How to implement inheritance in JavaScript

Introduction

If you have done Object Oriented Programming in JavaScript, you will know that you can create a class as follows:

Person = function(id, name, age){
    this.id = id;
    this.name = name; 
    this.age = age;

    alert('A new person has been accepted');
}

So far our class Person only has two properties and we are going to give it some methods. A clean way of doing this is to use its 'prototype' object.

Starting from JavaScript 1.1, the prototype object was introduced in JavaScript. This is a built in object that simplifies the process of adding custom properties and methods to all instances of an object.

Let's add two methods to our class using its 'prototype' object as follows:

Person.prototype = {
    /** wake person up */
    wake_up: function() {
        alert('I am awake');
    },


    /** retrieve person's age */
    get_age: function() {
        return this.age;
    }
}

Now we have defined our class Person. What if we wanted to define another class called Manager which inherits some properties from Person? There is no point redefining all these properties again when we define our Manager class, we can just set it to inherit from the class Person.

JavaScript doesn't have built-in inheritance but we can use a technique to implement inheritance as follows:

Inheritance_Manager = {};//We create an inheritance manager class (the name is arbitrary)

Now let's give our inheritance class a method called Extend which takes the baseClass and subClassas arguments. Within the Extend method, we will create an inner class with a function inheritance() { }. The reason why we are using this inner class is to avoid confusion between the baseClass and subClass prototypes.

Next we make the prototype of our inheritance class point to the baseClass prototype as in the following code:

inheritance.prototype = baseClass. prototype;

Then we copy the inheritance prototype into the subClass prototype as follows:

subClass.prototype = new inheritance();

The next thing is to specify the constructor for our subClass as follows:

subClass.prototype.constructor = subClass;

Once finished with our subClass prototyping, we can specify the next two lines of code to set some base class pointers.

subClass.baseConstructor = baseClass;
subClass.superClass = baseClass.prototype;

Here is the full code for our Extend function:

Inheritance_Manager.extend = function(subClass, baseClass) {
    function inheritance() { }
    inheritance.prototype = baseClass.prototype;
    subClass.prototype = new inheritance();
    subClass.prototype.constructor = subClass;
    subClass.baseConstructor = baseClass;

    subClass.superClass = baseClass.prototype;
}

Now that we have implemented our inheritance, we can start using it to extend our classes. In this case, we are going to extend our Person class into a Manager class as follows:

We define the Manager class:

Manager = function(id, name, age, salary) {
    Person.baseConstructor.call(this, id, name, age);
    this.salary = salary;
    alert('A manager has been registered.');
}

We make it inherit form Person:

Inheritance_Manager.extend(Manager, Person);

If you have noticed, we have just called the Extend method of our Inheritance_Manager class and passed the subClass Manager in our case and then the baseClass Person. Note that the order is very important here. If you swap them, the inheritance will not work as you intended if at all.

Also note that you will need to specify this inheritance before you can actually define the subClass.

Now let us define the subClass:

We can add more methods as shown below. Our Manager class will always have the methods and properties defined in the Person class because it inherits from it.

Manager.prototype.lead = function(){
   alert('I am a good leader');
}

Now to test it, let us create two objects, one from the class Person and one from the inherited class Manager:

var p = new Person(1, 'Joe Tester', 26);
var pm = new Manager(1, 'Joe Tester', 26, '20.000');

Here is the full JavaScript code you can copy:

Person = function(id, name, age){
    this.id = id;
    this.name = name;
    this.age = age;
    alert('A new person has been accepted');
}

Person.prototype = {
    /** wake person up */
    wake_up: function() {
        alert('I am awake');
    },

    /** retrieve person's age */
    get_age: function() {
        return this.age;
    }
}

Inheritance_Manager = {};

Inheritance_Manager.extend = function(subClass, baseClass) {
    function inheritance() { }
    inheritance.prototype = baseClass.prototype;
    subClass.prototype = new inheritance();
    subClass.prototype.constructor = subClass;
    subClass.baseConstructor = baseClass;
    subClass.superClass = baseClass.prototype;
}

Manager = function(id, name, age, salary) {
    Manager.baseConstructor.call(this, id, name, age);
    this.salary = salary;
    alert('A manager has been registered.');
}

Inheritance_Manager.extend(Manager, Person);

Manager.prototype.lead = function(){
   alert('I am a good leader');
}

var p = new Person(1, 'Joe Tester', 26);
var pm = new Manager(1, 'Joe Tester', 26, '20.000');

You can manipulate these objects by accessing some of their properties as follows:

alert(p.name);
alert(pm.salary);

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