MDN may have intermittent access issues April 18 13:00 - April 19 01:00 UTC. See whistlepig.mozilla.org for all notifications.

mozilla
Your Search Results

    Inheritance and the prototype chain

    JavaScript is a bit confusing for developers experienced in class-based languages (like Java or C++), as it is dynamic and does not provide a class implementation per se (the class keyword is introduced in ES6, but is syntactical sugar, JavaScript will remain prototype-based).

    When it comes to inheritance, JavaScript only has one construct: objects. Each object has an internal link to another object called its prototype. That prototype object has a prototype of its own, and so on until an object is reached with null as its prototype. null, by definition, has no prototype, and acts as the final link in this prototype chain.

    While this is often considered to be one of JavaScript's weaknesses, the prototypal inheritance model is in fact more powerful than the classic model. It is, for example, fairly trivial to build a classic model on top of a prototypal model, while the other way around is a far more difficult task.

    Inheritance with the prototype chain

    Inheriting properties

    JavaScript objects are dynamic "bags" of properties (referred to as own properties). JavaScript objects have a link to a prototype object. When trying to access a property of an object, the property will not only be sought on the object but on the prototype of the object, the prototype of the prototype, and so on until either a property with a matching name is found or the end of the prototype chain is reached.

    Following the ECMAScript standard, the notation someObject.[[Prototype]] is used to designate the prototype of someObject. This is equivalent to the JavaScript property __proto__ (now deprecated). Since ECMAScript 5, the [[Protoype]] is accessed using the accessors Object.getPrototypeOf() and Object.setPrototypeOf().

    Here is what happens when trying to access a property:

    // Let's assume we have object o, with its own properties a and b:
    // {a: 1, b: 2}
    // o.[[Prototype]] has properties b and c:
    // {b: 3, c: 4}
    // Finally, o.[[Prototype]].[[Prototype]] is null.
    // This is the end of the prototype chain as null,
    // by definition, has no [[Prototype]].
    // Thus, the full prototype chain looks like:
    // {a:1, b:2} ---> {b:3, c:4} ---> null
    
    console.log(o.a); // 1
    // Is there an 'a' own property on o? Yes, and its value is 1.
    
    console.log(o.b); // 2
    // Is there a 'b' own property on o? Yes, and its value is 2.
    // The prototype also has a 'b' property, but it's not visited. 
    // This is called "property shadowing"
    
    console.log(o.c); // 4
    // Is there a 'c' own property on o? No, check its prototype.
    // Is there a 'c' own property on o.[[Prototype]]? Yes, its value is 4.
    
    console.log(o.d); // undefined
    // Is there a 'd' own property on o? No, check its prototype.
    // Is there a 'd' own property on o.[[Prototype]]? No, check its prototype.
    // o.[[Prototype]].[[Prototype]] is null, stop searching,
    // no property found, return undefined
    

    Setting a property to an object creates an own property. The only exception to the getting and setting behavior rules is when there is an inherited property with a getter or a setter.

    Inheriting "methods"

    JavaScript does not have "methods" in the form that class-based languages define them. In JavaScript, any function can be added to an object in the form of a property. An inherited function acts just as any other property, including property shadowing as shown above (in this case, a form of method overriding).

    When an inherited function is executed, the value of this points to the inheriting object, not to the prototype object where the function is an own property.

    var o = {
      a: 2,
      m: function(b){
        return this.a + 1;
      }
    };
    
    console.log(o.m()); // 3
    // When calling o.m in this case, 'this' refers to o
    
    var p = Object.create(o);
    // p is an object that inherits from o
    
    p.a = 12; // creates an own property 'a' on p
    console.log(p.m()); // 13
    // when p.m is called, 'this' refers to p.
    // So when p inherits the function m of o, 
    // 'this.a' means p.a, the own property 'a' of p
    

    Different ways to create objects and the resulting prototype chain

    Objects created with syntax constructs

    var o = {a: 1};
    
    // The newly created object o has Object.prototype as its [[Prototype]]
    // o has no own property named 'hasOwnProperty'
    // hasOwnProperty is an own property of Object.prototype. 
    // So o inherits hasOwnProperty from Object.prototype
    // Object.prototype has null as its prototype.
    // o ---> Object.prototype ---> null
    
    var a = ["yo", "whadup", "?"];
    
    // Arrays inherit from Array.prototype 
    // (which has methods like indexOf, forEach, etc.)
    // The prototype chain looks like:
    // a ---> Array.prototype ---> Object.prototype ---> null
    
    function f(){
      return 2;
    }
    
    // Functions inherit from Function.prototype 
    // (which has methods like call, bind, etc.)
    // f ---> Function.prototype ---> Object.prototype ---> null
    

    With a constructor

    A "constructor" in JavaScript is "just" a function that happens to be called with the new operator.

    function Graph() {
      this.vertices = [];
      this.edges = [];
    }
    
    Graph.prototype = {
      addVertex: function(v){
        this.vertices.push(v);
      }
    };
    
    var g = new Graph();
    // g is an object with own properties 'vertices' and 'edges'.
    // g.[[Prototype]] is the value of Graph.prototype when new Graph() is executed.
    

    With Object.create

    ECMAScript 5 introduced a new method: Object.create(). Calling this method creates a new object. The prototype of this object is the first argument of the function:

    var a = {a: 1}; 
    // a ---> Object.prototype ---> null
    
    var b = Object.create(a);
    // b ---> a ---> Object.prototype ---> null
    console.log(b.a); // 1 (inherited)
    
    var c = Object.create(b);
    // c ---> b ---> a ---> Object.prototype ---> null
    
    var d = Object.create(null);
    // d ---> null
    console.log(d.hasOwnProperty); 
    // undefined, because d doesn't inherit from Object.prototype
    

    With the class keyword

    ECMAScript 6 introduced a new set of keywords implementing classes. Although these constructs look like those familiar to developers of class-based languages, they are not. JavaScript remains prototype-based. The new keywords include class, constructor, static, extends, and super.

    "use strict";
    
    class Polygon {
      constructor(height, width) {
        this.height = height;
        this.width = width;
      }
    }
    
    class Square extends Polygon {
      constructor(sideLength) {
        super(sideLength, sideLength);
      }
      get area() {
        return this.height * this.width;
      }
      set sideLength(newLength) {
        this.height = newLength;
        this.width = newLength;
      }
    }
    
    var square = new Square(2);
    

    Performance

    The lookup time for properties that are high up on the prototype chain can have a negative impact on performance, and this may be significant in code where performance is critical. Additionally, trying to access nonexistent properties will always traverse the full prototype chain.

    Also, when iterating over the properties of an object, every enumerable property that is on the prototype chain will be enumerated.

    To check whether an object has a property defined on itself and not somewhere on its prototype chain, it is necessary to use the hasOwnProperty method which all objects inherit from Object.prototype.

    hasOwnProperty is the only thing in JavaScript which deals with properties and does not traverse the prototype chain.

    Note: It is not enough to check whether a property is undefined. The property might very well exist, but its value just happens to be set to undefined.

    Bad practice: Extension of native prototypes

    One mis-feature that is often used is to extend Object.prototype or one of the other built-in prototypes.

    This technique is called monkey patching and breaks encapsulation. While used by popular frameworks such as Prototype.js, there is still no good reason for cluttering built-in types with additional non-standard functionality.

    The only good reason for extending a built-in prototype is to backport the features of newer JavaScript engines; for example Array.forEach, etc.

    Example

    B shall inherit from A:

    function A(a){
      this.varA = a;
    }
    
    // What is the purpose of including varA in the prototype when A.prototype.varA will always be shadowed by
    // this.varA, given the definition of function A above?
    A.prototype = {
      varA : null,  // Shouldn't we strike varA from the prototype as doing nothing?
          // perhaps intended as an optimization to allocate space in hidden classes?
          // https://developers.google.com/speed/articles/optimizing-javascript#Initializing instance variables
          // would be valid if varA wasn't being initialized uniquely for each instance
      doSomething : function(){
        // ...
      }
    }
    
    function B(a, b){
      A.call(this, a);
      this.varB = b;
    }
    B.prototype = Object.create(A.prototype, {
      varB : {
        value: null, 
        enumerable: true, 
        configurable: true, 
        writable: true 
      },
      doSomething : { 
        value: function(){ // override
          A.prototype.doSomething.apply(this, arguments); // call super
          // ...
        },
        enumerable: true,
        configurable: true, 
        writable: true
      }
    });
    B.prototype.constructor = B;
    
    var b = new B();
    b.doSomething();
    

    The important parts are:

    • Types are defined in .prototype
    • You use Object.create() to inherit

    prototype and Object.getPrototypeOf

    JavaScript is a bit confusing for developers coming from Java or C++, as it's all dynamic, all runtime, and it has no classes at all. It's all just instances (objects). Even the "classes" we simulate are just a function object.

    You probably already noticed that our function A has a special property called prototype. This special property works with the JavaScript new operator. The reference to the prototype object is copied to the internal [[Prototype]] property of the new instance. For example, when you do var a1 = new A(), JavaScript (after creating the object in memory and before running function A() with this defined to it) sets a1.[[Prototype]] = A.prototype. When you then access properties of the instance, JavaScript first checks whether they exist on that object directly, and if not, it looks in [[Prototype]]. This means that all the stuff you define in prototype is effectively shared by all instances, and you can even later change parts of prototype and have the changes appear in all existing instances, if you wanted to.

    If, in the example above, you do var a1 = new A(); var a2 = new A(); then a1.doSomething would actually refer to Object.getPrototypeOf(a1).doSomething, which is the same as the A.prototype.doSomething you defined, i.e. Object.getPrototypeOf(a1).doSomething == Object.getPrototypeOf(a2).doSomething == A.prototype.doSomething.

    In short, prototype is for types, while Object.getPrototypeOf() is the same for instances.

    [[Prototype]] is looked at recursively, i.e. a1.doSomething, Object.getPrototypeOf(a1).doSomething, Object.getPrototypeOf(Object.getPrototypeOf(a1)).doSomething etc., until it's found or Object.getPrototypeOf returns null.

    So, when you call

    var o = new Foo();

    JavaScript actually just does

    var o = new Object();
    o.[[Prototype]] = Foo.prototype;
    Foo.call(o);

    (or something like that) and when you later do

    o.someProp;

    it checks whether o has a property someProp. If not it checks Object.getPrototypeOf(o).someProp and if that doesn't exist it checks Object.getPrototypeOf(Object.getPrototypeOf(o)).someProp and so on.

    In conclusion

    It is essential to understand the prototypal inheritance model before writing complex code that makes use of it. Also, be aware of the length of the prototype chains in your code and break them up if necessary to avoid possible performance problems. Further, the native prototypes should never be extended unless it is for the sake of compatibility with newer JavaScript features.

    Hide Sidebar