Javascript for loop efficiency

Question

Is

for (var i=0, cols=columns.length; i

more efficient than

for (var i=0; i

        

Answer 1

Any expression that's in the second portion of a for will be evaluated once per loop.

So, here, with your second proposition, yes, columns.length will be calculated each time the condition is checked -- which would make the first proposition faster than the second one.

(That's true for many other languages, btw)