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 an image library, but that feels a bit of overkill.
Could I get away with making an <img> element then synchronously check the response somehow?
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.
Do this. In this example, it will replace any non-image with a default image
<img src="xx" onerror="this.src = '/images/default.png'>