handler.setPrototypeOf()

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

Syntax

const p = new Proxy(target, {
  setPrototypeOf: function(target, prototype) {
  }
});

Parameters

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

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

Return value

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

Description

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

Interceptions

This trap can intercept these operations:

Invariants

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).

Examples

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.

Approach 1: Returning false

This approach means that any mutating operation that throws an exception on failure to mutate, must 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.

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

const newProto = {}, target = {};

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

Approach 2: Throwing an Exception

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

const handlerThrows = {
    setPrototypeOf(target, newProto) {
        throw new Error('custom error');
    }
}; 

const newProto = {}, target = {};

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

Specifications

Specification
ECMAScript Latest Draft (ECMA-262)
The definition of '[[SetPrototypeOf]]' in that specification.

Browser compatibility

Update compatibility data on GitHub
DesktopMobileServer
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidFirefox for AndroidOpera for AndroidSafari on iOSSamsung InternetNode.js
setPrototypeOfChrome Full support 49Edge Full support 12Firefox Full support 49IE No support NoOpera Full support 36Safari Full support 10WebView Android Full support 49Chrome Android Full support 49Firefox Android Full support 49Opera Android Full support 36Safari iOS Full support 10Samsung Internet Android Full support 5.0nodejs Full support 6.0.0

Legend

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See also