Iterating over result of getElementsByClassName using Array.forEach

javascript foreach getelementsbyclassname

I want to iterate over some DOM elements, I'm doing this:

document.getElementsByClassName( "myclass" ).forEach( function(element, index, array) {
  //do stuff
});

but I get an error:

document.getElementsByClassName("myclass").forEach is not a function

I am using Firefox 3 so I know that both getElementsByClassName and Array.forEach are present. This works fine:

[2, 5, 9].forEach( function(element, index, array) {
  //do stuff
});

Is the result of getElementsByClassName an Array? If not, what is it?

isherwood

No, it's not an array. As specified in DOM4, it's an HTMLCollection (in modern browsers, at least. Older browsers returned a NodeList).

In all modern browsers (pretty much anything other IE <= 8), you can call Array's forEach method, passing it the list of elements (be it HTMLCollection or NodeList) as the this value:

var els = document.getElementsByClassName("myclass");

Array.prototype.forEach.call(els, function(el) {
    // Do stuff here
    console.log(el.tagName);
});

// Or
[].forEach.call(els, function (el) {...});

If you're in the happy position of being able to use ES6 (i.e. you can safely ignore Internet Explorer or you're using an ES5 transpiler), you can use Array.from:

Array.from(els).forEach((el) => {
    // Do stuff here
    console.log(el.tagName);
});

No need to convert it to an Array first. Just use [].forEach.call(elsArray, function () {...}).

It's NOT a NodeList. It's an array-like object. I don't even think it has an instance type. querySelectorAll method returns a NodeList though.

@MaksimVi. You're absolutely right: DOM4 specifies that document.getElementsByClassName() should return an HTMLCollection (which is very similar but not a NodeList). Thanks for pointing out the mistake.

@MaksimVi.: I wonder whether that changed at some point. I usually check these things.

@TimDown, Thanks for HTMLCollection tip. Now I finally can use HTMLCollection.prototype.forEach = Array.prototype.forEach; in my code.

Athari

You can use Array.from to convert collection to array, which is way cleaner than Array.prototype.forEach.call:

Array.from(document.getElementsByClassName("myclass")).forEach(
    function(element, index, array) {
        // do stuff
    }
);

In older browsers which don't support Array.from, you need to use something like Babel.

ES6 also adds this syntax:

[...document.getElementsByClassName("myclass")].forEach(
    (element, index, array) => {
        // do stuff
    }
);

Rest destructuring with ... works on all array-like objects, not only arrays themselves, then good old array syntax is used to construct an array from the values.

While the alternative function querySelectorAll (which kinda makes getElementsByClassName obsolete) returns a collection which does have forEach natively, other methods like map or filter are missing, so this syntax is still useful:

[...document.querySelectorAll(".myclass")].map(
    (element, index, array) => {
        // do stuff
    }
);

[...document.querySelectorAll(".myclass")].map(element => element.innerHTML);

Note: without transpilling as suggested (Babel), this is NOT compatible in IE < Edge, Opera, Safari < 9, Android browser, Chrome for Android, ...etc) Source: mozilla dev docs

icl7126

Or you can use querySelectorAll which returns NodeList:

document.querySelectorAll('.myclass').forEach(...)

Supported by modern browsers (including Edge, but not IE):
Can I use querySelectorAll
NodeList.prototype.forEach()

MDN: Document.querySelectorAll()

Bear in mind the performance penalty over getElementByClassName

The performance penalty is negligible compared to other more intensive tasks like modifying DOM. If I execute 60,000 of these in 1 millisecond, I'm pretty sure it won't be an issue for any reasonable usage :)

You linked the wrong benchmark. Here is the correct one measurethat.net/Benchmarks/Show/4076/0/… Just ran it on my low-end phone, got 160k/s vs 380k/s. Since you mentioned DOM manipulation, here is that too measurethat.net/Benchmarks/Show/5705/0/… Got 50k/s vs 130k/s. As you see it's even slower to manipulate DOM, likely due to NodeList being static (as mentioned by others). Still neglegible in most use cases, but almost 3 fold slower nonetheless.

IE doesn't support forEach method for NodeList too. But there is a work around like using spread operator or Array.from

james.garriss

Edit: Although the return type has changed in new versions of HTML (see Tim Down's updated answer), the code below still works.

As others have said, it's a NodeList. Here's a complete, working example you can try:

<!DOCTYPE html>
<html>
    <head>
        <meta charset="UTF-8">
        <script>
            function findTheOddOnes()
            {
                var theOddOnes = document.getElementsByClassName("odd");
                for(var i=0; i<theOddOnes.length; i++)
                {
                    alert(theOddOnes[i].innerHTML);
                }
            }
        </script>
    </head>
    <body>
        <h1>getElementsByClassName Test</h1>
        <p class="odd">This is an odd para.</p>
        <p>This is an even para.</p>
        <p class="odd">This one is also odd.</p>
        <p>This one is not odd.</p>
        <form>
            <input type="button" value="Find the odd ones..." onclick="findTheOddOnes()">
        </form>
    </body>
</html>

This works in IE 9, FF 5, Safari 5, and Chrome 12 on Win 7.

Verified Dec 2020 that this code still works.

Kloptikus

The result of getElementsByClassName() is not an Array, but an array-like object. Specifically it's called an HTMLCollection, not to be confused with NodeList (which has it's own forEach() method).

One simple way with ES2015 to convert an array-like object for use with Array.prototype.forEach() that hasn't been mentioned yet is to use the spread operator or spread syntax:

const elementsArray = document.getElementsByClassName('myclass');

[...elementsArray].forEach((element, index, array) => {
    // do something
});

I feel this is really the right way to do it in modern browsers. This is the exact use case spread syntax was created to solve.

Haritsinh Gohil

getElementsByClassName returns HTMLCollection in modern browsers.

which is array-like object similar to arguments which is iteratable by for...of loop see below what MDN doc is saying about it:

The for...of statement creates a loop iterating over iterable objects, including: built-in String, Array, Array-like objects (e.g., arguments or NodeList), TypedArray, Map, Set, and user-defined iterables. It invokes a custom iteration hook with statements to be executed for the value of each distinct property of the object.

Javascript Example

for (const element of document.getElementsByClassName("classname")){
   element.style.display="none";
}

Typescript Example

let elements = document.getElementsByClassName('classname');
let i;

for (i = 0; i < elements.length; i++) {

  if (elements[i] instanceof HTMLElement) {
    elements[i].style.display = "none";
  }

}

Not so, according to Typescript: error TS2488: Type 'HTMLCollectionOf<Element>' must have a '[Symbol.iterator]()' method that returns an iterator.

@TurtlesAreCute, Here OP is using javascript not typescript and i have answered in according to vanilla js recommendation so in typescript it can be different solution for the problem.

@TurtlesAreCute, By the way it is also working in typescript also, but you have to mention right type of variable which holds element of particular css class, so it can cast it accordingly, for detail see this answer.

This is easily the best answer and it works fine in Typescript.

@NateS what you can do is get this HTMLCollection into the variable and declare it type any and it will work as it works in javascript.

Quentin

Is the result of getElementsByClassName an Array?

If not, what is it?

As with all DOM methods that return multiple elements, it is a NodeList, see https://developer.mozilla.org/en/DOM/document.getElementsByClassName

Community

As already said, getElementsByClassName returns a HTMLCollection, which is defined as

[Exposed=Window]
interface HTMLCollection {
  readonly attribute unsigned long length;
  getter Element? item(unsigned long index);
  getter Element? namedItem(DOMString name);
};

Previously, some browsers returned a NodeList instead.

[Exposed=Window]
interface NodeList {
  getter Node? item(unsigned long index);
  readonly attribute unsigned long length;
  iterable<Node>;
};

The difference is important, because DOM4 now defines NodeLists as iterable.

According to Web IDL draft,

Objects implementing an interface that is declared to be iterable support being iterated over to obtain a sequence of values. Note: In the ECMAScript language binding, an interface that is iterable will have “entries”, “forEach”, “keys”, “values” and @@iterator properties on its interface prototype object.

That means that, if you want to use forEach, you can use a DOM method which returns a NodeList, like querySelectorAll.

document.querySelectorAll(".myclass").forEach(function(element, index, array) {
  // do stuff
});

Note this is not widely supported yet. Also see forEach method of Node.childNodes?

Chrome 49 return forEach in not a function

@VitalyZdanevich Try Chromium 50

On Chrome 50 I am getting document.querySelectorAll(...).forEach is not a function

@VitalyZdanevich It worked on Chromium 50, and still works on Chromium 53. Maybe it wasn't considered stable enough to be shipped to Chrome 50.

gildniy

This is the safer way:

var elements = document.getElementsByClassName("myclass");
for (var i = 0; i < elements.length; i++) myFunction(elements[i]);

StefanSL

Here is a test I created on jsperf: https://jsperf.com/vanillajs-loop-through-elements-of-class

The most perfomant version in Chrome and Firefox is the good old for loop in combination with document.getElementsByClassName:

var elements = document.getElementsByClassName('testClass'), elLength = elements.length;
for (var i = 0; i < elLength; i++) {
    elements.item(i).textContent = 'Tested';
};

In Safari this variant is the winner:

var elements = document.querySelectorAll('.testClass');
elements.forEach((element) => {
    element.textContent = 'Tested';
});

If you want the most perfomant variant for all browsers it might be this one:

var elements = document.getElementsByClassName('testClass');
Array.from(elements).map(
    (element) => {
        return element.textContent = 'Tested';
    }
);

reko_t

It does not return an Array, it returns a NodeList.

Андрей

Use this code to add forEach method to HTMLCollection

/**
 *
 * @type {Function}
 */
HTMLCollection.prototype.forEach = HTMLCollection.prototype.forEach ||
    function (callBack) {
        for (let i = 0; i < this.length; i++) {
            callBack(this[i])
        }
    };

Then your code will work:

document.getElementsByClassName( "myclass" ).forEach( function(element, index, array) {
  //do stuff
});

While this works, it is a bad practice to modify prototype.

Konstantin

The simplest solution:

Object.assign(HTMLCollection.prototype, { forEach(event) { Array.prototype.forEach.call(this, (element) => event(element)); }});

After that you can write like this:

document.getElementsByClassName("disable").forEach((element) => alert(element.classList[0]));

🙂

Please do some formatting.

Iterating over result of getElementsByClassName using Array.forEach

Follow WeChat

Want to stay one step ahead of the latest teleworks?

相似问题

Platform

Support

Contact US