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Possible Duplicate:
Use of 'prototype' vs. 'this' in Javascript?

I am confused with these two type of adding a method to a function. Let me explain with an example.

var foo = function(){
    this.bar = function(){alert('I am a method')}
}

foo.prototype.baz = function(){alert('I am another method')}

var car = new foo();

Now at this point we can use baz and bar methods for car. Well, but what is the difference between them. What is the nuance adding a method to function's prototype or it's constructor.

Thanks..

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Fatih Acet
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2 Answers2

9

Functions assigned to the prototype will be shared by all instances; functions assigned in the constructor will have a separate function object per instance.

Also, functions assign in the constructor can use the constructor's variables and parameters.

For example:

var foo = function(param){
    this.bar = function() { alert('I can see a parameter: ' + param); };
}

foo.prototype.baz = function() { alert('I can't see foo's parameters'); };

var car = new foo("Hi there!");
car.bar();
SLaks
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1
var foo = function(){
    this.bar = function(){alert('I am a method')}
}

This gives each foo instance a bar method of its own. So,

 var car1 = new foo();
 var car2 = new foo();
 car2.bar = //new function definition here.
 //car1.bar doesn't change.

In the second case:

foo.prototype.baz = function(){alert('I am another method')}

You are attaching a function to the prototype chain of the function foo. So, each instance gets the same method. Changing baz would change the function on each instance as each instance inherits the same function.

The advantage of the second approach is clear in terms of inheritance. Also, in the first approach, since you are attaching a method to each specific instance, it won't scale if you are creating several foo instances as each instance would unnecessarily have a copy of the same method.

SLaks
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Rajat
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  • **Wrong**. Re-assigning an instance member will hide the prototype for that instance. – SLaks Nov 24 '10 at 19:02
  • So after these two satisfactory answer, I will understand that, if adding a method with prototype all instances shares the method and this is about memory management issues. Am I right? – Fatih Acet Nov 24 '10 at 19:05
  • @Lorenzo: Unless you're creating millions of instances, don't worry about it. – SLaks Nov 24 '10 at 19:06
  • @SLaks I understand that reassigning an instance member will hide the prototype. But i meant that changes to the function baz are live meaning the instances automatically inherit the new function definition. – Rajat Nov 24 '10 at 20:00
  • @Rajat: Do you mean changes to the prototype? If so, you should clarify. – SLaks Nov 24 '10 at 20:04
  • @SLaks Yeah, thats what I meant. In hindsight, I think my sentence construction was a little confusing. Thanks for the correction though. – Rajat Nov 24 '10 at 20:19