How Prototype Works in JavaScript

By Mathieu Larose

July 2014

A prototype in JavaScript is an object with a special role: it’s the parent of another object. And the role of a parent is to provide properties to its children.

Let’s see how a prototype is linked to another object.

Internal Property [[Prototype]]

Every object has an internal property named [[Prototype]]. The value of [[Prototype]] is either a reference to another object or null. Example:

Observations:

a.[[Prototype]]   // => null
b.[[Prototype]]   // => a

Note that this is not valid JavaScript code as we cannot access directly an internal property of an object. We will see later how to retrieve the value of [[Prototype]].

Let’s now see what’s the purpose of the prototype chain.

Delegation

When a reference is made to a property in an object, the interpreter first looks if a property of that name is contained in the object. In that case, its value is returned. Otherwise, the interpreter checks its object’s prototype (referenced by the [[Prototype]] internal property), then the object’s prototype’s prototype and so on. This process continues until the property is found or the interpreter reaches the end of the prototype chain. In that case, undefined is returned.

Example:

a.p1   // => 'a-1'
a.p2   // => 'a-2'
a.p3   // => undefined
a.p4   // => undefined

b.p1   // => 'b-1'
b.p2   // => 'a-2'
b.p3   // => 'b-3'
b.p4   // => undefined

Assignment of [[Prototype]]

Every function comes with a prototype property (not to be confused with the [[Prototype]] internal property) whose value is an empty object.

When a function is used as a constructor (i.e. called as part of a new expression) the value of the [[Prototype]] internal property of the newly created object is set to the value of the prototype property of the function. Example:

var A = function () {};
A.prototype        // => {}

var a1 = new A();  // A is invoked as a constructor.
a1.[[Prototype]]   // => A.prototype

var a2 = new A();
a2.[[Prototype]]   // => A.prototype

Let’s add properties to those objects:

var A = function() {};
A.p1 = 'A-1';
A.p4 = 'A-4';

A.prototype.p1 = 'A-P-1';
A.prototype.p3 = 'A-P-3';

var a1 = new A();
a1.p1 = 'a1-1';

var a2 = new A();
a2.p2 = 'a2-2';

And let’s see how they are looked up:

A.p1   // => 'A-1'
A.p2   // => undefined
A.p3   // => undefined
A.p4   // => 'A-4'

A.prototype.p1  // => 'A-P-1'
A.prototype.p2  // => undefined
A.prototype.p3  // => 'A-P-3'
A.prototype.p4  // => undefined

a1.p1  // => 'a1-1'
a1.p2  // => undefined
a1.p3  // => 'A-P-3'
a1.p4  // => undefined

a2.p1  // => 'A-P-1'
a2.p2  // => 'a2-2'
a2.p3  // => 'A-P-3'
a2.p4  // => undefined

A is not part of the prototype chains starting at a1 and a2. Thus, it has no influence when a property is looked up in a1 and a2.

Overall View

To keep things simple, some objects were omitted in the previous examples. Let’s add the native objects (bold) to the previous example.

The properties of the native objects are documented in the ECMAScript Language Specification. Below are links to an annotated version of the specification for the corresponding objects:

By following the dashed lines, we know exactly what properties are inherited by each object. For instance, properties defined in Object.prototype (such as toString and hasOwnProperty) are inherited by all objects, because every prototype chain ends with Object.prototype.

Accessing [[Prototype]]

There is currently one standard way to access the [[Prototype]] internal property of an object and it’s by using the function Object.getPrototypeOf. Example:

var A = function() {};
var a = new A();

Object.getPrototypeOf(a) === A.prototype  // => true

Although most implementations currently support the __proto__ property, it is not part of the current standard (ECMAScript 5). But it will most likely be part of the next standard. Example:

a.__proto__ === A.prototype  // => true

Summary

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