JavaScript: that vs this

Question

I am trying to understand better the use of that and this in JavaScript. I am following Douglas Crockford's tutorial here: http://javascript.crockford.com/private.html but I am confused regarding a couple of things. I have given an example below, and I would like to know if I am making a correct use of them:

function ObjectC()
{
   //...
}

function ObjectA(givenB)
{
   ObjectC.call(this); //is the use of this correct here or do we need that?

   var aa = givenB;
   var that = this;

   function myA ()
   {
      that.getA(); //is the use of that correct or do we need this?
   }

   this.getA = function() //is the use of this correct?
   {
       console.log("ObjectA");
   };


}

 function ObjectB()
 {

    var that = this;

    var bb = new ObjectA(that); //is the use of that correct or do we need this?

    this.getB = function()
    {
        return bb;
    };

    that.getB(); //is the use of that correct or do we need this?


 }

Note this is just an example.

Answer 1

this in JavaScript always refers to current object, method of which was called. But sometimes you need to access this of your object in deeper. For example, in callbacks. Like so:

function MyClass() {
    this.a = 10;
    this.do = function() {
        http.get('blablabla', function(data) {
            this.a = data.new_a;
        });
    };
}

It will not work, because this in callback may refer to http, to some dom element or just window(which is really common). So, it is common solution to define self or that, an alias for this or your object, so you can refer it anywhere inside.

function MyClass() {
    var self = this;
    this.a = 10;
    this.do = function() {
        http.get('blablabla', function(data) {
            self.a = data.new_a;
        });
    };
}

This should give you vision why it is used and how it should be used.

There is no other reasons(currect me if I'm wrong) to create special variable, you can use this to send your object to other objects and do things, many assignments, such logic, wow...