How can I test if a URL is a valid image (in javascript)?

Question

When submitting a form, I want to make sure a field is a valid image URL.

I could make an AJAX endpoint on my server which CURLs the URL and parses the output with a

Answer 1

Do this, the following code will replace any non-image with a default image.

<img src="xx" onerror="this.src = '/images/default.png'">

Answer 2

You can make an <img> element and handle its onerror and onload events.

If the load event fires, it's a valid image; if the error event fires, it isn't.
This even works across domains.