DOM Traversals

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DOM Traversals

Let’s say you want to go to your neighbor’s house. What’s the fastest and most efficient way to get there?

  1. Move from your house to their house (since you already know their address)
  2. Lookup their address on Google maps, then walk according to the directions Google gives you

If you move directly from your house to their house, you’re doing the equivalent of traversing the DOM—selecting one element from a neighboring element.

If you lookup their address on Google, you’re doing the equivalent of document.querySelector to find elements.

Can you guess which method is more efficient?

<div class="neighborhood">
  <div class="your-house">😎</div>
  <div class="neighbor-house">🎉</div>
</div>

You probably know the answer—it’s always easier to move from an element to another (compared do doing a full search). That’s why we traverse the DOM.

You can traverse in three directions:

  1. Downwards
  2. Sideways
  3. Upwards

Traversing downwards

There are three ways to traverse downwards:

  1. querySelector or querySelectorAll
  2. children
  3. firstElementChild

querySelector or querySelectorAll

To traverse downwards from a specific element, you can use element.querySelector or element.querySelectorAll. You already know this.

If we put element.querySelector into the house analogy, we search for a specific room in your house. It’s faster than searching for the same room from outer space (the document).

<div class="component">
  <h2 class="component__title">Component title</h2>
</div>

const component = document.querySelector('.component')
const title = component.querySelector('.component__title')

console.log(title) // <h2 class="component__title"> ... </h2>

children

children is a property that lets you select direct descendants (elements that are immediately nested in another element). It returns a HTML Collection that updates when children elements are changed.

Note: HTML Collections are live collections

<ul class="list">
  <li><a href="#">Link 1</a></li>
  <li><a href="#">Link 2</a></li>
  <li><a href="#">Link 3</a></li>
  <li><a href="#">Link 4</a></li>
  <li><a href="#">Link 5</a></li>
</ul>

const list = document.querySelector('.list')
const listItems = list.children

console.log(listItems)
Selecting all list items with the children property
Selecting all list items with the children property

A HTML Collection is a list of HTML Elements. A NodeList is a list of nodes. You can say that a HTML Collection is a subset of a NodeList.

Functions wise, HTML Collections don’t have the forEach method. If you want to loop over a HTML Collection with forEach, you need to convert it into an array with Array.from.

const array = Array.from(HTMLCollection)
array.forEach(el => { /* do whatever you want */})

Selecting a specific child

You can select the nth-item in the list from both NodeLists (result from querySelectorAll) and HTML Collections (result from children). To do so, you use the index of the element, just like how you select a specific item from an Array.

<ul class="list">
  <li><a href="#">Link 1</a></li>
  <li><a href="#">Link 2</a></li>
  <li><a href="#">Link 3</a></li>
  <li><a href="#">Link 4</a></li>
  <li><a href="#">Link 5</a></li>
</ul>

const listItems = document.querySelectorAll('li')

const firstItem = listItems[0]
const secondItem = listItems[1]
const thirdItem = listItems[2]
const fourthItem = listItems[3]
const fifthItem = listItems[4]

console.log(firstItem)
console.log(secondItem)
console.log(thirdItem)
console.log(fourthItem)
console.log(fifthItem)
Select a specific child with [index]
Select a specific child with [index]

Try the above code with a HTML Collection. You’ll get the same result.

firstElementChild

You can select the first element with children like this:

<ul class="list">
  <li><a href="#">Link 1</a></li>
  <li><a href="#">Link 2</a></li>
  <li><a href="#">Link 3</a></li>
  <li><a href="#">Link 4</a></li>
  <li><a href="#">Link 5</a></li>
</ul>

const list = document.querySelectorAll('li')
const firstItem = list[0]

You can also select the first element with firstElementChild.

const list = document.querySelector('.list')
const firstItem = list.firstElementChild

Traversing upwards

There are two ways to traverse upwards:

  1. parentElement
  2. closest

parentElement

parentElement is a property that lets you select the parent element. The parent element is the element that encloses the current element.

In the following HTML, .list is the parent element of all <li>. Each <li> is the parent element of their respective <a>.

<ul class="list">
  <li><a href="#">Link 1</a></li>
  <li><a href="#">Link 2</a></li>
  <li><a href="#">Link 3</a></li>
  <li><a href="#">Link 4</a></li>
  <li><a href="#">Link 5</a></li>
</ul>

const firstListItem = document.querySelector('li')
const list = firstListItem.parentElement

console.log(list)
// <ul class="list">...</ul>

closest

parentElement is great for selecting one level upwards. To find an element that can be multiple levels above the current element, you use the closest method.

closest lets you select the closest ancestor element that matches a selector. Here’s the syntax:

const closestAncestor = Element.closest(selector)

As you may suspect, selector is the same selector you pass to querySelector and querySelectorAll.

In the following HTML, you can select .list from the <a> effortlessly with Element.closest:

<ul class="list">
  <li><a href="#">Link 1</a></li>
  <li><a href="#">Link 2</a></li>
  <li><a href="#">Link 3</a></li>
  <li><a href="#">Link 4</a></li>
  <li><a href="#">Link 5</a></li>
</ul>

const firstLink = document.querySelector('a')
const list = firstLink.closest('.list')

console.log(list)
// <ul class="list"> ... </ul>

Note: closest starts searching from the current element, then proceeds upwards until it reaches the document. It stops and returns the first element it finds.

const firstLink = document.querySelector('a')
const firstLinkThroughClosest = firstLink.closest('a')

console.log(firstLinkThroughClosest)
// <a href="#">Link 1</a>

closest is pretty new. It doesn’t work on IE Edge 14 and below. It doesn’t work on Opera mini too. If you need to support older browsers, you may want to use a polyfill.

Traversing sideways

There are three ways to traverse sideways:

  1. nextElementSibling
  2. previousElementSibling
  3. Combining parentElement, children, and index

nextElementSibling

You can select the next element with nextElementSibling.

const nextElem = Node.nextElementSibling

<ul class="list">
  <li><a href="#">Link 1</a></li>
  <li><a href="#">Link 2</a></li>
  <li><a href="#">Link 3</a></li>
  <li><a href="#">Link 4</a></li>
  <li><a href="#">Link 5</a></li>
</ul>

const firstListItem = document.querySelector('li')
const secondListItem = firstListItem.nextElementSibling

console.log(secondListItem)
// <li><a href="#">Link 2</a></li>

previousElementSibling

Likewise, you can select the previous element with previousElementSibling.

<ul class="list">
  <li><a href="#">Link 1</a></li>
  <li><a href="#">Link 2</a></li>
  <li><a href="#">Link 3</a></li>
  <li><a href="#">Link 4</a></li>
  <li><a href="#">Link 5</a></li>
</ul>

const secondListItem = document.querySelectorAll('li')[1]
const firstListItem = secondListItem.previousElementSibling

console.log(firstListItem)
// <li><a href="#">Link 1</a></li>

Combining parentElement, children, and index

This method lets you select a specific sibling. It’s easier to explain how it works with an example, so let’s do that. Say you want to select the fourth item from the first item in this HTML.

<ul class="list">
  <li><a href="#">Link 1</a></li>
  <li><a href="#">Link 2</a></li>
  <li><a href="#">Link 3</a></li>
  <li><a href="#">Link 4</a></li>
  <li><a href="#">Link 5</a></li>
</ul>

You already have the first item:

const firstItem = document.querySelector('li')

To select the fourth item, you can use firstItem.parentElement to get the list, then list.children to get a HTML Collection. Once you have the HTML Collection, you can find the fourth item by using a index of 3. (Remember, zero-based index!).

const firstItem = document.querySelector('li')
const list = firstItem.parentElement
const allItems = list.children
const fourthItem = allItems[3]

console.log(fourthItem)
// <li><a href="#">Link 4</a></li>

Putting everything together in one step:

const firstITem = document.querySelector('li')
const fourthItem = firstITem.parentElement.children[3]

console.log(fourthItem)
// <li><a href="#">Link 4</a></li>

Wrapping up

You learned how to traverse the DOM in three directions—downwards, upwards, and sideways—in this lesson. Here’s a quick bullet point to summarize the methods you learned:

  1. Traversing downwards
    1. element.querySelector
    2. element.querySelectorAll
    3. element.children
  2. Traversing upwards
    1. element.parentElement
    2. element.closest
  3. Traversing sideways
    1. element.nextElementSibling
    2. element.previousElementSibling
    3. Combine parentElement, children, and index

Exercise

Practice traversing the DOM with the methods taught in this lesson. With the HTML given below, do these tasks:

  1. Select .characters with document.querySelector
  2. Select .humans from .characters
  3. Select all humans with querySelectorAll, starting from .humans
  4. Select all hobbits with children
  5. Select the Merry (the hobbit)
  6. Select .elves from Merry
  7. Select Glorfindel from .elves
  8. Select Elrond from Glorfindel
  9. Select Legolas from Glorfindel
  10. Select the .characters div from Nazgûl
<div class="characters">
  <ul class="hobbits">
    <li>Frodo Baggins</li>
    <li>Samwise "Sam" Gamgee</li>
    <li>Meriadoc "Merry" Brandybuck</li>
    <li>Peregrin "Pippin" Took</li>
    <li>Bilbo Baggins</li>
  </ul>
  <ul class="humans">
    <li>Gandalf</li>
    <li>Saruman</li>
    <li>Aragorn</li>
    <li>Boromir</li>
    <li>Faramir</li>
  </ul>
  <ul class="elves">
    <li>Legolas</li>
    <li>Glorfindel</li>
    <li>Elrond</li>
    <li>Arwen Evenstar</li>
  </ul>
  <ul class="enemies">
    <li>Sauron</li>
    <li>Nazgûl</li>
  </ul>
</div>

Answers

  • Select .characters with document.querySelector
const characters = document.querySelector('.characters')
  • Select .humans from .characters
const humansList = characters.querySelector('.humans')
  • Select all humans with querySelectorAll, starting from .humans
const humans = humansList.querySelectorAll('li')
  • Select all hobbits with children
  • Select the Merry (the hobbit)
const hobbitsList = characters.querySelector('.hobbits')
const hobbits = hobbitsList.children
const merry = hobbits[2]
  • Select .elves from Merry
const elvesList = merry.parentElement.nextElementSibling.nextElementSibling
  • Select Glorfindel from .elves
const glorfindel = elvesList.children[1]
  • Select Elrond from Glorfindel
const elrond = glorfindel.nextElementSibling
  • Select Legolas from Glorfindel
const legolas = glorfindel.previousElementSibling
  • Select the .characters div from Nazgûl
const nazgul = document.querySelector('.enemies').chlidren[1]
const characters = nazgul.closest('.characters')