Prototype Objects in JavaScript

We have been hearing this term, Prototype Object, in JavaScript and so on. But how does a Prototype Object play an important role in JavaScript? Let's try to understand that and implement it.


A Prototype Object is an object that simplifies the process of adding custom properties/methods to all instances of an object. Basicly, in JavaScript you are allowed to add custom properties to Prebuilt and Custom objects.

The following briefly describes Prebuilt and Custom Objects in JavaScript:

  • Prebuilt: These are the objects that are created with the new keyword such as image, string, date, array object and so on.
  • Custom: These are the objects that are created by the developer to hold the properties or other information, for example:


  1. var Person = {};  
When we talk about Object Oriented Programming, we always think of how to create objects in JavaScript. Let's see come code in action to create and invoke objects in JavaScript.
  1. <script language="javascript" type="text/javascript">  
  2.     function Greet(mode) {  
  3.         this.mode = mode;  
  4.         this.callgreet = function() {  
  5.             alert( 'Good ' + this.mode)  
  6.         }  
  7.     }  
  8.     obj1 = new Greet("Morning")  
  9.     obj1.callgreet() //alerts "Good Morning"  
  10.     obj2 = new Greet("Evening")  
  11.     obj2.callgreet() //alerts "Good Evening"  
  12. </script>  
As we can see, the preceding structure is called an “Object Constructor” where the object is instantiated using the new keyword. Methods and properties are declared inside.

This flexibility allows us to create multiple instances of the Object (Greet).

But the limitation with the preceding approach is to define everything inside the function/class. Every time I need to pass an object with different values in the constructor, it becomes unreliable or tedious to write everything in the same place.

The approach that I will be taking is Prototyping that logically allows attachment of a method to an object after it's been defined. Let's see the code above re-structured in Prototype style.
  1. function Greet(mode) {  
  2.     this.mode = mode;  
  3.     this.callgreet = function() {  
  4.         alert( 'Good ' + this.mode)  
  5.     }  
  6. }   
  7. // Attaching changeGreet function with existing object using Prototype.  
  8. Greet.prototype.changeGreet = function(greetmode){  
  9.     this.mode = greetmode;  
  10. }  
  12. obj1 = new Greet("Morning")  
  13. obj1.callgreet() //alerts "Good Morning"  
  14. obj2 = new Greet("Evening")  
  15. obj2.callgreet() //alerts "Good Evening"  
  16. obj2.changeGreet("Afternoon");  
  17. obj2.callgreet();  
  1. Easy to attach a new custom method to an object that is already defined/created.
  2. The new Custom Method is now shared/accessed by all instances of the object. In our case a new custom method (changeGreet) is accessible by obj1 and obj2.

I hope this article tried to explain the concept of prototyping, including how to use Prototyping to attach custom methods and properties.

Thanks for reading.

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