue 编辑

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This feature is obsolete.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 JavaScript engine calls the JSObjectOps.defaultValue and JSClass.convert callbacks to convert objects to primitive values.

Syntax

typedef JSBool (*JSConvertOp)(JSContext *cx, JSObject *obj, JSType type,
    jsval *vp);

Description

The JSObjectOps.defaultValue callback corresponds to the [[DefaultValue]] method defined in ECMA 262-3 §8.6.2.6. Many expressions in the ECMAScript language, such as a+b, involve implicit conversion of the operands using [[DefaultValue]].

The default [[DefaultValue]] behavior for ordinary objects is as described in ES5 §8.12.8. It calls the JavaScript methods obj.valueOf() and/or obj.toString(). Provide a custom JSObjectOps.defaultValue callback to implement customized [[DefaultValue]] behavior.

The JSClass.convert callback should convert obj to the given type, returning JS_TRUE with the resulting value in *vp on success, and returning JS_FALSE on error or exception. On success, *vp must be a primitive value: per ES5 §8.12.8, every object "must ensure that its [[DefaultValue]] internal method can return only primitive values." Debug builds of SpiderMonkey will assert if a convert callback is successful but leaves *vp holding a primitive value.

JS_ConvertStub implements the default behavior for the JSClass.convert hook, which is to call obj.valueOf() and obj.toString() in accordance with the algorithm in ES5 §8.12.8.