CIS 119 Rollover using preloading of images

Code

"use strict";
let pic1 = new Image();
let pic2 = new Image();
pic1.src = 'th_musicCat.jpg';
pic2.src = 'th_computerCat.jpg';

window.onload = function() {
    let elem = document.getElementById('rollover');
    elem.src = pic2.src;
    elem.onmouseover = function() { elem.src = pic1.src; };
    elem.onmouseout = function() { elem.src = pic2.src; };
};