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Welcome to lucky day number 7. Today we're talking about Stimulus: a small, easy-to-understand JavaScript library that lets us create super-organized code that... just always works. It is one of my favorite reasons to use the Internet.

Installing StimulusBundle

But even though Stimulus is a JavaScript library... Symfony has a bundle to help us load it, get it set up, and use it. So, find your terminal and run:

composer require symfony/stimulus-bundle

One of the most important things about StimulusBundle is its recipe. After it finishes, run:

git status

The Recipe Changes

Oooh. It made a number of changes. The first is over here in assets/app.js. On top - I'll remove that comment - we're now importing a new bootstrap.js:

16 lines | assets/app.js
import './bootstrap.js';
// ... lines 2 - 16

That file starts the Stimulus application.

Notice that this imports an @symfony/stimulus-bundle module:

6 lines | assets/bootstrap.js
import { startStimulusApp } from '@symfony/stimulus-bundle';
// ... lines 2 - 6

The @ symbol isn't important: that's just a character namespaced JavaScript packages use. The important thing is that this is a bare import, which means the browser will try to find this package by looking at our importmap.

Ok! Open up importmap.php. The recipe added two new entries here:

31 lines | importmap.php
// ... lines 1 - 15
return [
// ... lines 17 - 23
'@hotwired/stimulus' => [
'version' => '3.2.2',
'@symfony/stimulus-bundle' => [
'path' => './vendor/symfony/stimulus-bundle/assets/dist/loader.js',

The first is for Stimulus itself - that now lives in the assets/vendor/ directory. The second is... a kind of "fake" 3rd party package. It says that @symfony/stimulus-bundle should resolve to a file in our vendor/ directory. This is a bit of fanciness: we say import '@symfony/stimulus-bundle'... and that will ultimately import this loader.js file from vendor/.

The recipe also added a controllers/ directory - the home for our custom Stimulus controllers - and a controllers.json file, which we'll talk about tomorrow.

Oh, and in base.html.twig, it added this ux_controller_link_tags() line:

49 lines | templates/base.html.twig
<!DOCTYPE html>
// ... lines 4 - 7
{% block stylesheets %}
{{ ux_controller_link_tags() }}
{% endblock %}
// ... lines 11 - 14
// ... lines 16 - 47

Delete it! That was needed with AssetMapper 6.3, but not anymore. We'll talk about what that did tomorrow anyway.

Using Stimulus

Ok: so, all we've done is composer require this new bundle. And yet, when we refresh the page and look at the console, Stimulus is already working! This application #starting and application #start come from Stimulus. That's awesome.

With StimulusBundle, anything we put into the controllers/ directory will automatically be available as a Stimulus controller. So, the fact that we have a hello_controller.js means that we can use a controller named hello:

import { Controller } from '@hotwired/stimulus';
* This is an example Stimulus controller!
* Any element with a data-controller="hello" attribute will cause
* this controller to be executed. The name "hello" comes from the filename:
* hello_controller.js -> "hello"
* Delete this file or adapt it for your use!
export default class extends Controller {
connect() {
this.element.textContent = 'Hello Stimulus! Edit me in assets/controllers/hello_controller.js';

In fact, we can see it right now. When this controller is activated, it replaces the text of the element it's attached to. To prove Stimulus is working, inspect any element on the page... and hack in a data-controller="hello".

When I hit enter, boom! It activates the controller.

Creating a Custom Controller

That was fun, but let's create our own, real controller. Copy hello_controller.js and create a new file called celebrate_controller.js. I'll remove the comments and the connect method:

10 lines | assets/controllers/celebrate_controller.js
import { Controller } from '@hotwired/stimulus';
// ... lines 2 - 3
export default class extends Controller {
// ... lines 5 - 8

Here's the goal: when we hover over the logo, I want to call a method on the controller that triggers the js-confetti library. Start by creating the method. It could be called anything, but poof() sure is a fun name!

Head over to app.js, copy the js-confetti code and delete it:

16 lines | assets/app.js
// ... lines 1 - 9
import JSConfetti from 'js-confetti';
const jsConfetti = new JSConfetti();
// ... lines 14 - 16

Pop that into celebrate controller... and move the import statement to the top:

import { Controller } from '@hotwired/stimulus';
import JSConfetti from 'js-confetti';
export default class extends Controller {
poof() {
const jsConfetti = new JSConfetti();

Cool! The last step is to activate this on an element. Do that in base.html.twig. Let's see... here's the logo. Add data-controller="celebrate". And to trigger the action on hover, say data-action=""... and the suggestion is almost correct. The format is, first: the JavaScript event that we want to listen to. Instead of click, we want mouseover. Then ->, the name of our controller, # and the method name: poof:

52 lines | templates/base.html.twig
// ... line 1
// ... lines 3 - 14
<body class="bg-black text-white font-mono">
<div class="container mx-auto min-h-screen flex flex-col">
<header class="my-8 px-4">
<nav class="flex items-center justify-between mb-4">
<div class="flex items-center">
href="{{ path('app_homepage') }}"
<img src="{{ asset('images/logo.png') }}" width="50" alt="Space Inviters Logo" >
// ... lines 27 - 29
// ... lines 31 - 36
// ... lines 39 - 48

That's it! Refresh and celebrate!!! Each time we mouseover, it calls the method. You can see this liberally in the console.

Wow, so, as soon as we add a controller to the controllers/ directory, it's loaded and ready to go. Remember, all with no build.

Lazy-Loading Controllers

But sometimes you might have a controller that's only used on certain pages... so you don't want to force your user to download it immediately on every page. If you have that situation, you can make your controller lazy. It's the best.

To do that, add this special comment above it: stimulusFetch: 'lazy':

11 lines | assets/controllers/celebrate_controller.js
// ... lines 1 - 3
/* stimulusFetch: 'lazy' */
export default class extends Controller {
// ... lines 6 - 9

Yes, that is pretty crazy. But as soon as we do that, instead of downloading this file on page load, it will wait until it sees an element on the page with data-controller"celebrate".

Watch: delete the data-controller temporarily. Then go over, refresh, and on the network tools, if I search for celebrate, there's nothing there. If I search for confetti - since our controller imports - js-confetti, that's also not there. Those have not been downloaded.

Clear out your network tools. Then go up to the logo and hack in that data-controller. We're imitating what would happen if we loaded some fresh HTML via AJAX and... that fresh HTML includes an element with data-controller"celebrate".

As soon as that appears on the page, go back to the network tools. Two new items showed up! It noticed the data-controller and downloaded the controller and js-confetti... since that's imported from the controller. And it works brilliantly. I absolutely love this.

Keep this controller lazy, but back in base.html.twig, re-add data-controller.

One of the great things about Stimulus is that it's used by people all over the Interwebs! And there are many pre-made Stimulus controllers out there to help us solve problems. One group of them is called Symfony-UX. We'll dive into one of its packages tomorrow.