best intuitive solution so far

Awesome thanks!

for...in loops over enumerable properties. That means if any method was added to Array.prototype incorrectly or lazily (or on purpose) that was enumerable, it would show up in the loop (and is probably undesired).

Just curious, what is the danger in using for...in with arrays?

A lot of repeated code here.. remember the DRY principle.

w isn't declared anywhere, so you're creating a global variable. And using for...in with arrays isn't usually a good idea (even if it worked in your solution). Either use a normal for loop or one of the Array.prototype methods.

Thanks.
For the performance: jsperf
It seems to be faster Math.max, on Chrome and Firefox.

You could use Math.max to calculate the length. I'm not sure which is faster tho.