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Installing AssetMapper


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We now know that we can run modern JavaScript directly in our browser. But to help smooth the process, we're going to install a new Symfony component called AssetMapper.

Find your terminal and run:

composer require symfony/asset-mapper symfony/asset

I'm including the second package because it gives us that nice asset() function in Twig. It's already installed in this project - just make sure you have it in yours.

Before we start: AssetMapper is experimental in Symfony 6.3, so there will likely be some backwards compatibility breaks before 6.4. But as we will see, the concepts are solid, and you can deploy a super-performant site with AssetMapper today.

Changes from the Flex Recipe

Ok, run:

git status

Oooh: the Flex recipe for AssetMapper added several things. Time for a quick tour! First, it gave us an assets/ directory... which looks pretty much identical to what you would get if you installed WebpackEncore. We have an app.js file - this will be the main, one file that's executed - and also app.css: the main CSS file.

8 lines | assets/app.js
* Welcome to your app's main JavaScript file!
* This file will be included onto the page via the importmap() Twig function,
* which should already be in your base.html.twig.
console.log('This log comes from assets/app.js - welcome to AssetMapper! 🎉')

body {
background-color: skyblue;

In templates/base.html.twig, the recipe also added a link tag to point to app.css. We're going to talk more about stylesheets later, but you can already see that the CSS setup is perfectly straightforward.

70 lines | templates/base.html.twig
// ... lines 1 - 2
// ... lines 4 - 11
{% block stylesheets %}
<link rel="stylesheet" href="{{ asset('styles/app.css') }}">
{% endblock %}
// ... line 15
{% block javascripts %}
{{ importmap() }}
// ... line 18
{% endblock %}
// ... lines 21 - 70

The recipe added one more important line to this file: {{ importmap() }}. That partners with a new importmap.php file. Those are important, and we'll go into detail about them soon.

The takeaway is that the recipe created a few files in the assets/ and added a link tag to base.html.twig. But otherwise, there's not a lot going on yet.

AssetMapper "Paths"

Looking back at the terminal, the recipe also created a new asset_mapper.yaml file. Let's open that up: config/packages/asset_mapper.yaml.

# The paths to make available to the asset mapper.
- assets/

AssetMapper has one, main concept: you point it at a directory or set of directories, like assets/, and it makes all the files inside available publicly, as if they lived in the public/ directory. We'll see how that's accomplished in a minute.

But before we do anything else, refresh the page and... the background turned blue! That's coming from the app.css file. And, in the console log, we see a message that's coming from assets/app.js. So, somehow, magically, just by running a composer require command, these two files are already exposed publicly and are being loaded onto the page. Next, let's learn how this is all working.