function ease(startValue, endValue, durationMs) {
  const stepCount = durationMs / 16; // how many steps will we take
  const stepSize = (endValue - startValue) / stepCount; // the size of each step
  let currentValue = startValue;

  function step() {
    currentValue += stepSize;

    if (currentValue < endValue) {
      // do something with currentValue
      window.requestAnimationFrame(step); // recursive call to step()
    }
  }

  step(); // kick things off
}