Event Listeners
var onButtonClick = function(event) {
console.log(event.target);
}
var onKeyUp = function(event) {
console.log(event.keyCode);
}Register Event Listeners
window.addEventListener('load', onInitialize);
button.addEventListener('click', onButtonClick);
div.addEventListener("mouseover", onMouseOver);
div.addEventListener("mouseout", onMouseOut);
document.addEventListener('keyup', onKeyUp);