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

Question

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?

Answer 1

Here's a nice form of a loop I often use. You create the iterated variable from the for statement and you don't need to check the length property, which can be expensive specially when iterating through a NodeList. However, you must be careful, you can't use it if any of the values in array could be "falsy". In practice, I only use it when iterating over an array of objects that does not contain nulls (like a NodeList). But I love its syntactic sugar.

var list = [{a:1,b:2}, {a:3,b:5}, {a:8,b:2}, {a:4,b:1}, {a:0,b:8}];

for (var i=0, item; item = list[i]; i++) {
  // Look no need to do list[i] in the body of the loop
  console.log("Looping: index ", i, "item" + item);
}

Note that this can also be used to loop backwards.

var list = [{a:1,b:2}, {a:3,b:5}, {a:8,b:2}, {a:4,b:1}, {a:0,b:8}];
    
for (var i = list.length - 1, item; item = list[i]; i--) {
  console.log("Looping: index ", i, "item", item);
}

ES6 Update

for...of can be used for a cleaner syntax. Available since ES6

const a = ["a", "b", "c"];

for (const [index, element] of a.entries()) {
  console.log({index, element});
}

// If you don't need the index
for (const element of a) {
  console.log({element});
}
 

Answer 2

Note that in some cases, you need to loop in reverse order (but then you can use i-- too).

For example somebody wanted to use the new getElementsByClassName function to loop on elements of a given class and change this class. He found that only one out of two elements was changed (in FF3).
That's because the function returns a live NodeList, which thus reflects the changes in the Dom tree. Walking the list in reverse order avoided this issue.

var menus = document.getElementsByClassName("style2");
for (var i = menus.length - 1; i >= 0; i--)
{
  menus[i].className = "style1";
}

In increasing index progression, when we ask the index 1, FF inspects the Dom and skips the first item with style2, which is the 2nd of the original Dom, thus it returns the 3rd initial item!

Answer 3

I like doing:

 
var menu = document.getElementsByTagName('div');
for (var i = 0; menu[i]; i++) {
     ...
}

There is no call to the length of the array on every iteration.

Answer 4

At the risk of getting yelled at, i would get a javascript helper library like jquery or prototype they encapsulate the logic in nice methods - both have an .each method/iterator to do it - and they both strive to make it cross-browser compatible

EDIT: This answer was posted in 2008. Today much better constructs exist. This particular case could be solved with a .forEach.

Answer 5

I had a very similar problem earlier with document.getElementsByClassName(). I didn't know what a nodelist was at the time.

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

My issue was that I expected that elements would be an array, but it isn't. The nodelist Document.getElementsByTagName() returns is iterable, but you can't call array.prototype methods on it.

You can however populate an array with nodelist elements like this:

var myElements = [];
for (var i=0; i<myNodeList.length; i++) {                               
    var element = myNodeList[i];
    myElements.push(element);
};

After that you can feel free to call .innerHTML or .style or something on the elements of your array.

Answer 6

I think using the first form is probably the way to go, since it's probably by far the most common loop structure in the known universe, and since I don't believe the reverse loop saves you any time in reality (still doing an increment/decrement and a comparison on each iteration).

Code that is recognizable and readable to others is definitely a good thing.

Answer 7

I too advise to use the simple way (KISS !-)

-- but some optimization could be found, namely not to test the length of an array more than once:

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

Answer 8

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.

Answer 9

Form of loop provided by Juan Mendez is very useful and practical, I changed it a little bit, so that now it works with - false, null, zero and empty strings too.

var items = [
    true,
    false,
    null,
    0,
    ""
];

for(var i = 0, item; (item = items[i]) !== undefined; i++)
{
    console.log("Index: " + i + "; Value: " + item);
}

Answer 10

I know that you don't want to hear that, but: I consider the best practice is the most readable in this case. As long as the loop is not counting from here to the moon, the performance-gain will not be uhge enough.

Answer 11

I know this question is old -- but here's another, extremely simple solution ...

var elements = Array.from(document.querySelectorAll("div"));

// Loop.
elements.forEach(function(value) {
  yourFunction(value);
});

Answer 12

I prefer the for loop as it's more readable. Looping from length to 0 would be more efficient than looping from 0 to length. And using a reversed while loop is more efficient than a foor loop as you said. I don't have the link to the page with comparison results anymore but I remember that the difference varied on different browsers. For some browser the reversed while loop was twice as fast. However it makes no difference if you're looping "small" arrays. In your example case the length of elements will be "small"

Answer 13

I think you have two alternatives. For dom elements such as jQuery and like frameworks give you a good method of iteration. The second approach is the for loop.

Answer 14

I like to use a TreeWalker if the set of elements are children of a root node.