00_index.html.erb

<div data-controller="clipboard" class="mt-4 bg-gray-50 p-6">
  <input type="text" class="form-input bg-grey-50" value="SeCrEtKeY-42!" data-clipboard-target="source" readonly />

  <button class="btn" data-clipboard-target="button" data-action="clipboard#copy">
    Copy
  </button>
</div>

01_clipboard_controller.js

import { Controller } from "stimulus";

export default class extends Controller {
  static targets = ["source", "button"];

  connect() {
    if (!document.queryCommandSupported("copy")) {
      this.buttonTarget.classList.add("hidden");
    }
  }

  copy() {
    this.sourceTarget.select();
    document.execCommand("copy");
  }
}