text: let playSound = e => {  
    const audio = document.querySelector(`audio[data-key="${e.keyCode}"`);
    const key = document.querySelector(`.key[data-key="${e.keyCode}"`);
    if(!audio || !key ) return;
    audio.currentTime = 0;
    audio.play();
    key.classList.add('clicked');
  }
  
  removeTransition = function(e) {
    if ( e.propertyName !== 'transform' ) return;
    this.classList.remove('clicked'); 
  }
  
  // let removeTransition = e => {
  //   if (e.propertyName !== 'transform') return;
  //   this.classList.remove('clicked');
  // };
  
  const keys = document.querySelectorAll('.key');
  keys.forEach(key => key.addEventListener('transitionend', removeTransition));
  
  window.addEventListener('keydown', playSound);
  