Use Import Maps and Stimulus with Rails 6

I'm currently on Rails 6.0.4 and I want to use Stimulus to build a new page. There's a lot of work still to do before upgrading to Rails 7, so I want to use importmaps and stimulus with Rails 6 if possible. But so far I haven't been able to get a stimulus controller working. I followed these steps:

1. Update the Gemfile and bundle:

gem 'importmap-rails'
gem 'stimulus-rails'

2. Run rails importmap:install

This gave me config/importmap.rb, and app/javascript/application.js, which was pinned as "application" in the config file.

3. Rename the current application.js file

My current application.js is inside app/assets/application.js.coffee, so I renamed this to old_application.js.coffee for the time being and updated application.html.haml to the following:

= javascript_include_tag "old_application", defer: true
= javascript_importmap_tags

This seemed to work and it gave me the following import map when I rendered the page:

{
  "imports": {
    "application": "/assets/application-920fceca960b509c1e98c9b27d167fa368d4b588ceb1be42d1667552714f94d5.js"
  }
}

4.Run rails stimulus:install

This gave me the app/javascript/controllers folder with index.js, application.js and hello_controller.js inside.

It also updated config/importmap.rb to the following:

pin "application", preload: true
pin "@hotwired/stimulus", to: "stimulus.min.js", preload: true
pin "@hotwired/stimulus-loading", to: "stimulus-loading.js", preload: true
pin_all_from "app/javascript/controllers", under: "controllers"

... and my app/javascript/application.js to the following:

import "controllers"

This then gives me an error, saying that controllers/hello_controller.js has not been precompiled:

5. Update asset paths and precompiled files

We're on version 3 of sprockets, and upgrading to version 4 will also take some time, due to this issue. So instead I updated the existing initializers/assets.rb file to include the new javascript folder and files:

Rails.application.config.assets.paths << Rails.root.join('app', 'javascript', 'controllers')
Rails.application.config.assets.precompile += ['application.js', 'controllers/index.js', 'controllers/application.js', 'controllers/hello_controller.js']

This fixed the issue and gives me the following import map on the rendered page:

{
  "imports": {
    "application": "/assets/application-920fceca960b509c1e98c9b27d167fa368d4b588ceb1be42d1667552714f94d5.js",
    "@hotwired/stimulus": "/assets/stimulus.min-c5a29ae8ade968ede2149a7e76a29d0756a87cef66959113681098e5923ba26b.js",
    "@hotwired/stimulus-loading": "/assets/stimulus-loading-6b29b7e0382bd421ac62df99b46f45072fd3962208fbf0b2522041b4d037bbd9.js",
    "controllers/hello_controller": "/assets/hello_controller-8cddac086717993eeda2cc8af3be73bc3a6ecfb07212600aeed8ce8122fffa34.js",
    "controllers": "/assets/index-7445df4dc00776a5e3496034da4bc5906891c342c89c0e6a7775b1498b8222d8.js",
    "controllers/application": "/assets/application-5703baa647baaa5a3da21d4e61aa5686824d0e0ab82c2ba11f5c963cc8414a8f.js"
  }
}

6. Test stimulus controller with an alert

I changed hello_controller.js to show an alert when it's connected:

import { Controller } from "@hotwired/stimulus"

export default class extends Controller {
  connect() {
    alert('Hello World!')
  }
}

But when I attach this to an element with data-controller="hello" I don't see anything, suggesting that stimulus hasn't been correctly loaded.

This is the partial for that:

.home-container{data: { controller: 'hello' }}
  %section
    .container

Which gives me this HTML:

<div class="home-container" data-controller="hello">
  <section>
    <div class="container">
    </div>
  </section>
<div>

Can anyone suggest what I might be missing? If I start a new rails app on Rails 7 and set everything up like that, it works without a problem.