Microsoft® JScript®
Conditional Compilation
 Language Reference 
Version 3 

See Also

Allows the use of new JScript language features without sacrificing compatibility with browsers that don't support the features.
Conditional compilation is activated by using the @cc_on statement, or using an @if or @set statement outside of a comment. Some typical uses for conditional compilation are using new features in JScript, embedding debugging support into a script, and tracing code execution.

It is strongly recommended that conditional compilation code be placed in comments:

/*@cc_on @*/
/*@if (@_jscript_version == 4)
   alert("JScript version 4");
   @else @*/
   alert("You need a more recent script engine.");
/*@end @*/
This example uses special comment delimiters that are only used if conditional compilation is activated by the @cc_on statement. Scripting engines that do not understand conditional compilation only see the message informing of the need for a new scripting engine.