Stimulus Controllers

Overview

Stimulus controllers provide modular JavaScript functionality connected to HTML via data attributes. After creating a new controller, you must register it in the index.js file.

Creating a New Controller

1. Create the Controller File

Create a new controller in app/javascript/controllers/:

// app/javascript/controllers/example_controller.js
import { Controller } from "@hotwired/stimulus"

export default class extends Controller {
  static targets = ["element"]
  static values = { name: String }

  connect() {
    // Called when controller is connected to DOM
  }

  disconnect() {
    // Called when controller is disconnected from DOM
  }

  // Action methods
  handleClick(event) {
    event.preventDefault()
    // Your logic here
  }
}

2. Register the Controller

CRITICAL: After creating a new controller, run:

bin/rails stimulus:manifest:update

This automatically updates app/javascript/controllers/index.js to register your controller.

Manual Registration (if needed):

import ExampleController from "./example_controller"
application.register("example", ExampleController)

Controller name in HTML uses kebab-case: data-controller="example"

3. Use in HTML

Connect the controller to HTML elements:

.container data-controller="example" data-example-name-value="test"
  button data-action="click->example#handleClick" Click Me
  div data-example-target="element" Target Element

Naming Conventions

• File: example_controller.js (snake_case)
• Class: export default class extends Controller
• Registration: "example" (kebab-case)
• HTML: data-controller="example" (kebab-case)
• Multi-word: bulk_submit_controller.js → "bulk-submit"

Key Concepts

Targets

Reference specific DOM elements:

static targets = ["input", "output"]

// Access in methods:
this.inputTarget        // First matching element
this.inputTargets       // All matching elements
this.hasInputTarget     // Boolean check

Values

Type-safe data attributes:

static values = {
  url: String,
  count: Number,
  active: Boolean,
  items: Array,
  config: Object
}

// Access in methods:
this.urlValue
this.countValue

// Watch for changes:
urlValueChanged(newUrl, oldUrl) {
  // Called when value changes
}

Actions

Connect events to methods:

<!-- Basic action -->
data-action="click->example#save"

<!-- Multiple actions -->
data-action="click->example#save submit->example#submit"

<!-- Custom events -->
data-action="example:refresh->example#reload"

<!-- Event modifiers -->
data-action="submit->example#save:prevent"

Classes

Manage CSS classes:

static classes = ["active", "hidden"]

// Use in methods:
this.element.classList.add(this.activeClass)
this.element.classList.remove(this.hiddenClass)

Common Patterns

Form Validation

export default class extends Controller {
  static targets = ["form", "submit"]

  validate() {
    const isValid = this.formTarget.checkValidity()
    this.submitTarget.disabled = !isValid
  }
}

Toggle Visibility

export default class extends Controller {
  static targets = ["content"]
  static classes = ["hidden"]

  toggle() {
    this.contentTarget.classList.toggle(this.hiddenClass)
  }
}

AJAX Updates

export default class extends Controller {
  static values = { url: String }

  async refresh() {
    const response = await fetch(this.urlValue)
    const html = await response.text()
    this.element.innerHTML = html
  }
}

Testing

Test Stimulus controllers in system specs:

it 'handles interaction', :js do
  visit page_path

  click_button 'Toggle'

  expect(page).to have_css('[data-controller="example"]')
end

Troubleshooting

Controller not working?

1. Verify controller is registered in index.js
2. Run bin/rails stimulus:manifest:update
3. Check browser console for errors
4. Verify data attribute spelling (kebab-case)
5. Ensure JavaScript is enabled in tests (:js tag)

Targets not found?

• Check target name in static targets matches HTML
• Use hasXxxTarget to verify existence before accessing
• Ensure target element is in controller scope

Related Skills

• frontend-patterns – HTML and CSS patterns
• turbo-fetch – Dynamic form updates
• testing-patterns – Testing JavaScript features