Detect when an image fails to load in Javascript

function testImage(URL) {
    var tester=new Image();
    tester.onload=imageFound;
    tester.onerror=imageNotFound;
    tester.src=URL;
}

function imageFound() {
    alert('That image is found and loaded');
}

function imageNotFound() {
    alert('That image was not found.');
}

testImage("http://foo.com/bar.jpg");

References
https://stackoverflow.com/questions/9815762/detect-when-an-image-fails-to-load-in-javascript
https://www.w3schools.com/jsref/event_onerror.asp