Javascript Variables initialization types

Question

I need to know the difference between these three types of variable initialization.

1)- The first type is using the "this" keyword.

var x=function (data1,data2){
this.data1=data1;
this.data2=data2;
}

2)- Second is using the "var" keyword...from what i know the var keyword makes the scope of a variable local which means that var can only be accessible when we are inside the function.

var x=function (data1,data2){
var data1=data1;
var data2=data2;
}

3)- Third type is without using any keyword..from what i know is that we don't specify the var keyword with a variable then it becomes global for the complete application. So you can access it any where you want...but in this case i think this would throw an error...because how would javascript know if data1 is a variable or a parameter (the passed on argument) ?

var x=function (data1,data2){
data1=data1;
data2=data2;
}

Answer 1

1) this.foo = bar

The first kind you mention makes your variables properties of a (constructor) function. In JavaScript, functions are objects. Thus, using this makes a variable a property of an object (your constructor function, in this case).

2) var x = 7

Let me start off saying that anytime you do not use var in front of a variable (save for the this stuff above), that variable will become a member of the global context object (window, for most user-agents).

Thought leaders in JavaScript and programming see global variables as a sin, or code smell. Avoid global variables as much as possible.

The issue with the first method (this) is that prior to ECMAScript 5, there was no good way to enforce privacy on object properties (as you might find in the classical C++ based languages like Java, PHP, etc). ECMAScript 5 and above allow you to assign attributes to properties for things like writability, enumerability, etc...

However, as it pertains to var, well, var always makes its variables private to the function object. You cannot access a var variable from external, client code like this

dog.age = 5 //Trying to assign the number five to a var variable.

You can make a function that has access (known as an accessor), though. In effect, when you see var, think private to this object's scope. That's the gist of it. Closure (from enclosure) is an important subject in JavaScript. When you get into using var, inevitably you start to learn about closure.

3) No key word.

Finally, using naked variables attaches all of them to the global object (window, for most user-agents). This tends to lead to confusion and debugging issues. You may inadvertently stage a collision or conflict where a value is being changed across many instances of your constructor function. It's just not the way to go.

In summary, a fantastic book on the subject of JavaScript variables is The Principles of Object Oriented JavaScript. Mr. Zakas explains things much better than Crockford and Flannagan. I highly recommend it. Know your goal. Know your problem. Avoid global variables. Learn about JavaScript's object mode and scope. I recommend JavaScript: The Definitive Guide when it comes to learning about scope and context in JavaScript.