The handler.setPrototypeOf() method is a trap for Object.setPrototypeOf().


var p = new Proxy(target, {
  setPrototypeOf: function(target, prototype) {


The following parameters are passed to the setPrototypeOf method. this is bound to the handler.

The target object.
The object's new prototype or null.

Return value

The setPrototypeOf method returns true if the [[Prototype]] was successfully changed, otherwise false.


The handler.setPrototypeOf method is a trap for Object.setPrototypeOf().


This trap can intercept these operations:


If the following invariants are violated, the proxy will throw a TypeError:

  • If target is not extensible, the prototype parameter must be the same value as Object.getPrototypeOf(target).


If you want to disallow setting a new prototype for your object, your handler's setPrototypeOf method can either return false, or it can throw an exception.

The former approach means that any operation that performs such mutation, that throws an exception on failure to mutate, will have to create the exception itself.  For example, Object.setPrototypeOf() will create and throw a TypeError itself.  If the mutation is performed by an operation that doesn't ordinarily throw in case of failure, such as Reflect.setPrototypeOf(), no exception will be thrown.

var handlerReturnsFalse = {
    setPrototypeOf(target, newProto) {
        return false;

var newProto = {}, target = {};

var p1 = new Proxy(target, handlerReturnsFalse);
Object.setPrototypeOf(p1, newProto); // throws a TypeError
Reflect.setPrototypeOf(p1, newProto); // returns false

The latter approach will cause any operation that attempts to mutate, to throw.  This approach is required if you want even non-throwing operations to throw on failure, or you want to throw a custom exception value.

var handlerThrows = {
    setPrototypeOf(target, newProto) {
        throw new Error("custom error");

var newProto = {}, target = {};

var p2 = new Proxy(target, handlerThrows);
Object.setPrototypeOf(p2, newProto); // throws new Error("custom error")
Reflect.setPrototypeOf(p2, newProto); // throws new Error("custom error")


Specification Status Comment
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of '[[SetPrototypeOf]]' in that specification.
Standard Initial definition.
ECMAScript 2017 Draft (ECMA-262)
The definition of '[[SetPrototypeOf]]' in that specification.

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support ? 49 (49) ? ? ?
Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support ? ? 49.0 (49) ? ? ?

See also


© 2016 Mozilla Contributors
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.

ECMAScript6 JavaScript Method Prototype Proxy