Logging is something that you can employ both server-side, and client-side. There are several useful logging API methods that can be employed for various purposes, and in different situations: info(), warn(), and error(), each with respectively higher levels of significance. These three types of logging methods are available both from the GlideSystem API (available within all server-side scripts as the gs object) on the server, and from the console API on the client.
An informational message (using gs.info() on the server, or console.info() on the client) ...