Microsoft® JScript®
@cc_on Statement
 Language Reference 
Version 3 

See Also

Activates conditional compilation support.
The @cc_on statement activates conditional compilation in the scripting engine.

It is strongly recommended that you use the @cc_on statement in a comment, so that browsers that do not support conditional compilation will accept your script as valid syntax:

(remainder of script)
Alternatively, an @if or @set statement outside of a comment also activates conditional compilation.