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Migrating Encore -> AssetMapper


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Symfony 6.3 came with a new component called AssetMapper... and I love it! Okay, I work on it... so I'm totally not objective... but trust me it's amazing! It lets us write modern JavaScript and css with no build system. We have an Asset Mapper tutorial and a more recent LAST Stack tutorial where we build cool stuff with it.

AssetMapper Vs Webpack Encore?

AssetMapper is a replacement for Webpack Encore. Encore isn't going to die super soon, but I definitely caught it browsing some retirement brochures!

So I know what you're wondering:

Should I convert my app from Webpack Encore to AssetMapper?

The short, but not satisfying answer is... it's up to you. AssetMapper is more modern, it's easier to use and if you're frustrated with slow builds from Encore, that's a great reason to switch. But if Encore is working fine, there's no huge reason to do all the work of converting to AssetMapper. Also, if you use React or Vue, you'll want to stay with Encore because those do still require a build step.

Removing Webpack Encore

But let's convert! Head over to your terminal and find that tab where yarn watch is doing its thing. Stop that with Ctrl+C and close that tab. We do not need a build system - so that second tab is not coming back.

Then run:

composer remove symfony/webpack-encore-bundle

This will remove that package... but more important: its recipe will uninstall itself! It feels great: package.json gone, webpack.config.js gone, the encore_entry_ functions in base.html.twig gone.

But... it also deleted app.js and app.css. We do want those files, so run

git checkout assets/

to get them back. But everything else looks good! Run:

git diff

In the old package.json, the dependencies here were related to Webpack Encore and we will not need those. But some of these are for our frontend, and we will re-add those via AssetMapper.

Ok, lock in those changes with a commit... then throw a party by removing node_modules, public/build/ and the yarn error file. Oh, we can also remove yarn.lock. Gorgeous!

Installing AssetMapper

Now let's install AssetMapper:

composer require symfony/asset-mapper

Its recipe does a bunch of interesting things. We won't go too deep into how AssetMapper works - we have other tutorials for that - but let's explore. In .gitignore:

15 lines | .gitignore
// ... lines 1 - 10
###> symfony/asset-mapper ###

it ignores the final location of the built assets and where the vendor files live. And in templates/base.html.twig, it added an importmap() function that will output CSS and JavaScript.

84 lines | templates/base.html.twig
// ... line 1
// ... lines 4 - 9
{% block stylesheets %}
{% endblock %}
{% block javascripts %}
{% block importmap %}{{ importmap('app') }}{% endblock %}
{% endblock %}
// ... lines 17 - 82

It also gave us an importmap.php file.

31 lines | importmap.php
* Returns the importmap for this application.
* - "path" is a path inside the asset mapper system. Use the
* "debug:asset-map" command to see the full list of paths.
* - "entrypoint" (JavaScript only) set to true for any module that will
* be used as an "entrypoint" (and passed to the importmap() Twig function).
* The "importmap:require" command can be used to add new entries to this file.
* This file has been auto-generated by the importmap commands.
return [
'app' => [
'path' => './assets/app.js',
'entrypoint' => true,
'@hotwired/stimulus' => [
'version' => '3.2.2',
'@symfony/stimulus-bundle' => [
'path' => './vendor/symfony/stimulus-bundle/assets/dist/loader.js',
'@hotwired/turbo' => [
'version' => '7.3.0',

This is, effectively, the new package.json: the home for 3rd party packages. And hey! It already added Stimulus and Turbo! Those are two of the packages from package.json that we do need.

Will this work? Refresh and... kinda? We don't have Bootstrap CSS... which is why it looks terrible. But I can see that assets/styles/app.css is being loaded: that's giving us some basic styles. But we need to fix these imports.

40 lines | assets/styles/app.css
@import '~bootstrap';
@import '~@fortawesome/fontawesome-free/css/all.css';
@import '~@fontsource/roboto-condensed';
// ... lines 4 - 40

Onwards we go! Let's roll up our sleeves and nail down the last few steps to get AssetMapper up and running next.