Catching an Access-Control-Allow-Origin error in JavaScript

While browsers will log a more-detailed error message to the console, you can’t access that from your code. See https://bugs.chromium.org/p/chromium/issues/detail?id=118096#c5:

The details of errors of XHRs and Fetch API are not exposed to JavaScript for security reasons.

As far as the what specs actually require here, the Fetch spec is what defines the details of the “status message” to provide in case of an error — even if XHR is used instead of the Fetch API (the XHR spec references the Fetch spec). And for any network error or response blocked by the browser, the Fetch spec requires that the status message be “the empty byte sequence”:

A network error is a response whose status is always 0, status message is always the empty byte sequence, header list is always empty, body is always null, and trailer is always empty.

So all you can get back from any error you can catch is “TypeError: Failed to fetch” or such.

If you’re using XHR, all you have for handling an error is the onerror event handler:

xhr.onerror = function() { console.log("Error occurred but I dunno what exactly.")}