Part of the entire reason that JavaScript exists is to interact with HTML and CSS. Learn how all three web languages intersect in this article!

Getting started

Create an HTML file with the following code, then open it up in your browser:

<html>
    <head>
        <title>The DOM :D</title>
    </head>
    <body>
        <h1>An h1</h1>
        <p id="myId">I'm a paragraph, look at me!</p>
        <h1 class="title second-h1" id="classesExample">Another h1</h1>
        <h1>A third h1</h1>
        <img src="https://codetheweb.blog/assets/img/posts/javascript-dom/llama.jpg">
        <script>
            // Exciting code coming soon!
        </script>
    </body>
</html>

If you want to try out anything in this article, you can put it in the <script> tag, save, reload the page and see what happens! In this tutorial, I’ll be using console.log — this prints stuff to the console so you can see it. To open up the console:

  1. Right-click
  2. Go to ‘Inspect Element’
  3. Go to the ‘Console’ tab

That’s all! I hope you enjoy…

Why would I want to change HTML anyway?

I’m assuming you already have some reason because otherwise you wouldn’t be looking at this article, but just in case you don’t…

For most of this series, we’ve been displaying the values of certain bits of JavaScript by either using alert or console.log. But what if we wanted to show the values on the actual web page? Here’s where we need to start changing HTML with JavaScript…

What is the DOM?

The DOM stands for the Document Object Model. It is a programming interface for HTML — in other words, it lets you modify HTML using another programming language (JavaScript). All of the stuff I’ll teach you about in this article is all part of the DOM — it is a way of being able to change the HTML. You can also change CSS using the DOM because you can make CSS as inline styles (with the style attribute).

Selecting an element

To modify an HTML element, you need to start by selecting it. To do this, we use CSS selectors (here’s a refresher) combined with the document.querySelector function. Here’s an example (a console.log added so we can see the result):

document.querySelector('#myId');

This will return our <p> element because it has an ID of myId. We can see the output of this by console.log-ing it:

console.log( document.querySelector('#myId') );

How about if there are multiple elements that fit the selector? For example:

console.log( document.querySelector('h1') );

There are <h1> elements on the page — it would just select the first. I’ll be talking about how to select multiple elements near the end of this article.

Remember to put these into your <script> tag and take a look at what happens in the console.

Changing the contents of an element

Now that we’ve selected out element, let’s do something with it! We’ll change the HTML inside it. Remember that HTML can just be text, so using this method we can do anything from changing text to creating new elements inside something. Here’s what it looks like:

document.querySelector('h1').innerHTML = 'Hello World!';

Put this in your script tag — you will see that the first <h1> on the page changes to Hello World! immediately.

You can also insert HTML into an element — this is why it’s innerHTML! Try it out…

document.querySelector('#myId').innerHTML = 'This is <b>bold</b>.';

Often, you want to add something to an element’s innerHTML instead of replacing it. We can do this by using += instead of =:

document.querySelector('body').innerHTML += '<h2>I am now on the page!</h2>';

Using += is just a much quicker way of saying:

// DON'T USE THIS, USE THE ONE ABOVE
document.querySelector('body').innerHTML = document.querySelector('body').innerHTML + '<h2>I am now on the page!</h2>';

Try adding some elements to the page yourself!

Classes

Modifying an element’s class attribute is another common use for JavaScript. It can be useful if there’s a bunch of CSS which you want to apply to multiple elements — you can just give it a class! Let’s use this element from our HTML:

<h1 class="title second-h1" id="classesExample">Another h1</h1>

This element currently has two classes — title and second-h1. Here are some things we can do with its classes…

Getting a list of classes

console.log( document.querySelector('#classesExample').classList );

Checking if the element has a class

console.log( document.querySelector('#classesExample').classList.contains('title') );
// Logs true

Adding a class

document.querySelector('#classesExample').classList.add('another-class');

Removing a class

document.querySelector('#classesExample').classList.remove('title');

Toggling a class

Toggling simply means removing it if it’s already there, or adding it if it’s not…

document.querySelector('#classesExample').classList.toggle('second-h1');

Getting / setting any attribute

In the section above, I showed you how to modify an element’s class attribute. But how about changing any attribute?

We can use the getAttribute function to get an attribute’s value — here’s the syntax:

document.querySelector('element').getAttribute('name');

Try to get the src of the image on our page…

document.querySelector('img').getAttribute('src');
// Logs 'https://codetheweb.blog/assets/img/posts/javascript-dom/llama.jpg'

Now, let’s try changing the URL! We want to change it from this:

https://codetheweb.blog/assets/img/posts/javascript-dom/llama.jpg

To this:

https://codetheweb.blog/assets/img/posts/javascript-dom/llama-changed.jpg

First of all, take a look at the original image on your page.

We can use the setAttribute function. It looks like this:

document.querySelector('element').setAttribute('name', 'value');

See if you can work out how to change our image’s src attribute to the new URL…

document.querySelector('img').setAttribute('src', 'https://codetheweb.blog/assets/img/posts/javascript-dom/llama-changed.jpg');

Put it in your script and watch the magic occur!

This is how to get and set attributes.

Changing an element’s CSS

We can modify an element’s CSS using it’s .style values. Here’s an example:

document.querySelector('h1').style.color = 'red';

Try it out! You will see the first title turn red…

We can set any CSS property — not just the color. Go ahead and try changing another one yourself! It could be height, font-weight, etc.

You can have dashes as variable names in JavaScript — so you may be wondering how to change properties such as background-color! What we do is we write it in camelCase.

camelCase is where you stick the words together (without spaces) and capitalize the first letter of every word except the first. For example, background-color would become backgroundColor and border-bottom-left-radius would become borderBottomLeftRadius.

Try changing the background-color of our first h1! Remember camelCase…

document.querySelector('h1').style.backgroundColor = 'orange';

Selecting multiple elements

For this entire article, I’ve been showing you how to get one element and then do something with it. But how about if you want to get multiple elements? Let’s say we want to get all the <h1>s on the page. We can use the document.querySelectorAll function:

document.querySelectorAll('h1')

Try console.log-ing it and see what it returns…

NodeList(3) [h1, h1#classesExample.title.second-h1, h1]

It looks like an array of all the elements that match that selector!

This is pretty much correct, except instead of an array it is a special data type called a NodeList. A NodeList is like an array except that it is made up of HTML elements. If you edit one of these elements with JavaScript (eg. change the style), it will show up on the page immediately.

You can pretty much treat it like an array — the main thing that you have to watch out for is that many array functions won’t work (eg. .join()).

Now that we’ve got a NodeList of all our <h1> elements, let’s do something to them! We need to loop through them using a for loop, and then change each one — you can loop through a NodeList.

Let’s try changing the font-family of all the <h1>s to sans-serif — here’s what it looks like:

var elements = document.querySelectorAll('h1');

for(var i = 0; i < elements.length; i++) {
    elements[i].style.fontFamily = 'sans-serif';
}

All your <h1>’s will change their font!

This is how you select and change multiple elements from the one query selector…

Conclusion

There we go! I hope you learned something today 😁

If you know someone else who I can help with this article, go along and share it with them — sharing is caring! If you have any feedback, need help or just wanna say hi, feel free to do so down in the comments (or contact me).

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See you next time, where I’ll be talking about URL parsing (processing) in JavaScript!

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