index.js

for(var i = 0;i < document.querySelectorAll("button").length;i++){
  document.querySelectorAll("button")[i].addEventListener("click",function (){
    play(this.innerHTML);
    animation(this.innerHTML);

  });
}
document.addEventListener("keydown",function(event){
  play(event.key);
  animation(event.key);
})

function play(key){
  switch (key) {
    case "w": var audio = new Audio('sounds/crash.mp3');
    audio.play();
      break;
    case "a": var audio = new Audio('sounds/kick-bass.mp3');
    audio.play();
    break;
    case "s": var audio = new Audio('sounds/snare.mp3');
    audio.play();
      break;
    case "d": var audio = new Audio('sounds/tom-1.mp3');
    audio.play();
    break;
    case "j": var audio = new Audio('sounds/tom-2.mp3');
    audio.play();
    break;
    case "k": var audio = new Audio('sounds/tom-3.mp3');
    audio.play();
      break;
    case "l": var audio = new Audio('sounds/tom-4.mp3');
    audio.play();
    break;
    default:console.log(key);

  }
}


function animation(currentKey){
  document.querySelector("." +currentKey).classList.add("pressed");

  setTimeout(function(){
    document.querySelector("."+currentKey).classList.remove("pressed");
  }, 100);
}