Are loops really faster in reverse?

Question

I've heard this quite a few times. Are JavaScript loops really faster when counting backward? If so, why? I've seen a few test suite examples showing that reversed loops are quicker, but I can't find any explanation as to why!

I'm assuming it's because the loop no longer has to evaluate a property each time it checks to see if it's finished and it just checks against the final numeric value.

I.e.

for (var i = count - 1; i >= 0; i--) {   // count is only evaluated once and then the comparison is always on 0. } 

Answer 1

It's not that i-- is faster than i++. Actually, they're both equally fast.

What takes time in ascending loops is evaluating, for each i, the size of your array. In this loop:

for(var i = array.length; i--;) 

You evaluate .length only once, when you declare i, whereas for this loop

for(var i = 1; i <= array.length; i++) 

you evaluate .length each time you increment i, when you check if i <= array.length.

In most cases you shouldn't even worry about this kind of optimization.

Answer 2

This guy compared a lot of loops in javascript, in a lot of browsers. He also has a test suite so you can run them yourself.

In all cases (unless I missed one in my read) the fastest loop was:

var i = arr.length; //or 10 while(i--) {   //... }