a11yist/app/javascript/controllers/unsaved_changes_controller.js

import { Controller } from "@hotwired/stimulus"

const LEAVE_ALERT = "Es gibt ungespeicherte Änderungen! Wirklich verlassen?"

// Connects to data-controller="unsaved-changes"
export default class extends Controller {
  initialState = null

  connect() {
    this.initialState = this.formState()

    window.addEventListener("beforeunload", (event) => this.leavingPage(event))
    document.addEventListener('turbo:before-visit', (e) => this.leavingPage(e))
    this.element.addEventListener("submit", (_) => this.initialState = this.formState())
  } 

  formState() {
    return JSON.stringify(Array.from(new FormData(this.element).entries())
                               .filter(x => x[1] != "")
                               .sort(x => x[0]))
  }
  
  leavingPage(event) {
    if(this.initialState == this.formState()) {
      return
    }

    if (!window.confirm(LEAVE_ALERT)) { 
      event.preventDefault()
    }
  }
}