Proxy.handler.getPrototypeOf

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 or null.
• If target is not extensible, Object.getPrototypeOf(proxy) method must return the same value as Object.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 2017 Draft (ECMA-262) The definition of '[[GetPrototypeOf]]' in that specification.	Draft

Browser compatibility

See also

• Proxy
• handler
• Object.getPrototypeOf()
• Reflect.getPrototypeOf()