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handler.getPrototypeOf()

The handler.getPrototypeOf() method is a trap for the [[GetPrototypeOf]] object internal method, which is used by operations such as Object.getPrototypeOf().

Try it

Syntax

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:

Or any other operation that invokes the [[GetPrototypeOf]] internal method.

Invariants

If the following invariants are violated, the trap throws a TypeError when invoked.

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

const obj = {};
const proto = {};
const handler = {
  getPrototypeOf(target) {
    console.log(target === obj); // true
    console.log(this === handler); // true
    return proto;
  },
};

const p = new Proxy(obj, handler);
console.log(Object.getPrototypeOf(p) === proto); // true

Five ways to trigger the getPrototypeOf trap

const obj = {};
const 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

const obj = {};
const p = new Proxy(obj, {
  getPrototypeOf(target) {
    return "foo";
  },
});
Object.getPrototypeOf(p); // TypeError: "foo" is not an object or null

const obj2 = Object.preventExtensions({});
const p2 = new Proxy(obj2, {
  getPrototypeOf(target) {
    return {};
  },
});
Object.getPrototypeOf(p2); // TypeError: expected same prototype value

Specifications

Specification
ECMAScript Language Specification # sec-proxy-object-internal-methods-and-internal-slots-getprototypeof

Browser compatibility

	Desktop						Mobile						Server
	Chrome	Edge	Firefox	Opera	Safari	Chrome Android	Firefox for Android	Opera Android	Safari on IOS	Samsung Internet	WebView Android	Deno	Node.js
`getPrototypeOf`	49	79	49	36	10	49	49	36	10	5.0	49	1.0	6.0.0