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What's the best way to loop through a set of elements in JavaScript?

Asked 2008-10-01T11:57:49.010
68

In the past and with most my current projects I tend to use a for loop like this:

var elements = document.getElementsByTagName('div');
for (var i=0; i<elements.length; i++) {
    doSomething(elements[i]);
}

I've heard that using a "reverse while" loop is quicker but I have no real way to confirm this:

var elements = document.getElementsByTagName('div'), 
    length = elements.length;

while(length--) {
    doSomething(elements[length]);
}

What is considered as best practice when it comes to looping though elements in JavaScript, or any array for that matter?

javascript arrays loops element
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1 Answer

4

Also see my comment on Andrew Hedges' test ...

I just tried to run a test to compare a simple iteration, the optimization I introduced and the reverse do/while, where the elements in an array was tested in every loop.

And alas, no surprise, the three browsers I tested had very different results, though the optimized simple iteration was fastest in all !-)

Test:

An array with 500,000 elements build outside the real test, for every iteration the value of the specific array-element is revealed.

Test run 10 times.

IE6:

Results:

Simple: 984,922,937,984,891,907,906,891,906,906

Average: 923.40 ms.

Optimized: 766,766,844,797,750,750,765,765,766,766

Average: 773.50 ms.

Reverse do/while: 3375,1328,1516,1344,1375,1406,1688,1344,1297,1265

Average: 1593.80 ms. (Note one especially awkward result)

Opera 9.52:

Results:

Simple: 344,343,344,359,343,359,344,359,359,359

Average: 351.30 ms.

Optimized: 281,297,297,297,297,281,281,297,281,281

Average: 289.00 ms

Reverse do/while: 391,407,391,391,500,407,407,406,406,406

Average: 411.20 ms.

FireFox 3.0.1:

Results:

Simple: 278,251,259,245,243,242,259,246,247,256

Average: 252.60 ms.

Optimized: 267,222,223,226,223,230,221,231,224,230

Average: 229.70 ms.

Reverse do/while: 414,381,389,383,388,389,381,387,400,379

Average: 389.10 ms.

answered 2008-10-02T10:22:23.663

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