Conditional compilation allows the use of new JScript language features without sacrificing compatibility with older versions that do not support the features.

Conditional compilation is activated by using the @cc_on statement, or using an @if or @set statement. Some typical uses for conditional compilation include using new features in JScript, embedding debugging support into a script, and tracing code execution.

Always place conditional compilation code in comments, so that hosts (like Netscape Navigator) that do not understand conditional compilation will ignore it. Here is an example.

/*@cc_on @*/
/*@if (@_jscript_version >= 4)
   alert("JScript version 4 or better");
   @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 support conditional compilation only see the message informing of the need for a new scripting engine.