How to detect on-page 404 errors using JavaScript?

To capture all error events on the page, you can use addEventListener with the useCapture argument set to true. The reason window.onerror will not do this is because it uses the bubble event phase, and the error events you want to capture do not bubble.

If you add the following script to your HTML before you load any external content, you should be able to capture all the error events, even when loading offline.

<script type="text/javascript">
window.addEventListener('error', function(e) {
    console.log(e);
}, true);
</script>

You can access the element that caused the error through e.target. For example, if you want to know what file did not load on an img tag, you can use e.target.src to get the URL that failed to load.

NOTE: This technically will not detect the error code, it detects if the image failed to load, as it technically behaves the same regardless of the status code. Depending on your setup this would probably be enough, but for example if a 404 is returned with a valid image it will not trigger an error event.