Deprecated: This feature is no longer recommended. Though some browsers might still support it, it may have already been removed from the relevant web standards, may be in the process of being dropped, or may only be kept for compatibility purposes. Avoid using it, and update existing code if possible; see the compatibility table at the bottom of this page to guide your decision. Be aware that this feature may cease to work at any time.

Warning: Changing the [[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 the time spent in the obj.__proto__ = ... statement, but may extend to any code that has access to any object whose [[Prototype]] has been altered. You can read more in JavaScript engine fundamentals: optimizing prototypes.

Warning: While Object.prototype.__proto__ is supported today in most browsers, its existence and exact behavior has only been standardized as a legacy feature to ensure compatibility for web browsers. For better support, use Object.getPrototypeOf() and Object.setPrototypeOf() instead.

The __proto__ property of Object.prototype is an accessor property (a getter function and a setter function) that exposes the internal [[Prototype]] (either an object or null) of the object through which it is accessed.

The use of __proto__ is controversial and discouraged. It was never originally included in the ECMAScript language spec, but modern browsers implemented it anyway. Its semantics are standardized for compatibility with web browsers, so it will be supported into the future — nevertheless, it is deprecated in favor of Object.getPrototypeOf/Reflect.getPrototypeOf and Object.setPrototypeOf/Reflect.setPrototypeOf (though still, setting the [[Prototype]] of an object is a slow operation that should be avoided if performance is a concern).

The __proto__ property can also be used in an object literal definition to set the object [[Prototype]] on creation, as an alternative to Object.create(). See: object initializer / literal syntax. That syntax is standard and optimized for in implementations, and quite different from Object.prototype.__proto__.



Return value

If used as a getter, returns the object's [[Prototype]]. If used as a setter, returns undefined.



Thrown if attempting to set the prototype of a non-extensible object or an immutable prototype exotic object, such as Object.prototype or window.


The __proto__ getter function exposes the value of the internal [[Prototype]] of an object. For objects created using an object literal, this value is Object.prototype. For objects created using array literals, this value is Array.prototype. For functions, this value is Function.prototype. You can read more about the prototype chain in Inheritance and the prototype chain.

The __proto__ setter allows the [[Prototype]] of an object to be mutated. The value provided must be an object or null. Providing any other value will do nothing.

The __proto__ property is a simple accessor property on Object.prototype consisting of a getter and setter function. A property access for __proto__ that eventually consults Object.prototype will find this property, but an access that does not consult Object.prototype will not. If some other __proto__ property is found before Object.prototype is consulted, that property will hide the one found on Object.prototype. If the object has null as its prototype, the __proto__ accessor won't exist at all.


Using __proto__

function Circle() {}
const shape = {};
const circle = new Circle();

// Set the object prototype.
// DEPRECATED. This is for example purposes only. DO NOT DO THIS in real code.
shape.__proto__ = circle;

// Get the object prototype
console.log(shape.__proto__ === Circle);  // false
const ShapeA = function () {};
const ShapeB = {
  a() {

ShapeA.prototype.__proto__ = ShapeB;
console.log(ShapeA.prototype.__proto__); // { a: [Function: a] }

const shapeA = new ShapeA();
shapeA.a(); // aaa
console.log(ShapeA.prototype === shapeA.__proto__); // true
const ShapeC = function () {};
const ShapeD = {
  a() {

const shapeC = new ShapeC();
shapeC.__proto__ = ShapeD;
shapeC.a(); // a
console.log(ShapeC.prototype === shapeC.__proto__); // false
function Test() {}
Test.prototype.myName = function () {

const test = new Test();
console.log(test.__proto__ === Test.prototype); // true
test.myName(); // myName

const obj = {};
obj.__proto__ = Test.prototype;
obj.myName(); // myName


ECMAScript Language Specification
# sec-additional-properties-of-the-object.prototype-object

Browser compatibility

BCD tables only load in the browser

See also