CIS 119 Text link rollovers changing images

Code

"use strict";
let pic1 = new Image();
let pic2 = new Image();
let pic3 = new Image();
let pic4 = new Image();
pic1.src = '../images/th_musicCat.jpg';
pic2.src = '../images/th_computerCat.jpg';
pic3.src = '../images/th_phoneCat.jpg';
pic4.src = '../images/th_attackCat.jpg';

window.onload = function() {
    let link1 = document.getElementById('a1');
    let link2 = document.getElementById('a2');
    let elem1 = document.getElementById('r1');
    let elem2 = document.getElementById('r2');
    elem1.src = pic2.src;
    elem2.src = pic4.src;
    link1.onmouseover = function() { elem1.src = pic1.src; };
    link1.onmouseout = function() { elem1.src = pic2.src; };
    link2.onmouseover = function() { elem2.src = pic3.src; };
    link2.onmouseout = function() { elem2.src = pic4.src; };
};