Which is more efficient? (Variables for JavaScript loop)

Question

To keep it simple, sat at my desk wondering if there is a difference in efficiency between (in JavaScript):

var i = 0;

for(i=0; i<Something.length; i++) foo();

for(i=0; i<Something.length; i++) foo();

And...

for(var i=0; i<Something.length; i++) foo();

for(var i=0; i<Something.length; i++) foo();

And what results did you get with performance tests before giving up and posting on SO? — skyfoot
– skyfoot, Commented Oct 29, 2013 at 9:21
Just looking at the JSperfs now. Wow! People reply quick on here. Will assess and accept an answer in abit. — User2
– User2, Commented Oct 29, 2013 at 9:51

krasu · Accepted Answer · 2013-10-29 09:46:36Z

4

This one will be faster, you will cache Something.length in variable so it will not be interpreted during loop:

for(var i=0, len = Something.length; i<len; i++) foo();

Here is a test

But moving var definition from loop will be really a bit faster without caching

edited Oct 29, 2013 at 9:46

answered Oct 29, 2013 at 9:22

krasu

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User2 Over a year ago

Accepted this, because of the JSperf. Helpful, thanks. Seems the fastest is creating the variables out side of the for loop. Thanks

Ovilia · Accepted Answer · 2013-10-29 09:24:54Z

2

I tried with 1000000 loops, the first runs for 2.7sec, the second runs for 2.418sec.

Apparently, the first one is faster.

But as JavaScript has no block scope, i in both cases will still be available out of the loop and you may have error if you forget to init it later.

answered Oct 29, 2013 at 9:24

Ovilia

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