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Is there a more efficient way to convert an HTMLCollection to an Array, other than iterating through the contents of said collection and manually pushing each item into an array?

Is there a more efficient way to convert an HTMLCollection to an Array, other than iterating through the contents of said collection and manually pushing each item into an array?

• javascript
• arrays
• object

Share Improve this question Follow asked Oct 21, 2008 at 18:04 TomTom 15.9k55 gold badges5050 silver badges6363 bronze badges 5

• 14 What is meant by "efficient"? If best performing, a for loop is generally faster than Array.prototype.slice. A loop also works in a wider variety of browsers (i.e. all), so by those criteria it is the "most efficient way". And it's very little code: for (var a=[], i=collection.length; i;) a[--i] = collection[i]; so not much of a "con" there :-) – RobG Commented Jan 28, 2015 at 20:18
• 1 Looking at current browser performance, slice has mostly caught up with loops in terms of performance, except in Chrome. Using a larger number of elements and slight optimisation of the loop, the results are almost identical, except in Chrome where a loop is very much faster. – RobG Commented Jan 4, 2016 at 23:17
• I created a jsperf test that looks at both methods that @harpo mentioned as well as a jquery test for performance. I've found jquery is slightly slower than both javascript methods and top performance varies between the js test cases. Chrome 59.0.3071 / Mac OS X 10.12.5 prefers using Array.prototype.slice.call and Brave (based on Chrome 59.0.3071) has virtually no difference between the two javascript tests over multiple runs. See jsperf.com/htmlcollection-array-vs-jquery-children – NuclearPeon Commented Jul 11, 2017 at 23:31
• 1 jsben.ch/h2IFA => performance test for the most common ways to do this – EscapeNetscape Commented Feb 13, 2020 at 15:24
• If my HTMLCollection includes dynamic links, how do I convert into an array while maintaining the links? When I convert into an array using a for loop, it comes in as text only. I lose the links because of innerHTML – bfoley_teamug Commented Jul 7, 2020 at 18:51

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10 Answers 10

Sorted by: Reset to default 1135

var arr = Array.prototype.slice.call( htmlCollection )

will have the same effect using "native" code.

Edit

Since this gets a lot of views, note (per @oriol's comment) that the following more concise expression is effectively equivalent:

var arr = [].slice.call(htmlCollection);

But note per @JussiR's comment, that unlike the "verbose" form, it does create an empty, unused, and indeed unusable array instance in the process. What compilers do about this is outside the programmer's ken.

Edit

Since ECMAScript 2015 (ES 6) there is also Array.from:

var arr = Array.from(htmlCollection);

Edit

ECMAScript 2015 also provides the spread operator, which is functionally equivalent to Array.from (although note that Array.from supports a mapping function as the second argument).

var arr = [...htmlCollection];

I've confirmed that both of the above work on NodeList.

A performance comparison for the mentioned methods: http://jsben.ch/h2IFA

not sure if this is the most efficient, but a concise ES6 syntax might be:

let arry = [...htmlCollection] 

---

Edit: Another one, from Chris_F comment:

let arry = Array.from(htmlCollection)

I saw a more concise method of getting Array.prototype methods in general that works just as well. Converting an HTMLCollection object into an Array object is demonstrated below:

[].slice.call( yourHTMLCollectionObject );

And, as mentioned in the comments, for old browsers such as IE7 and earlier, you simply have to use a compatibility function, like:

function toArray(x) {
    for(var i = 0, a = []; i < x.length; i++)
        a.push(x[i]);

    return a
}

I know this is an old question, but I felt the accepted answer was a little incomplete; so I thought I'd throw this out there FWIW.

For a cross browser implementation I'd sugguest you look at prototype.js $A function

copyed from 1.6.1:

function $A(iterable) {
  if (!iterable) return [];
  if ('toArray' in Object(iterable)) return iterable.toArray();
  var length = iterable.length || 0, results = new Array(length);
  while (length--) results[length] = iterable[length];
  return results;
}

It doesn't use Array.prototype.slice probably because it isn't available on every browser. I'm afraid the performance is pretty bad as there a the fall back is a javascript loop over the iterable.

This works in all browsers including earlier IE versions.

var arr = [];
[].push.apply(arr, htmlCollection);

Since jsperf is still down at the moment, here is a jsfiddle that compares the performance of different methods. https://jsfiddle.net/qw9qf48j/

I suppose that calling Array.prototype functions on instances of HTMLCollection is a much better option than converting collections to arrays (e.g.,[...collection] or Array.from(collection)), because in the latter case a collection is unnecessarily implicitly iterated and a new array object is created, and this eats up additional resources. Array.prototype iterating functions can be safely called upon objects with consecutive numeric keys starting from [0] and a length property with a valid number value of such keys' quantity (including, e.g., instances of HTMLCollection and FileList), so it's a reliable way. Also, if there is a frequent need in such operations, an empty array [] can be used for quick access to Array.prototype functions. A runnable example:

alert(
    Array.prototype.reduce.call(
        document.querySelector('ol').children,
        (acc, { textContent }, i) => `${acc}${i + 1}) ${textContent}` + `\n`,
        '',
    ),
);

<ol>
    <li>foo</li>
    <li>bar</li>
    <li>bat</li>
    <li>baz</li>
</ol>

To convert array-like to array in efficient way we can make use of the jQuery makeArray :

makeArray: Convert an array-like object into a true JavaScript array.

Usage:

var domArray = jQuery.makeArray(htmlCollection);

---

A little extra:

If you do not want to keep reference to the array object (most of the time HTMLCollections are dynamically changes so its better to copy them into another array, This example pay close attention to performance:

var domDataLength = domData.length //Better performance, no need to calculate every iteration the domArray length
var resultArray = new Array(domDataLength) // Since we know the length its improves the performance to declare the result array from the beginning.

for (var i = 0 ; i < domDataLength ; i++) {
    resultArray[i] = domArray[i]; //Since we already declared the resultArray we can not make use of the more expensive push method.
}

What is array-like?

HTMLCollection is an "array-like" object, the array-like objects are similar to array's object but missing a lot of its functionally definition:

Array-like objects look like arrays. They have various numbered elements and a length property. But that’s where the similarity stops. Array-like objects do not have any of Array’s functions, and for-in loops don’t even work!

This is my personal solution, based on the information here (this thread):

var Divs = new Array();    
var Elemns = document.getElementsByClassName("divisao");
    try {
        Divs = Elemns.prototype.slice.call(Elemns);
    } catch(e) {
        Divs = $A(Elemns);
    }

Where $A was described by Gareth Davis in his post:

function $A(iterable) {
  if (!iterable) return [];
  if ('toArray' in Object(iterable)) return iterable.toArray();
  var length = iterable.length || 0, results = new Array(length);
  while (length--) results[length] = iterable[length];
  return results;
}

If browser supports the best way, ok, otherwise will use the cross browser.

Sometimes, Even You have written code the correct way, But still it doesn't work properly.

var allbuttons = document.getElementsByTagName("button");
console.log(allbuttons);

var copyAllButtons = [];
for (let i = 0; i < allbuttons.length; i++) {
  copyAllButtons.push(allbuttons[i]);
}
console.log(copyAllButtons);

you get empty array. Like, This

HTMLCollection []
[]

Console_javascript

For Solving this problem, You have to add link of javascript file after body tag in html file.

<script src="./script.js"></script>

As you can see below, html_file

Final Output

HTMLCollection(6) [button.btn.btn-dark.click-me, button.btn.btn-dark.reset, button#b, button#b, button#b, button#b, b: button#b]
(6) [button.btn.btn-dark.click-me, button.btn.btn-dark.reset, button#b, button#b, button#b, button#b]

I'm not sure about efficiency but, from a purely aesthetic point of view, I find this pleasing.

HTMLCollection.prototype.toArray = function() { return Array.from(this); }

now you can use it like this.

document.getElementsByClassName('tax-status').toArray().forEach(console.log);

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