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Day 5 already? We're flying! It's time to add some CSS to our site. So how does that work inside AssetMapper?

Well, we already have an assets/styles/app.css file. And... there's nothing stopping us from going into base.html.twig, and adding a link tag: link, rel="stylesheet", href then asset() and the logical path: styles/app.css.

Swell! When we refresh... and look at the page source, there it is! It works great and it's super boring. The kind of boring I like.

However, if we remove this line... and go and refresh the page. Huh, we still have this blue background: a blue background that's coming from app.css:

body {
background-color: skyblue;

Take another peek at the page source. There is still a link tag pointing to that file? Back over in base.html.twig, hmm, nothing here. Where is that coming from?

The answer - I bet you guessed - is the importmap() function:

21 lines | templates/base.html.twig
// ... line 1
// ... lines 4 - 10
{% block javascripts %}
{{ importmap('app') }}
{% endblock %}
// ... lines 15 - 19

And it's because it's being imported from app.js:

15 lines | assets/app.js
// ... lines 1 - 6
import './styles/app.css';
// ... lines 8 - 15

How CSS Works

Importing a CSS file from JavaScript is probably something you got used to with Webpack Encore. You import a CSS file... and ultimately, it's rendered on the page as a link tag. However, this is not something that ECMAScript modules actually support. The only thing you can import are JavaScript files. So this should fail spectacularly: like it should download the CSS file and try to parse it as JavaScript.

However, as you may have noticed, it doesn't fail! I love mysteries!

This is a totally extra feature that we added to AssetMapper. Here's how it works. In base.html.twig, we say importmap('app'). The app is known as the entrypoint: the one file the browser will execute directly. And we know that refers to assets/app.js.

So what AssetMapper does is, it goes into this file and finds all the import statements for JavaScript and CSS files. For every CSS import it finds, it adds that as a link tag. It's... basically just that simple.

The CSS Importmap Trick

Well, there is one little, fascinating complication. Go to the network tab in your browser and search for app. This is the app.js file that's being executed by the browser. Notice: it does still have the import statement to the CSS file! If you think about it, when our browser executes this line, it should fail! It should download the CSS file, try to parse it as JavaScript & hit a syntax error. But it doesn't.

The reason is a trick inside AssetMapper. When you import a CSS file, AssetMapper adds an importmap entry for it. So even though this starts with ./, our browser does look to see if there's a matching path inside the importmap. And there is. Because of that, it downloads this file.... which is not a real file. It's a fake file that does.... absolutely nothing. So it makes that line not error out and... not do anything.

Importing CSS from Other JavaScript Files

To see how powerful this is, let's create a second CSS file to support our alien greeting. Call it alien-greeting.css and make the body background darkgreen. Though, personally, I'm hoping aliens are rainbow colored:

body {
background: darkgreen;

Over in alien-greeting.js, import that: ../styles/alien-greeting.css:

6 lines | assets/lib/alien-greeting.js
import '../styles/alien-greeting.css';
// ... lines 2 - 6

Will this work? Try it! Refresh and... green background! In the source, we have a second link tag and a second new item in the importmap. So that's awesome! Because app.js imports alien-greeting.js, AssetMapper also finds any CSS files that it imports.

Lazy-Loading CSS

Here's where things get really spooky-cool. JavaScript modules have a dynamic import syntax that allows you to import modules asynchronously. That lets us load a file later when we need it, instead of on page load. And we can use this trick with CSS.

Copy this. Pretend that we only want to load that CSS file when inPeace equals false. So I'll say, if not inPeace, then use setTimeout() to wait for 4 seconds. After 4 seconds, import the CSS file. Except, as soon as you need an import to not live at the top of your file, you need to call it like a function:

10 lines | assets/lib/alien-greeting.js
export default function (message, inPeace = false) {
if (!inPeace) {
setTimeout(() => {
}, 4000);
// ... lines 7 - 8

That's pretty cool. Try it. At first, blue background! 2, 3, 4, green background! The CSS file loaded lazily. How? Well, there's no alien-greeting.css link tag in the page source anymore. Instead, we wait for the browser to execute this JavaScript line. When it does, it looks for this in the importmap, finds it and downloads this fake file. But this time, instead of it being a line that does nothing, this fake file adds a new link tag to the head section with rel="stylesheet" and href set to alien-greeting.css.

Heck, we can watch this in real time! Over here, under the head tag, we see the stylesheet. If I refresh and quickly open that, it's not there. And... then it gets added. So stinkin' cool.

Now that we've conquered how CSS works, tomorrow, we'll use it to bring our site to life! But I want to do it with an extra fun angle: I want to use Tailwind CSS.