flash-controller.js

import { Controller } from 'stimulus';

export default class extends Controller {
  static targets = ['message']

  connect() {
    this.timeout = setTimeout(this.closeFlash, 2500);
  }

  disconnect() {
    clearTimeout(this.timeout);
  }

  closeFlash = () => {
    this.flashBox.fadeOut(300, () => {
      this.flashBox.remove();
    });
  }

  get flashBox() {
    return $(this.element);
  }
}