Introduction to the DOM

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In this tutorial, I am going to introduce you to a fundamental concept in front end web development called the Document Object Model, or the DOM for short.

Table of Contents

You can skip to a specific section of this tutorial using the table of contents below:

What is the DOM?

So far in this course, we have only written plain JavaScript. This is somewhat useful, but the real power of JavaScript comes from its ability to integrate with HTML and CSS, which are the two basic languages that specify the elements that appear on any web page.

So what is the DOM and how does it play into this? Well, the DOM is what you see in the elements panel of your browser’s developer tools. The DOM looks like this:

An example of the DOM in a modern web browser

The DOM contains the HTML and CSS that create the website. JavaScript allows us to actually build interactive functionality into these HTML and CSS elements.

Here are a few examples of how JavaScript has influenced web applications that you have interacted with in the past:

  • When you add an item to your cart while shopping on Amazon, JavaScript changes the appearance of your cart when you navigate to it on the website
  • Starting a YouTube video when you click the “Play” button
  • Adding the thumbs-up emoji when you like a photo on Facebook

There are a number of ways that JavaScript can interact with HTML and CSS elements on a page, including:

  • Creating new HTML elements or removing existing ones
  • Adding or removing CSS classes to existing HTML elements
  • Playing music or video in the webpage

We will learn about more ways to integrate JavaScript with HTML and CSS throughout the rest of this course.

Important Web Development Terminology

Before we dig into how to integrate JavaScript into existing HTML and CSS code, there are a few important terms that you should understand first:

  • The window: in JavaScript, the window is an object where all of your global variables are stored as attributes. This means that if you have a global variable, you can either access it using its variable name (like we have so far in this course) or with the dot operator. In general, everything you know about your window (including its height, width, etc.) is available from the window object. Here’s an example using the global variable location:

console.log(location);

console.log(window.location);

//These will each log the same attribute to the console

  • The document: this refers to all of the HTML that is contained between your opening <html> tag and your closing <html> tag. Everything within the document is accessible using the document keyword combined with a dot operator.
  • The navigator: this keyword provides information about the actual device that the browser is running on. Think of attributes like battery life, webcam information, audio access, and other device-specific attributes.

The HTML Document That We Will Be Using In This Course

In order to demonstrate how to use JavaScript to interact with existing HTML and CSS files, you will first need to have some HTML and CSS to work with.

I have provided some basic HTML and CSS files in this repository for you to work with. The easiest way to proceed is to fork this to your own GitHub account and download the files from there.

Here’s a summary of the repository in its current state:

  • The index.html file contains some basic HTML elements that we will be manipulating with JavaScript throughout the rest of this course.
  • The style.css document contains some basic CSS styling. One of the main uses of this style.css file is to create some CSS classes that we will add to and remove from HTML elements using JavaScript. The index.html file is already linked to the style.css file using a <style> tag by default.
  • The script.js file is an empty JavaScript file. You should code in this file while working your way through this course.

Here are some high-level guidelines on how you should work through these lessons:

  • Download a new copy of the index.html, style.css, and script.js files to your computer
  • Open the index.html file in a browser window. This will allow you to actually see the webpage rendered in its final form, and not the underlying code.
  • Open the script.js file in your code editor. Every time you make changes to this file, you can save it and refresh the index.html file in your browser to see how the new JavaScript code changes your webpage in real-time.

Final Thoughts

In this tutorial, you received your first exposure to working with the document object model - or the DOM - in JavaScript.

Here is a brief summary of the content we covered in this lesson:

  • What is the DOM
  • Examples of how JavaScript can change the appearance or functionality of live websites
  • How to connect CSS and JS files to an HTML document using <style> and <script> tags
  • The best way to work through this course so that you can see how your JavaScript interacts with your HTML and CSS code