handler.getPrototypeOf()

The handler.getPrototypeOf() method is a trap for the [[GetPrototypeOf]] internal method.

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Syntax

const 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

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

Specifications

Browser compatibility

See also

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