From the Source
It Just Makes Sense
JScript IntelliSense in Visual Studio 2008
By Erik Reitan
Creating full IntelliSense for JScript has been problematic to develop, because JScript is a language that features dynamic typing. However, Visual Studio 2008 provides the solution. In addition to displaying the programming elements you expect, JScript IntelliSense lets you add code descriptions that IntelliSense will display when you code or when anyone develops Web applications with your code. Adding your own JScript IntelliSense details and comments is especially useful if you re developing a client-script library.
Before Visual Studio can provide JScript IntelliSense, the JScript IntelliSense engine must scan a project to determine what client code is available (based on files and on code links in the project). This scanning process creates the current coding context on which IntelliSense suggestions are based.
By default, the JScript IntelliSense context is generated on idle in Visual Studio or Visual Web Developer Express, but you also can manually invoke it. The context can be manually generated by selecting Edit | IntelliSense | Update JScript IntelliSense.
Understanding the IntelliSense Context
The JScript IntelliSense engine determines the coding context by scanning your current file, script file references, assembly script references, client-side Web service references, and page references. The script used to establish the IntelliSense context within Visual Studio includes: