Sample JavaScript example

[Arjun Reddy]

Hi All,

Why does the following example not work? The class and the methods both are declared by the keyword function here in JavaScript? Please also look at the comments I have put, if they are right or wrong.

function Test()  //Similar to a class
{
this.value1  //Similar to instance variables and are always represented by this.something syntax
this.value2
}

function sum() //Similar to a method?
{
document.write(obj1.value1 + obj1.value2)
}

var obj1 = new Test() //Instantiating the class Test here and creatin an object obj1
obj1.value1=10
obj1.value2=20
obj1.sum() //Calling obj1.sum does not work either

Thanks.

[Bear Bibeault]

Arjun Reddy wrote:Why does the following example not work?

Making us guess what "not working" means isn't conducive to getting your question answered.

However, I think I'm guessing that you somehow expect sum() to magically act as a "method".

Firstly, trying to think of JavaScript as having "classes" is an analogy that breaks down rapidly. JavaScript is prototype-based rather than class-based, and a functional rather than an OO language.

So if you want a function to act as a "method" of the constructor, it needs to be a property of the constructor -- either transient or prototyped.

So...

Test.prototype.sum = function() { ...whatever... }
    

[Arjun Reddy]

Thanks for replying Bibeault. I got it. Below code works fine now.

function Test()  {
this.value1
this.value2
this.result=sum
}

function sum() 
{
document.write(obj1.value1 + obj1.value2)
}

var obj1 = new Test()
obj1.value1=10
obj1.value2=20
obj1.result()

[Arjun Reddy]

I however have one more question though.

The following codes both are giving the same output so why use prototype actually? because I read that prototype object is used to make sure all the instances of an object have same property value.

Code 1:

   function Test()  {
this.value1=10
}


var obj1 = new Test()
var obj2 = new Test()
document.write(obj1.value1)
document.write(obj2.value1)
 

Code 2:

  function Test()  {
}

Test.prototype.value1=10

var obj1 = new Test()
var obj2 = new Test()
document.write(obj1.value1)
document.write(obj2.value1)  

Thanks.

[Bear Bibeault]

Either way.

[Arjun Reddy]

Bear Bibeault wrote:Either way.

Ok, In this case we can do it either way but is there any other use of the prototype object?

Thanks.

[Bear Bibeault]

The prototype is basically a blueprint from which new versions of instances created via the constructor will be copied. By placing something in the prototype, you guarantee it will be part of every instance. Adding it, ad hoc, in the constructor has the same outward effect just through a different mechanism.

[Arjun Reddy]

Ok. Thanks Bibeault.