[insin]

NaN is passed to Array.prototype.join() as the separator to join strings with, so it gets coerced to String ("NaN"). It's deliberately working backward from the behaviour of Array.prototype.join() to create a contrived example for giggles, not a WTF.

It's no more a WTF than this, which follows the same principle: https://gist.github.com/insin/1183916

[moron4hire]

Oh, I understand that. I was more trying to explain why someone might think it was a WTF.

And I'm pretty sure your example returns the decimal 15. Comma operator returns the last element in the list, and parseInt will truncate strings with non-number-like text to the number-like part. Here, the number-like part is a hexadecimal code, triggering another feature of parseInt that it can figure out the radix on the fly.