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HTMLCollection, NodeList and array of objects

Assuming the DOM is as described in the snippet below, my requirement is to get a javascript array of all the child nodes of container div.

<div id=”container”>
<div class="divy">...</div>
<div class="divy">...</div>
<div class="divy">...</div>
<div class="divy">...</div>
</div>

During the good jQuery days, you could just do $('.divy') or $('#container').children()

I was trying to do the same thing using the native DOM selector API and was in for a surprise

const childDivs = document.querySelectorAll('.divy')
Array.isArray(childDivs) //=> false
childDivs.constructor.name //=> NodeList

Okay let’s try one of the getElementBy% methods and check what is returned

const childDivsAgain = document.getElementsByClassName('divy')
Array.isArray(childDivs) //=> false
childDivs.constructor.name //=> HTMLCollection

Now what are NodeList and HTMLCollection objects and why are we not getting the plain vanilla javascript array from these methods?

Let’s try to understand the difference between HTMLCollection and NodeList first.

An HTMLCollection is a list of nodes. An individual node may be accessed by either ordinal index or the node’s name or id attributes.
Collections in the HTML DOM are assumed to be live meaning that they are automatically updated when the underlying document is changed.

HTML Collections are always “in the DOM”, whereas a NodeList is a more generic construct that may or may not be in the DOM.

A NodeList object is a collection of nodes. The NodeList interface provides the abstraction of an ordered collection of nodes, without defining or constraining how this collection is implemented. NodeList objects in the DOM are live or static based on the interface used to retrieve them

Let’s test the specification with relevant code to understand more

let parentDiv = document.getElementById('container')
let nodeListDivs = document.querySelectorAll('.divy')
let htmlCollectionDivs = document.getElementsByClassName('divy')
nodeListDivs.length //=> 4
htmlCollectionDivs.length //=> 4
//append new child to container
var newDiv = document.createElement('div');
newDiv.className = 'divy'
parentDiv.appendChild(newDiv)
nodeListDivs.length //=> 4
htmlCollectionDivs.length //=> 5

We can see that the HTMLCollection is literally live, in the sense, any change to DOM is updated automatically and available in the collection.

Not all NodeList objects are static. For example, document.getElementByName will return a live NodeList.

But remember that neither HTMLCollectionnor NodeList support the array prototype methods like push pop or splice methods . Iterator methods like forEach have been recently added though and latest browsers might support them.

I came to know about this difference, when I was trying to use the dragula drag-n-drop library to add selected components to the library

let draggableLists = document.querySelectorAll('div.draggable')
dragula(draggableLists); //will not work

This will not work as expected as the dragula() constructor expects a javascript array and we are passing a NodeList to it.

To convert the NodeList or HTMLCollection object to a javascript array, you can do one of the following:

Use Array.from method

const nodelist = document.querySelectorAll(‘.divy’)
const divyArray = Array.from(nodelist)

Use Array.prototype.slice

const nodelist = document.querySelectorAll(‘.divy’)
const divyArray = Array.prototype.slice.call(nodelist)

ES6 way

And I like this one. If you are using ES6, you can just use the spread operator

const divyArray = […document.querySelectorAll(‘.divy’)]

Now, let’s apply that to the dragula api to make it work

dragula([...document.querySelectorAll('div.draggable')])

So when do you really convert NodeList to an array? Well, it depends on your usecase. If you really want an iterator to the latest updated DOM at all times, you should use the NodeList or HTMLCollection as it is without converting it to an array.

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