index.js

// When buttons are clicked
for (var i = 0; i < document.querySelectorAll(".drum").length; i++)
{
  document.querySelectorAll(".drum")[i].addEventListener("click", function() {

    var buttonSound = this.innerHTML;
    makeSound(buttonSound);
    buttonAnimation(buttonSound);
  })
}

// When keys are pressed
document.addEventListener("keydown", function(event){
  makeSound(event.key);
  buttonAnimation(event.key);
})

function makeSound(key) {
  switch (key) {

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

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

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

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

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

    case "k":
      var crash = new Audio("sounds/crash.mp3");
      crash.play();
      break;

    case "l":
      var kick = new Audio("sounds/kick-bass.mp3");
      kick.play();
      break;

    default:
      console.log(key);

  }

}

function buttonAnimation(currentKey){
  var activeButton = document.querySelector("." + currentKey);
  activeButton.classList.add("pressed");

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