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The Object.setPrototypeOf() method sets the prototype (i.e., the internal [[Prototype]] property) of a specified object to another object or null.

Warning: Changing the [[Prototype]] of an object is, by the nature of how modern JavaScript engines optimize property accesses, a very slow operation, in every browser and JavaScript engine. The effects on performance of altering inheritance are subtle and far-flung, and are not limited to simply the time spent in obj.__proto__ = ... statement, but may extend to any code that has access to any object whose [[Prototype]] has been altered. If you care about performance you should avoid setting the [[Prototype]] of an object. Instead, create a new object with the desired [[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 [[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 [[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.

Examples

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:

// Only works in Chrome and FireFox, does not work in IE:
Object.setPrototypeOf = Object.setPrototypeOf || function(obj, proto) {
  obj.__proto__ = proto;
  return obj; 
}

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!!'

Specifications

Specification Status Comment
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'Object.setProtoypeOf' in that specification.
Standard Initial definition.
ECMAScript Latest Draft (ECMA-262)
The definition of 'Object.setProtoypeOf' in that specification.
Draft  

Browser compatibility

Feature Chrome Edge Firefox (Gecko) Internet Explorer Opera Safari
Basic support 34 (Yes) 31 (31) 11 (Yes) 9

 

Feature Android Chrome for Android Edge Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support No support No support (Yes) 31.0 (31) ? No support 9

See also

Document Tags and Contributors

 Last updated by: jameshkramer,