Polymorphism in JavaScript

Introduction

Polymorphism in Object-Oriented Programming is the ability to create a variable, a function, or an object that has more than one form.

Background

The primary usage of Polymorphism in Object-Oriented Programming is the ability of objects belonging to different types to respond to methods, fields, or property calls of the same name, each one according to an appropriate type-specific behaviour.

The programmer does not have to know the exact type of the object in advance, and so the exact behaviour is determined at run-time. This is what we call late binding or dynamic binding (this make Polymorphism a very cool feature). Requirements are such that, there must be properties with the same name and the same parameter sets in all the superclasses, subclasses and interfaces.

Using the code

As far as JavaScript is concerned, we really don’t have interfaces as in other programming languages but we do have superclasses and subclasses. Going back to my article on JavaScript Inheritance, we have created classes that inherit from others.

Let’s reuse the same example, this time we will create a class Person then another class called Manager that inherits from Person. Both classes will include a method called wake_up(). We will then create objects of both types and put them in an array.

When we loop through that array, the system will call the function wake_up() on each of the objects and the implementation will be determined dynamically.

Let’s define our class Person.

Person = function (id, name, age) {
          this.id = id;
          this.name = name;
          this.age = age;
         // alert('A new person has been accepted');
      }
      /* definition of our Person class */
      Person.prototype = {
          /** wake person up */
          wake_up: function () {
              alert('A person is awake');
          },
          /** retrieve person's age */
          get_age: function () {
              return this.age;
          }
      }

Now let us create an Inheritance class so that other classes can inherit from our class person. For more details and demo on inheritance, refer to my article on JavaScript Inheritance by clicking here.

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; }

Ok we are then going to define our class Manager as follows:

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

Our class Manager will inherit from Person as follows:

Inheritance_Manager.extend(Manager, Person);

Let’s add more functionality and overwrite our wake_up() function

Manager.prototype = {
   wake_up: function () {
       alert('I am in control with Polymorphism');
   }
}

Now we create an array and store in objects of type Person and Manager.

var arrPeople = new Array();
arrPeople[0] = new Person(1, 'Joe Tester', 26);
arrPeople[1] = new Manager(1, 'Joe Tester', 26, '20.000');

See how the function wake_up() behaves based on the different objects we have. This is what we call Polymorphism. Polymorphism makes things so simple when you have many objects that present the same interface but different implementations.

for (var i in arrPeople) {
   arrPeople[i].wake_up();
   alert(arrPeople[i].get_age());
}

Many people don’t believe that JavaScript is fully-fledged Object Oriented Programming Language but I think JavaScript just does things its own way which is different from other languages. As we can see in the case of Polymorphism here, there is no difference with other programming languages. So it is fully polymorphic.

Here is the complete code:

/** This is our Person class */
       Person = function (id, name, age) {
           this.id = id;
           this.name = name;
           this.age = age;
          // alert('A new person has been accepted');
       }
       /* definition of our Person class */
       Person.prototype = {
           /** wake person up */
           wake_up: function () {
               alert('A person is 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 = {
           wake_up: function () {
               alert('I am in control');
           }
       }

       var arrPeople = new Array();
       arrPeople[0] = new Person(1, 'Joe Tester', 26);
       arrPeople[1] = new Manager(1, 'Joe Tester', 26, '20.000');

       for (var i in arrPeople) {
           arrPeople[i].wake_up();
           alert(arrPeople[i].get_age());
       }