The GlideAjax class allows a client-side script to make an AJAX call to the server, execute a server-side script include, and even return a value. GlideAjax might be one of the least well understood client-side APIs in ServiceNow, in part because it has both a client-side, and a server-side component.
First, let's discuss the server-side component. A GlideAjax call from a client-side script will ultimately execute a script include (which is a server-side script), so let's create a script include. For our example, we're going to create a simple script include that'll return the value of a system property, so we'll create a script include called GetPropertyAjax. Once we enter this name, the default scaffolding of the script include is populated into the Script field.
To make this script include accessible from client-side scripts via GlideAjax though, we need to check the Client callable tickbox. This will alter the contents of the Script field, so that our script include extends...