index.js

// Button click event listener.
var buttonNumber = document.querySelectorAll('.drum').length;

for(var i = 0; i < buttonNumber; i++){
  document.querySelectorAll('.drum')[i].addEventListener('click', function(){
    var buttonName = this.innerHTML;
    playSound(buttonName);
    buttonAnimation(buttonName);
  });
}

// Keyboard press event listener.
document.addEventListener('keydown', function(event){
  playSound(event.key);
  buttonAnimation(event.key);
});


// Function that makes the sounds.
function playSound(key){

  switch (key) {
    case 'w':
      var crash = new Audio('sounds/crash.mp3');
      crash.play();
      break;

    case 'a':
      var kickBass = new Audio('sounds/kick-bass.mp3');
      kickBass.play();
      break;

    case 's':
      var snare = new Audio('sounds/snare.mp3');
      snare.play();
      break;

    case 'd':
      var tom1 = new Audio('sounds/tom-1.mp3');
      tom1.play();
      break;

    case 'j':
      var tom2 = new Audio('sounds/tom-2.mp3');
      tom2.play();
      break;

    case 'k':
      var tom3 = new Audio('sounds/tom-3.mp3');
      tom3.play();
      break;

    case 'l':
      var tom4 = new Audio('sounds/tom-4.mp3');
      tom4.play();
      break;

    default:
      console.log(buttonName);
  }
}

// Function that makes animation on clicked button.
function buttonAnimation(key){
  var clickedButton = document.querySelector("." + key);
  clickedButton.classList.add("pressed");

  setTimeout(function(){
    clickedButton.classList.remove("pressed")
  }, 100);
}