CIS 119 Multiple text rollovers changing one image

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 link3 = document.getElementById('a3');
    let link4 = document.getElementById('a4');
    let elem = document.getElementById('pic');
    elem.src = pic4.src;
    link1.onmouseover = function() { elem.src = pic1.src; };
    link2.onmouseover = function() { elem.src = pic2.src; };
    link3.onmouseover = function() { elem.src = pic3.src; };
    link4.onmouseover = function() { elem.src = pic4.src; };
};