JS_GetProperty

Find a specified property and retrieve its value.

Syntax

bool
JS_GetProperty(JSContext *cx, JS::HandleObject obj, const char *name,
               JS::MutableHandleValue vp);

bool
JS_GetUCProperty(JSContext *cx, JS::HandleObject obj,
                 const char16_t *name, size_t namelen,
                 JS::MutableHandleValue vp);

bool
JS_GetPropertyById(JSContext *cx, JS::HandleObject obj, JS::HandleId id,
                   JS::MutableHandleValue vp); // Added in SpiderMonkey 1.8.1
Name Type Description
cx JSContext * A context. Requires request. In a JS_THREADSAFE build, the caller must be in a request on this JSContext.
obj JS::HandleObject Object to search on for the property.
name or id const char * or const char16_t * or JS::HandleId Name of the property to look up.
namelen size_t (in JS_GetUCProperty only) The length of name, in characters; or -1 to indicate that name is null-terminated.
vp JS::MutableHandleValue Out parameter. On success, *vp receives the current value of the property, or undefined if no such property is found.

Description

JS_GetProperty examines a specified JS object obj and its prototype chain for a property with the specified name. It behaves like the JavaScript expression obj[name]. JS_GetUCProperty is the Unicode version of the function. JS_GetPropertyById is the same but takes a JS::HandleId for the property name.

In the simplest case, JS_GetProperty stores the value of the property in *vp and returns true. However, several different hooks can affect property gets. The full algorithm is described below.

Details

First, a property lookup is performed. If the lookup proceeds without error, exactly one of the following cases applies:

  • If the property is not found, then *vp is set to undefined. Then the JSClass.getProperty hook of obj's class is called with the arguments (cx, obj, id, vp). For many objects, including objects of standard classes such as Object and Array, this hook does nothing and returns true, so the property get succeeds and undefined is left in *vp.
  • If the property is found on a non-native object, get handler of Proxy (only if the object is a Proxy) or its JSObjectOps.getProperty method is called.
  • Otherwise the property is found on a native object. If the property has a JavaScript getter, it is called. Otherwise *vp is set to the property's stored value, or undefined if the property does not have a stored value, and then the property's getter is called with the arguments (cx, obj, id, vp). For many properties, the getter does nothing and returns true, so the property get succeeds and the property's stored value is left in *vp.

Internally, property retrieval, including all the behavior described above, is implemented by obj's JSObjectOps.getProperty callback.

On success, these functions set *vp to the current value of the property, or undefined if obj has no such property, and return true. On an error or exception, these functions return false, and the value left in *vp is undefined.

See Also