app.js

(() => {
  const btn = document.getElementById("calculate_btn");
  const res = document.getElementById("result");
  btn.addEventListener("click", e => {
    res.innerHTML = "Loading...";
    const val = doLongCalculation();
    const shortenedData = val.slice(0, 50).join(", ") + "...";
    res.innerHTML = shortenedData;
  });
})();