Question

One interviewer asked me to explain what is this represented for in the following codes.
I had no idea about that.
Because I have not seen any codes like that before.
Please tell me is there any difference between these two and in what conditions such codes are useful?

function A(){
    this.x=1;//No.1
    function B(){
        this.y=2;//No.2
    }
}

I only know that when I use new to create a new object ,it has a property x equals to 1;

Answer 1

That depends on how you call the function. If you call it as a regular function, like this:

A();

then this will be the window object of the document.

If you use the function as an object constructor, like this:

var a = new A();

then this will be a reference to the newly created object.

The reference this in the B function will never be anything, as it’s not possible to call the function. It’s local inside the A function and as there is no code in the A function that could be use to call the B function, it will never be called.

If we make it possible to call the B function, the same thing applies as with the A function. What this is depends on how you use the function. Even if you call the B function from within the A function when the A function is used as a constructor (and thus this is an object reference in A), the B function doesn’t inherit this from A. It’s either a reference to window (when B is used as a function) or a reference to a newly created object (if B is used as a constructor).