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 = 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: https://jsperf.app/array-iteration-direction

Note that your backwards case should start at arr.length-1 and have i >= 0 rather than i > 0. — nnnnnn
– nnnnnn, Commented Jan 1, 2012 at 2:30
The title is misleading. It is no longer the case that directly using arr.length is slower in advanced browsers. — user2864740
– user2864740, Commented Oct 4, 2014 at 7:05
possible duplicate of JavaScript loop performance - Why is to decrement the iterator toward 0 faster than incrementing — Bergi
– Bergi, Commented Jun 27, 2015 at 16:50

Bergi · Accepted Answer · 2011-12-31 17:54:45Z

90

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.

answered Dec 31, 2011 at 17:54

Bergi

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12 Comments

Jeffrey Sweeney Over a year ago

Yep, this is how I do it for optimized functions. Just note that if the array is mutable (and it usually isn't), this may skip a resource or two. For example, this won't work if you're looping through elements that the loop itself is adding or deleting.

ajax333221 Over a year ago

you can even do for(... ; i-- ;)

Breno Inojosa Over a year ago

so basically it means that if I don't change the array and I keep array.length in a variable, then backwards speed = forward speed? Example: var length = array.length; for(var i = 0 ; i<length ; i++);

Bergi Over a year ago

afaik, i<l is a different computation than i>0, but yes they're rather equal (faster than uncached length): jsperf.com/array-iteration-direction/3 - yet some engines do recognize and optimize some of these cases

Gaurang Patel Over a year ago

Fast forward to 2018. it looks like the modern versions of browsers don't make loop slower even when accessing arr.length in each iteration (your forwards case)

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rabusmar · Accepted Answer · 2011-12-31 17:43:13Z

9

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.

answered Dec 31, 2011 at 17:43

rabusmar

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2 Comments

jfriend00 Over a year ago

@samccone - what result are you seeing (which test in which browser)? I'm seeing that caching the length is always at least as fast and usually faster.

samccone Over a year ago

alright you are correct... it looks like it is really a mixed bag of results tho but caching does help it... thanks XD

tcak · Accepted Answer · 2011-12-31 17:46:34Z

7

If you want to have them at same pace, you can do that for forward iteration;

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

So, your script won't need to take length of array on everystep.

answered Dec 31, 2011 at 17:46

tcak

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1 Comment

samccone Over a year ago

Ahhh... interesting your method now takes the crown for fastest iteration jsperf.com/array-iteration-direction

Gopal Nair · Accepted Answer · 2011-12-31 17:49:46Z

5

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!

answered Dec 31, 2011 at 17:49

Gopal Nair

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Comments

jfriend00 · Accepted Answer · 2011-12-31 17:43:55Z

3

i > 0 is faster than i < arr.length and is occurring on each iteration of the loop.

You can mitigate the difference with this:

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

This is still not as fast as the backwards item, but faster than your forward option.

answered Dec 31, 2011 at 17:43

jfriend00

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1 Comment

samccone Over a year ago

look at the hard coded test jsperf.com/array-iteration-direction ... using your logic it should be faster no?

dku.rajkumar · Accepted Answer · 2011-12-31 17:51:55Z

2

do it like below, it will perform in same way. because arr.length takes time in each iteration in forward.

int len = arr.length;

forward

for (var i = 0; i < len; ++i) {
}

backward

for (var i = len; i > 0; --i) {
}

answered Dec 31, 2011 at 17:51

dku.rajkumar

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2 Comments

Mike Dunlavey Over a year ago

The backward case isn't right. What I do is for(i=len; --i>=0;){

Vincent Over a year ago

for (let i = arr.length; i-- > 0;) {

Noctis · Accepted Answer · 2014-01-15 03:58:14Z

2

And these are equally good:

var arr= [], L= 10000;
while(L>-1) arr[L]= L--;

OR

var arr= [], i= 0;
while(i<10001) arr[i]=i++;

edited Jan 15, 2014 at 3:58

Noctis

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answered Dec 31, 2011 at 17:49

kennebec

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1 Comment

Mike Dunlavey Over a year ago

It should be while(--L>=0) arr[L] = L;

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