Prototype in JavaScript

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In this video we will discuss Prototype object in JavaScript. Let us understand it's use with an example.



The following Employee constructor function constructs Employee object. 
function Employee(name) 
{
    this.name = name;
}

There are several ways to add a function to the Employee object. One way is as shown below. This works but the problem with this approach is that, if you create 100 employee objects there will be 100 copies of getName() function. We don't want to be creating copies of functions, instead we want all the objects to share the same function code. This can be achieved using JavaScript prototype object.

function Employee(name) 
{
    this.name = name;

    this.getName = function () 
    {
        return this.name;
    }
}

var e1 = new Employee("Mark");
var e2 = new Employee("Sara");

document.write("e1.name = " + e1.getName() + "<br/>");
document.write("e2.name = " + e2.getName() + "<br/>");

Output : 
e1.name = Mark
e2.name = Sara

In this example, getName() function is added just to e1 object, and not to e2 object. So e2.getName() would throw an undefined error.

function Employee(name)

{

    this.name = name;

}

var e1 = new Employee("Mark");

e1.getName = function ()

{

    return this.name;

}

var e2 = new Employee("Sara");

document.write("e1.name = " + e1.getName() + "<br/>");

document.write("e2.name = " + e2.getName() + "<br/>");

In the following example getName() function is added to the Employee object using the name of the constructor function. This would also result in undefined error.

function Employee(name)

{

    this.name = name;

}

Employee.getName = function ()

{

    return this.name;

}

var e1 = new Employee("Mark");

var e2 = new Employee("Sara");

document.write("e1.name = " + e1.getName() + "<br/>");

document.write("e2.name = " + e2.getName() + "<br/>");

Using the prototype object to add functions : The following are the advantages of using the prototype object to add functions.
1. No matter how many objects you create, functions are loaded only once into memory
2. Allows you to override functions if required.

function Employee(name)

{

    this.name = name;

}

Employee.prototype.getName = function ()

{

    return this.name;

}

var e1 = new Employee("Mark");

var e2 = new Employee("Sara");

document.write("e1.name = " + e1.getName() + "<br/>");

document.write("e2.name = " + e2.getName() + "<br/>");

Output :
e1.name = Mark
e2.name = Sara