A diagnostic message is what the implementation presents when it detects an error.

These are normally presented as an alert box in web browsers.

In other implementations such as Web Server back ends, server-side JavaScript failures may be recorded in an error log or written into the pages being sent to the client. However, you should be careful to remove any debugging stubs before launching your service as these messages could potentially divulge some valuable security related information about your server (and how to hack it).

You should always know where the error logs are written for your implementation. On a UNIX platform, it is helpful to open an additional terminal console and pipe the output of the error log to the screen. This can be done with the tail command like this:

tail -f error.log