index.js

let numberOfButtons = document.querySelectorAll(".drum").length;

for (var i = 0; i < numberOfButtons; i++) {

  document.querySelectorAll(".drum")[i].addEventListener("click", function() {

    var buttonInnerHTML = this.innerHTML;

    makeSound(buttonInnerHTML);

    buttonAnimation(buttonInnerHTML);
  });
}

document.addEventListener("keydown", function(event) {
  makeSound(event.key);

  buttonAnimation(event.key)
})

function makeSound(key) {
  switch (key) {
    case "w":
      let crash = new Audio("sounds/crash.mp3")
      crash.play();
      break;
    case "a":
      let kick = new Audio("sounds/kick-bass.mp3")
      kick.play();
      break;
    case "s":
      let snare = new Audio("sounds/snare.mp3")
      snare.play();
      break;
    case "d":
      let tom1 = new Audio("sounds/tom-1.mp3")
      tom1.play();
      break;
    case "j":
      let tom2 = new Audio("sounds/tom-2.mp3")
      tom2.play();
      break;
    case "k":
      let tom3 = new Audio("sounds/tom-3.mp3")
      tom3.play();
      break;
    case "l":
      let tom4 = new Audio("sounds/tom-4.mp3")
      tom4.play();
      break;
    default:
  }
}

function buttonAnimation(currentKey) {
  var activeButton = document.querySelector("." + currentKey);
  activeButton.classList.add("pressed");
  setTimeout(function () {
    activeButton.classList.remove("pressed");
  }, 100);
}