A bookmarklet to add custom amount of jank to main thread. Crafted for those who 💜 jank!

jank-bookmarklet.js

javascript:(function(){
  var meter = `
  <label style="position:fixed; z-index:10000; top: 0; left: 50%; transform: translateX(-50%); background-color: pink; padding: 5px;">
  Main thread jank (0-500ms):  <input id="jankmeter" type="range" min=0 max=500 value=50>
  </label>
  `;

  const d = document.createElement('jank');
  d.innerHTML = meter;
  document.body.appendChild(d);

  requestAnimationFrame(function jank() {
      requestAnimationFrame(jank);
      const range = document.querySelector('input[type=range]#jankmeter');
      const start = performance.now();
      while(performance.now() - start < range.value);
  });
})();

Usage: Just add a bookmark with above text as its content.