Why is iterating through an array backwards faster than forwards

Question

Given this code:

var arr = [];

for (var i = 0; i < 10000; ++i)
    arr.push(1);

Forwards

for (var i =          


        

Answer 1

I am not entirely sure about this, but here is my guess:

For the following code:

for (var i = 0; i < arr.length; ++i) {;
}

During runtime, there is an arr.length calculation after each loop pass. This may be a trivial operation when it stands alone, but may have an impact when it comes to multiple/huge arrays. Can you try the following:

 var numItems = arr.length;
    for(var i=0; i< numItems; ++i)
    {
    }

In the above code, we compute the array length just once, and operate with that computed number, rather than performing the length computation over and over again.

Again, just putting my thoughts out here. Interesting observation indeed!