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

js

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

js

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

js

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

js

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

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See also