The handler.getPrototypeOf()
method is a trap for the [[GetPrototypeOf]]
internal method.
Syntax
var p = new Proxy(obj, { getPrototypeOf(target) { ... } });
Parameters
The following parameter is passed to the getPrototypeOf
method. this
is bound to the handler.
target
- The target object.
Return value
The getPrototypeOf
method must return an object or null
.
Description
Interceptions
This trap can intercept these operations:
Object.getPrototypeOf()
Reflect.getPrototypeOf()
__proto__
Object.prototype.isPrototypeOf()
instanceof
Invariants
If the following invariants are violated, the proxy will throw a TypeError
:
getPrototypeOf
method must return an object ornull
.- If
target
is not extensible,Object.getPrototypeOf(proxy)
method must return the same value asObject.getPrototypeOf(target)
.
Examples
Basic usage
var obj = {}; var proto = {}; var handler = { getPrototypeOf(target) { console.log(target === obj); // true console.log(this === handler); // true return proto; } }; var p = new Proxy(obj, handler); console.log(Object.getPrototypeOf(p) === proto); // true
Five ways to trigger the getPrototypeOf trap
var obj = {}; var p = new Proxy(obj, { getPrototypeOf(target) { return Array.prototype; } }); console.log( Object.getPrototypeOf(p) === Array.prototype, // true Reflect.getPrototypeOf(p) === Array.prototype, // true p.__proto__ === Array.prototype, // true Array.prototype.isPrototypeOf(p), // true p instanceof Array // true );
Two kinds of exceptions
var obj = {}; var p = new Proxy(obj, { getPrototypeOf(target) { return 'foo'; } }); Object.getPrototypeOf(p); // TypeError: "foo" is not an object or null var obj = Object.preventExtensions({}); var p = new Proxy(obj, { getPrototypeOf(target) { return {}; } }); Object.getPrototypeOf(p); // TypeError: expected same prototype value
Specifications
Specification | Status | Comment |
---|---|---|
ECMAScript 2015 (6th Edition, ECMA-262) The definition of '[[GetPrototypeOf]]' in that specification. |
Standard | Initial definition. |
ECMAScript Latest Draft (ECMA-262) The definition of '[[GetPrototypeOf]]' in that specification. |
Living Standard |
Browser compatibility
The compatibility table on this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request.
Feature | Chrome | Edge | Firefox | Internet Explorer | Opera | Safari |
---|---|---|---|---|---|---|
Basic support | No | No | 49 | No | No | No |
Feature | Android webview | Chrome for Android | Edge mobile | Firefox for Android | IE mobile | Opera Android | iOS Safari |
---|---|---|---|---|---|---|---|
Basic support | No | No | No | 49 | No | No | No |