Most DOM methods and collection properties aren't actually arrays, they're collections:

• querySelectorAll returns a static NodeList (a snapshot of matching elements as of when you call it).


• getElementsByTagName, getElementsByTagNameNS, getElementsByClassName, and the children property on a ParentNode (Elements are parent nodes) return live HTMLCollection instances (if you change the DOM, that change is reflected live in the collection).


• getElementsByName returns a live NodeList (not a snapshot).

NodeList only recently got forEach (and keys and a couple of other array methods). HTMLCollection didn't and won't; it turned out adding them would break too much code on the web.

Both NodeList and HTMLCollection are iterable, though, meaning that you can loop through them with for-of, expand them into an array via spread ([...theCollection]), etc. But if you're running on a browser where NodeList doesn't have forEach, it's probably too old to have any ES2015+ features like for-of or iteration.

Since NodeList is specified to have forEach, you can safely polyfill it, and it's really easy to do:

if (typeof NodeList !== "undefined" && NodeList.prototype && !NodeList.prototype.forEach) {

    // Yes, there's really no need for `Object.defineProperty` here

    NodeList.prototype.forEach = Array.prototype.forEach;

}

Direct assignment is fine in this case, because enumerable, configurable, and writable should all be true and it's a value property. (enumerable being true surprised me, but that's how it's defined natively on Chrome/Chromium/Edge/Etc., Firefox, the old Legacy Edge, and Safari).

In your own code, you can do that with HTMLCollection as well if you want, just beware that if you're using some old DOM libs like MooTools or YUI or some such, they may be confused if you add forEach to HTMLCollection.

As I said before, NodeList and HTMLCollection are both specified to be iterable (because of this Web IDL rule¹). If you run into a browser that has ES2015+ features but doesn't make the collections iterable for some reason, you can polyfill that, too:

if (typeof Symbol !== "undefined" && Symbol.iterator && typeof NodeList !== "undefined" && NodeList.prototype && !NodeList.prototype[Symbol.iterator]) {

    Object.defineProperty(NodeList.prototype, Symbol.iterator, {

        value: Array.prototype[Symbol.iterator],

        writable: true,

        configurable: true

    });

}

(And the same for HTMLCollection.)

Here's a live example using both, try this on (for instance) IE11 (although it will only demonstrate forEach), on which NodeList doesn't have these features natively:

// Using only ES5 features so this runs on IE11

function log() {

    if (typeof console !== undefined && console.log) {

        console.log.apply(console, arguments);

    }

}

if (typeof NodeList !== undefined && NodeList.prototype) {

    // forEach

    if (!NodeList.prototype.forEach) {

        // Yes, there's really no need for `Object.defineProperty` here

        console.log(Added forEach);

        NodeList.prototype.forEach = Array.prototype.forEach;

    }

    // Iterability - won't happen on IE11 because it doesn't have Symbol

    if (typeof Symbol !== undefined && Symbol.iterator && !NodeList.prototype[Symbol.iterator]) {

        console.log(Added Symbol.iterator);

        Object.defineProperty(NodeList.prototype, Symbol.iterator, {

            value: Array.prototype[Symbol.iterator],

            writable: true,

            configurable: true

        });

    }

}



log(Testing forEach);

document.querySelectorAll(.container div).forEach(function(div) {

    var html = div.innerHTML;

    div.innerHTML = html[0].toUpperCase() + html.substring(1).toLowerCase();

});



// Iterable

if (typeof Symbol !== undefined && Symbol.iterator) {

    // Using eval here to avoid causing syntax errors on IE11

    log(Testing iterability);

    eval(

        'for (const div of document.querySelectorAll(.container div)) { ' +

        '    div.style.color = blue; ' +

        '}'

    );

}

<div class=container>

  <div>one</div>

  <div>two</div>

  <div>three</div>

  <div>four</div>

</div>

¹ It's confusing, because HTMLCollection is iterable but it isn't marked with the iterable declaration, which bizarrely in the JavaScript DOM bindings doesn't mean that something is iterable, it means that it has forEach, entries, keys, values, and it's iterable. But HTMLCollection, which isn't marked with the iterable declaration, is still iterable. Instead, it's iterable because of this Web IDL rule as mentioned earlier.