how to change image on mouseover in javascript code example

Example: javascript onmouseover change image

let img = document.querySelector('img');
    let start = img.src;
    let hover = img.getAttribute('data-hover'); //specified in img tag

    img.onmouseover = () => { img.src = hover; }
    img.onmouseout = () => { img.src = start; } //to revert back to start