Intro to working with the DOM and client-side JavaScript
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The DOM & JavaScript

Learning Objectives

  • Explain what the DOM is and how it is structured
  • Target DOM elements using JavaScript selectors
  • Create, read, update, and delete DOM elements
  • Change the attributes or content of a DOM element
  • Explore JavaScript methods for DOM manipulation and traversal

Framing (10 minutes / 0:10)

In the Objects & Context lesson, you learned about objects as data structures and how we can use them to store data and methods. Today, we will learn about how JavaScript uses objects to represent what you see in the browser. Remember, everything is an object!

The Document Object Model, commonly referred to as the "DOM", is a programming interface for HTML. When you load HTML into the browser, it gets converted into a dynamic object-based structure. The visual representation of this is what you see when you open up Developer Tools in the browser.

The DOM is available for us to manipulate as an object, and this object is structured and stored like an upside down tree, like this...

lxf118 tut_grease diagram

Or this...

└── head
│   ├──title
│   ├──meta
│   ├──link[rel="stylesheet"]
|   └──script[type="text/javascript"]
└── body
    ├── header
    │   ├── h1
    │   └── nav
    └── section.simplicity
    |   └── h2
    │   └── article
    |   └── h2
    │   └── article
    │       └── block_quote
    │       └── block_quote
    └── footer

The Document Object

Each web page loaded in the browser has its own document object. The document interface serves as an entry point to the web page's content. The document is an example of a host object--that is, a JavaScript object provided by and unique to the browser environment.


Everything in the DOM exists as a node. HTML elements are called element nodes, attributes are called attribute nodes, the text inside elements are called text nodes. There are even comment nodes for <!-- html comments like this one --->. The document itself is called a document node.

You also can refer to nodes by their relationships to each other. For example, in the graphic above, you would say that the body element is the "parent" to the two div elements contained inside it, which are called child nodes. The two divs are also "siblings" to one another because they are on the same level in the tree structure.

Let's look at another example...

DOM Tree Relationships

Basics of Working with the DOM (5 min / 0:15)

Understanding the DOM is central to working in JavaScript. JavaScript uses the DOM to create dynamic HTML. This includes adding new HTML elements and attributes, changing CSS styles in a page, removing existing elements and attributes, and many more things.

Most of what you do with client-side javascript is going to revolve around manipulating the DOM.

Getting Data from the DOM

There are two groups of methods you can use to get elements from the DOM. We'll start with the oldest and end with the ones we recommend.

getElement(s)By (15 min / 0:30)

Each of these methods follows the same general naming convention:

Method Name Description
.getElementById() Gets a single element by an ID selector
.getElementsByClassName() Gets a list of elements with a class selector
.getElementsByTagName() Gets a list of elements with a tag (element) selector

Each of these three methods are part of the document object. We'll walk through each individually:


To use the getElementById method, we first need to reference the document object (where the method lives). Then, we pass in a string that matches the ID of an element in our HTML

let titleElement = document.getElementById("title");

The above code will start at the document (top of the tree), and look for an element with an id called title and save it in the variable titleElement


The getElementById method returns a single Node item; the getElementsByClassName returns a NodeList, which is like an Array of Nodes.

let paragraphElements = document.getElementsByClassName("paragraph");

The above code snippet returns a NodeList (like an Array) of every element with a class of 'paragraph' and saves it to the paragraphElements variable. Notice that Elements in the method name is plural here, where as in getElementById it's singular? This is to tell us that getElementByID only returns one Node while getElementsByClassName returns a list of Nodes.


The getElementsByTagName is a hand way of retrieving elements by their html tag (h1, span, a, li, etc). Elements is plural in the method name, meaning it too returns a list of Nodes.

let spanElements = document.getElementsByTagName("span");

The above snippet returns every span element on the page and saves it to the spanElements variable.

You Do: JS DOM Practice Part 1 (10 min / 0:40)

Clone down and open the practice exercise and work through the prompts in the getelements.js file.

querySelector (10 min / 0:50)

There are only two methods in this group: querySelector and querySelectorAll. Unlike the getElement(s)By group, these are simpler to understand - querySelector returns a single value, and querySelectorAll returns...well...everything that it can find. You can even select multiple IDs this way.

We'll walk through both querySelector and querySelectorAll, but first a note about selectors:


Unlike with the getElement(s)By family of methods, we need to pass a complete selector to both querySelector and querySelectorAll - it's in the name! What's a selector? A selector is a way of targeting a particular element, something we learned about when we first covered CSS.

The following is a list of CSS selectors and the JavaScript equivalents you would use with querySelector:

CSS Selector JS Selector
.class-name .class-name
#some-id #some-id
h1 h1

They're the same! Phew, that's lucky!

The querySelector methods were designed to mimic the way we target elements in CSS, so the selector we pass in is the same we'd use to style that element!


With querySelector, we'll pass in a selector for the element we want to retrieve from the DOM. The element that we get back will be the first element that matches that selector.

let title = document.querySelector(".title");

We'll only get one element back and it will always be the first element that matches the selector (in this case, .title). If we have more than one element in the page with that selector and we want to retrieve them all, then we'd use querySelectorAll


With querySelectorAll, we'll get back all elements on the page that match the selector we pass in.

let title = document.querySelectorAll("h2");

The above code snippet would return a list of all h2 elements on the page.

You Do: JS DOM Practice Part 2 (10 min / 1:00)

Open up the practice exercise and work through the prompts in the queryselector.js file.

Break (10 min / 1:10)

Setting Data in the DOM

Now that we know how to get elements from the DOM, it'd probably be helpful to learn what we can do with them. We'll soon learn about adding event listeners to DOM elements - a way for us to listen for when some event happens to a node (like it gets clicked) and then perform some response. But there are many other things we can do with nodes! Toggle, add or remove classes, change their styling, animate them, move them from one part of the page to another, replace their content with new content, etc. The list goes on!

Exploring DOM Nodes (45 min / 2:00)

20 minutes to research and prepare a demo, 25 minutes to present (5 minutes per group)

We're going to count off and break up in to 5 groups. Each group will be responsible for one of the topic areas below. Your goal is to research this topic area as a group and come up with a demo of how you would use it and why it might be helpful or important. We'll then go around the room and demo what we find!

1. Getting and Setting Attributes

Remember from our HTML lesson that some elements have attributes: the a tag has an href attribute and the img tag has a src attribute. In JavaScript, there are ways to access the list of attributes on a node and to get and set attributes.

Every node object has an attributes property where it lists it's attributes (like href and src). You can get and set data using the getAttribute and setAttribute method.

Look at the attributes property of a node. Also look up the getAttribute and setAttribute methods and how they work. Prepare a demo to showcase these. Your demo should show the following:

  • What is an attribute?
  • How do we access the list of attributes on a node?
  • How to we get the value of a particular attribute (like the href attribute)?
  • How do we add an attribute (like the name attribute)?

2. Class list API

A very common task in JavaScript is toggling CSS classes. We'll remove a .is-hidden class when the user clicks on something or we'll add an is-active class a navigation element when someone clicks on a hamburger menu.

The way we get and set classes on nodes is with the classList API. Every node has a classList property and there are methods we can use to add a class (addClass), remove a class (removeClass) or toggle a class (toggleClass).

Research these methods and think about how they work and why they're useful. Prepare a demo to showcase the following:

  • How can we see the list of classes a node has?
  • How can we check to see if a node has a class?
  • How can we add a class to a node?
  • How can we remove a class from a node?
  • How can we toggle a class from a node?

3. Traversing Nodes

We'll often have a particular node but need to check it's parents, children or siblings. Luckily, each node has this information stored within it!

Look through a node object (by console logging) and see if you can find the following:

  • children / childNodes
  • firstChild / firstElementChild
  • lastChild / lastElementChild
  • nextSibling / nextElementSibling and previousSibling / previousElementSibling
  • parentNode

What are these properties? What is the difference between children and child Nodes? What kinds of nodes do you see stored in these properties?

Think about these questions and explore the above list of properties. Prepare a demo to showcase how to access these and what the different options tell you.

4. Content

We'll sometimes have an element and want to change the text or html contained within that element. This is commonly called templating and there are libraries that will make it a little easier. With the new template literal syntax in ES6, we can often get away without a templating library. We could just use the list of properties below to reset the html or text of an element and interpolate data in to it.

Review these properties of a node:

  • innerHTML / outerHTML
  • innerText / outerText
  • textContent

What are they? How are they similar? How are they different?

Create a demo to showcase how you might use these and why they might be useful. Can we change the html inside of an element?

5. Dataset

Part of HTML5 includes the data-* attribute: a way for us to attach arbitrary data to an element. If we define a div element with a data-name="A Great Div" attribute, then our dataset property inside our node will be an object with a name key holding the string "A Great Div".

<div id="great" data-name="A Great Div"></div>

let div = document.getElementById('great') console.log(div.dataset)

Play around with it. Look up the data-* attribute and explore the dataset property inside of a node. See how you can create data attributes of your own and retrieve the data they hold from the dataset object.

What happens if you have a property with a name like this?

<div id="great" data-camel-case-is-fun="i agree"></div>


6. Changing the Styling

Something we may want to perform in JavaScript is updating or changing the styling of an element using JavaScript. A lot of web animation tools do that and there are tools for React (which we'll learn about later) that do this so you can write all your styles in JavaScript.

Explore the style property of a node. What do you see in there? How could we see the style of an element like, is it display: block? Can we change these style properties, like setting the background color?

7. Node Dimensions

There are a number of use cases where getting the height, width and position of a node are helpful, but the biggest is probable animation.

Explore this list of methods and properties:

  • getBoundingClientRect()
  • offsetHeight / offsetWidth and offsetLeft / offsetTop
  • clientHeight / clientWidth and clientLeft / clientTop

What are they? What information do they hold? What's the difference between offsetHeight and clientHeight? What data do you see in the result of getBoundingClientRect()?

You Do: Practice! (30 min / 2:30)

Choose between these two exercises and work on them for the rest of the class period.

JS DOM TOC - build a table of contents using things we've learned today.

JS DOM Slideshow - build a slideshow! A bit harder than the TOC, but has a more thorough walkthrough.