I recently signed up for Stephen Chapman's Javascript Guide weekly newsletter. I thought I knew a fair bit about javascript, I'd programmed most of the modern patterns used in javascript but apparently there were some gaping holes in my knowledge...

 

 

 
From Stephen Chapman, your Guide to JavaScript

The conventional way to write a for loop in javascript:

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

The problem with this is that evaluating the length of the array using myArray.length takes time and the way that we have coded the loop means that this evaluation has to be performed every time around the loop. If the array contains 1000 elements then the length of the array will be evaluated 1001 times.

 

What we can do to correct this for a loop where the size is fixed is to evaluate the length once at the start of the loop and save it in a variable. We can then test the variable to decide when to terminate the loop. This is much faster than evaluating the array length each time particularly when the array contains more than just a few entries or is part of the web page.

 

 

The code to do this is:

for (var i = 0, var j = myArray.length; i < j; i++) {...

So now we only evaluate the size of the array once and test our loop counter against the variable that holds that value each time around the loop. This extra variable can be accessed much faster than evaluating the size of the array and so our code will run much faster than before. We just have one extra variable in our script.