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That was my first thought. I ran the "slow loop" through the online compiler at:

And whether I checked "Simple" or "Advanced" I got:

for(var a = 0;a < arr.length;a++)

So it's not optimizing that pattern yet.

It would have to determine that arr.length doesn't change during the loop. Pretty difficult problem. :)

I'm unsure if the smiley means you're being sarcastic, but if not:

Such determinations are a pretty common thing for an optimizing compiler to attempt: