My code tweak for the Javascript30 by Wes Bos day 1

wesbos-drum-machine.js

let keys = document.querySelector('.keys');

  keys.addEventListener('transitionend', function(e) {
    if (e.propertyName === 'transform') {
      e.target.classList.remove('playing');
    }
  });

  function playPercussion(e) {
    const audioTrack = document.querySelector(`audio[data-key="${e.keyCode}"]`);
    const percussion = document.querySelector(`div[data-key="${e.keyCode}"]`);
    if (audioTrack) {
      audioTrack.currentTime = 0;
      audioTrack.play();
      percussion.classList.add('playing');
    }
  }

  window.addEventListener('keydown', playPercussion);