index.js

var numberOfDrums = document.querySelectorAll(".drum").length;

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

document.addEventListener("keydown", function(event){
   playSound(event.key);
   buttonAnimation(event.key);
});

function playSound(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(this.innerHTML)
            break;
    }
}

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