I'd say it's safe to eliminate the Object {} scope from the toString() call.

You can even eliminate the toString call completely, because .test() coerces its first argument into String.

Thanks for your ideas, @nikola and @atk!
However,
@nikola: This may lead to error in older IE versions.
@atk: Something like 'w]' will pass the test.

You could try a&&a.alert additionally

@atk: Thanks again! But an fake object {alert:1,toString:function(){return 'w]'}} still passes the test.

That's true. How about (a||0).constructor===this.Window? Works in all modern browsers (not in IE 7 or older).

Not in Chrome 14.

this.constructor !== window

Damn, there it's DOMWindow... /Wi/.test(a.constructor), maybe?

{constructor:'Wi'}

Let's try this:

function(a,b){b=this.Window||this.DOMWindow;return!!a&&b?a instanceof b:a==(b=a.document)&&b!=a}

It is sad that it won't work with iframes...

Rewritten to probably work with (i)frames:

function(a,b){return!!a&&(b=a.Window||a.DOMWindow)?a instanceof b:a==(b=a.document)&&b!=a}

It cannot work with my chromium 12...

Alas, it's not exposed in Chrome. OK, let's try this:

function(a,b){return!!a&&(b=top.constructor)?a instanceof b:a==(b=a.document)&&b!=a}

It won't work with (i)frame as well, because the scope of top differs from (i)frame.

Right. Any idea how to get DOMWindow in Chrome if its not exposed?

The magic __proto__?

Doesn't help very much, because it is different between frames.

I think it is hard. In fact, the original code is enough.

Try this:

function(a,b){return a&&a==a[b='window']&&a==a[b][b]}

Clever. Exploiting the fact that any global object is exposed as a property of window, and window itself is a global object.

@phoetry: It's clever but it won't work.

var a = {};
a.window = a;
(function(a,b){return a&&a==a[b='window']&&a==a[b][b]})(a) // true