Way to debug CORS errors

While browsers still aren't very helpful here, I recently built a webpage that can tell you exactly what's going on, and why, and exactly how to fix it. It's effectively an interactive self-explaining flowchart of the whole CORS system.

If you're stuck trying to debug your CORS setup, try it out: https://httptoolkit.tech/will-it-cors/

Which version of Chrome are you using? The latest versions have become much better at reporting CORS issues. For example, I am using Chrome version "32.0.1700.14 beta", and when I visit this page, I get the following error in my console:

Request header field X-Foo is not allowed by Access-Control-Allow-Headers.

This information is only available from the console, and cannot be programmatically accessed. Hopefully more browsers will follow suit. Note that if your request fails for some other, non-CORS reason, you may still see unhelpful error messages.