Extension of JavaScript "typeof"
I don't think much of it, to be honest.  It's not as reliable as you might think, because once you start passing objects between contexts (e.g. different windows) it will fail miserably.  Add to that the fact that 

Object.prototype.toString.call(&lt;any value&gt;);


could do most of your work for you and your extension starts to look a little redundant and unnecessary.

Here's a detailed article on this problem: Cross-context isArray and Internet Explorer