Question

var a = function() {

  alert( this ); // [object Window]
  alert(  a   ); // undefined

}.call( a );

Why does this still refer to the window object and a undefined. Alternatively, when I do it the following way, this is equal to the function a is not undefined.

var a = function() {

  return this === a; // true

};

a.call(a);

Can anyone explain why these two functions that are seemingly equivalent give different results?

Answer 1

Because in your first example a isn’t assigned until after the call to your function completes.

In detail, here’s what happens in the first example:

1. The anonymous function is defined.
2. The function is called using a as the this parameter.
3. The function executes, but because a is still undefined, this falls back to window instead.
4. The function returns undefined, which is assigned to a.