Object.prototype.__proto__

Summary

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.

Syntax

var proto = obj.__proto__;

Note: that is two underscores, followed by the five characters "proto", followed by two more underscores.

Description

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.  For objects created using new fun, where fun is one of the built-in constructor functions provided by JavaScript (Array, Boolean, Date, Number, Object, String, and so on -- including new constructors added as JavaScript evolves), this value is fun.prototype.  For objects created using new fun, where fun is a function defined in a script, this value is the value of fun.prototype at the time new fun is evaluated.  (That is, if a new value is assigned to fun.prototype, previously-created fun instances will continue to have the previous value as their [[Prototype]], and subsequent new fun calls will use the newly-assigned value as their [[Prototype]].)

The __proto__ setter allows the [[Prototype]] of an object to be mutated.  The object must be extensible according to Object.isExtensible: if it is not, a TypeError is thrown.  The value provided must be an object or null.  Providing any other value will do nothing.

To understand how prototypes are used for inheritance, see guide article Inheritance and the prototype chain.

There is nothing special about the __proto__ property.  It is simply an accessor property -- a property consisting of a getter function and a setter function -- on Object.prototype.  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 find it.  If some other "__proto__" property is found before Object.prototype is consulted, that property will hide the one found on Object.prototype.

var noProto = Object.create(null);

print(typeof noProto.__proto__); // undefined
print(Object.getPrototypeOf(noProto)); // null

noProto.__proto__ = 17;

print(noProto.__proto__); // 17
print(Object.getPrototypeOf(noProto)); // null

var protoHidden = {};
Object.defineProperty(protoHidden, "__proto__",
                      { value: 42, writable: true, configurable: true, enumerable: true });

print(protoHidden.__proto__); // 42
print(Object.getPrototypeOf(protoHidden) === Object.prototype); // true

Example

In the following, a new instance of Employee is created, then tested to show that its __proto__ is the same object as its constructor's prototype.

// Declare a function to be used as a constructor
function Employee() {
  /* initialise instance */
}

// Create a new instance of Employee
var fred = new Employee();

// Test equivalence
fred.__proto__ === Employee.prototype; // true

At this point, fred inherits from Employee, however assigning a different object to fred.__proto__ can change that:

// Assign a new object to __proto__
fred.__proto__ = Object.prototype;

Now fred no longer inherits from Employee.prototype, but directly from Object.prototype, and loses the properties it originally inherited from Employee.prototype.

However, this only applies to extensible objects, a non–extensible object's __proto__ property cannot be changed:

var obj = {};
Object.preventExtensions(obj);

obj.__proto__ = {}; // throws a TypeError

Note that even Object.prototype's __proto__ property can be redefined as long as the chain leads to null:

var b = {};

Object.prototype.__proto__ = {
    hi: function () {alert('hi');},
    __proto__: null
};

b.hi();

If Object.prototype's __proto__ had not been set to null, or had not been set to another object whose prototype chain did not eventually lead explicitly to null, a "cyclic __proto__ value" TypeError would result since the chain must eventually lead to null (as it normally does on Object.prototype).

Specifications

Browser compatibility