JavaScript this.window not equal to window

Question

Consider the following top-level javascript code:

if (this.window === window)
    alert('same');
else
    alert('different'); // alerts: different  

Why is this.window and window not strictly equal? I've also tried 'this' on the rhs of the expression and get the same result.

Answer 1

In Internet Explorer (8.0.7600 is what I've tested), this with no qualifier actually resolves to the global window object. In all other browsers I've tried (Chrome, Firefox, Opera), this.window === window in that context - and, helpfully, this === window as well.

Try this in IE to verify:

if (this === window)
  alert('same');
else
  alert('different');

Answer 2

It seems as though HTML elements do not contain a pointer back to their parent window, as it does for parentNode. Thus, this.window will return undefined when this is anything other than a window object.

The window object seems to be able to reference itself, perhaps because it is the only node high enough to "see" itself. Thus, window == window.window.window.window and so on.

The idiosyncrasies between browsers seem to do with how each implements the DOM structure, and in particular, how they interpret this at the top-level.

Seeing as how individual HTML elements can't reference their parent window with .window, I don't really see a point in ever using this.window, though I'd love to be proved wrong here.

If you're working on code that involves manipulating objects across two different windows, I would suggest assigning your new window to a variable, e.g. var newWin = window.open(...) and subsequently using this variable to reference new objects.