Why is iterating through an array backwards faster than forwards
Given this code:
var arr = [];
for (var i = 0; i < 10000; ++i)
arr.push(1);
Forwards
for (var i = 0; i < arr.length; ++i) {}
Backwards
for (var i = arr.length - 1; i >= 0; --i) {}
Hard-coded Forward
for (var i = 0; i < 10000; ++i) {}
Why is backwards so much faster?
Here is the test: http://jsperf.com/array-iteration-direction
Because your forwards-condition has to receive the length
property of your array each time, whilst the other condition only has to check for "greater then zero", a very fast task.
When your array length doesn't change during the loop, and you really look at ns-perfomance, you can use
for (var i=0, l=arr.length; i<l; i++)
BTW: Instead of for (var i = arr.length; i > 0; --i)
you might use for (var i = arr.length; i-- > 0; )
which really runs through your array from n-1 to 0, not from n to 1.
Because in the first form you are accessing the property length
of the array arr
once for every iteration, whereas in the second you only do it once.