Web/JavaScript/Reference/Global objects/Object/setPrototypeOf

The Object.setPrototypeOf() method sets the prototype (i.e., the internal Web/JavaScript/Reference/Global objects/Object/Prototype property) of a specified object to another object or null.

Warning: Changing the Web/JavaScript/Reference/Global objects/Object/Prototype of an object is, by the nature of how modern JavaScript engines optimize property accesses, currently a very slow operation in every browser and JavaScript engine. In addition, the effects of altering inheritance are subtle and far-flung, and are not limited to simply the time spent in the Object.setPrototypeOf(...) statement, but may extend to any code that has access to any object whose Web/JavaScript/Reference/Global objects/Object/Prototype has been altered.

Because this feature is a part of the language, it is still the burden on engine developers to implement that feature performantly (ideally). Until engine developers address this issue, if you are concerned about performance, you should avoid setting the Web/JavaScript/Reference/Global objects/Object/Prototype of an object. Instead, create a new object with the desired Web/JavaScript/Reference/Global objects/Object/Prototype using Object.create().

Syntax

Object.setPrototypeOf(obj, prototype)

Parameters

obj: The object which is to have its prototype set.
prototype: The object's new prototype (an object or null).

Return value

The specified object.

Description

Throws a TypeError exception if the object whose Web/JavaScript/Reference/Global objects/Object/Prototype is to be modified is non-extensible according to Object.isExtensible(). Does nothing if the prototype parameter isn't an object or null (i.e., number, string, boolean, or undefined). Otherwise, this method changes the Web/JavaScript/Reference/Global objects/Object/Prototype of obj to the new value.

Object.setPrototypeOf() is in the ECMAScript 2015 specification. It is generally considered the proper way to set the prototype of an object, vs. the more controversial Object.prototype.__proto__ property.

Appending Prototype Chains

A combination of Object.getPrototypeOf() and Object.prototype.__proto__ permits appending a whole prototype chain to a new prototype object:

/**
*** Object.appendChain(@object, @prototype)
*
* Appends the first non-native prototype of a chain to a new prototype.
* Returns @object (if it was a primitive value it will transformed into an object).
*
*** Object.appendChain(@object [, "@arg_name_1", "@arg_name_2", "@arg_name_3", "..."], "@function_body")
*** Object.appendChain(@object [, "@arg_name_1, @arg_name_2, @arg_name_3, ..."], "@function_body")
*
* Appends the first non-native prototype of a chain to the native Function.prototype object, then appends a
* new Function(["@arg"(s)], "@function_body") to that chain.
* Returns the function.
*
**/

Object.appendChain = function(oChain, oProto) {
  if (arguments.length < 2) { 
    throw new TypeError('Object.appendChain - Not enough arguments');
  }
  if (typeof oProto !== 'object' && typeof oProto !== 'string') {
    throw new TypeError('second argument to Object.appendChain must be an object or a string');
  }

  var oNewProto = oProto,
      oReturn = o2nd = oLast = oChain instanceof this ? oChain : new oChain.constructor(oChain);

  for (var o1st = this.getPrototypeOf(o2nd);
    o1st !== Object.prototype && o1st !== Function.prototype;
    o1st = this.getPrototypeOf(o2nd)
  ) {
    o2nd = o1st;
  }

  if (oProto.constructor === String) {
    oNewProto = Function.prototype;
    oReturn = Function.apply(null, Array.prototype.slice.call(arguments, 1));
    this.setPrototypeOf(oReturn, oLast);
  }

  this.setPrototypeOf(o2nd, oNewProto);
  return oReturn;
}

Usage

First example: Appending a chain to a prototype

function Mammal() {
  this.isMammal = 'yes';
}

function MammalSpecies(sMammalSpecies) {
  this.species = sMammalSpecies;
}

MammalSpecies.prototype = new Mammal();
MammalSpecies.prototype.constructor = MammalSpecies;

var oCat = new MammalSpecies('Felis');

console.log(oCat.isMammal); // 'yes'

function Animal() {
  this.breathing = 'yes';
}

Object.appendChain(oCat, new Animal());

console.log(oCat.breathing); // 'yes'

Second example: Transforming a primitive value into an instance of its constructor and append its chain to a prototype

function MySymbol() {
  this.isSymbol = 'yes';
}

var nPrime = 17;

console.log(typeof nPrime); // 'number'

var oPrime = Object.appendChain(nPrime, new MySymbol());

console.log(oPrime); // '17'
console.log(oPrime.isSymbol); // 'yes'
console.log(typeof oPrime); // 'object'

Third example: Appending a chain to the Function.prototype object and appending a new function to that chain

function Person(sName) {
  this.identity = sName;
}

var george = Object.appendChain(new Person('George'),
                                'console.log("Hello guys!!");');

console.log(george.identity); // 'George'
george(); // 'Hello guys!!'

Examples

Using Object.setPrototypeOf

var dict = Object.setPrototypeOf({}, null);

Polyfill

Using the older Object.prototype.__proto__ property, we can easily define Object.setPrototypeOf if it isn't available already:

if (!Object.setPrototypeOf) {
    // Only works in Chrome and FireFox, does not work in IE:
     Object.prototype.setPrototypeOf = function(obj, proto) {
         if(obj.__proto__) {
             obj.__proto__ = proto;
             return obj;
         } else {
             // If you want to return prototype of Object.create(null):
             var Fn = function() {
                 for (var key in obj) {
                     Object.defineProperty(this, key, {
                         value: obj[key],
                     });
                 }
             };
             Fn.prototype = proto;
             return new Fn();
         }
     }
}

Specifications

Specification
ECMAScript (ECMA-262)The definition of 'Object.setPrototypeOf' in that specification.

Browser compatibility

Update compatibility data on GitHub

	Desktop						Mobile						Server
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet	Node.js
`setPrototypeOf`	Chrome Full support 34	Edge Full support 12	Firefox Full support 31	IE Full support 11	Opera Full support 21	Safari Full support 9	WebView Android Full support 37	Chrome Android Full support 34	Firefox Android Full support 31	Opera Android Full support 21	Safari iOS Full support 9	Samsung Internet Android Full support 2.0	nodejs Full support 0.12

Legend

Full support: Full support