newsroom/assets/controllers/form-collection_controller.js

import { Controller } from '@hotwired/stimulus';

export default class extends Controller {
  static targets = ["collectionContainer"]

  static values = {
    index    : Number,
    prototype: String,
  }

  addCollectionElement(event)
  {
    const item = document.createElement('li');
    item.innerHTML = this.prototypeValue.replace(/__name__/g, this.indexValue);
    this.collectionContainerTarget.appendChild(item);
    this.indexValue++;
  }
}