Web Developers Pit: JavaScript Conditional Compilation.

Below are few inbuilt JS variable using which we can know some details of Client machine.

Predefined conditional compilation variables for JavaScript
Variable	Description
@_win32	Returns true if running on a Win32 system, otherwise NaN.
@_win16	Returns true if running on a Win16 system, otherwise NaN.
@_mac	Returns true if running on an Apple Macintosh system, otherwise NaN.
@_alpha	Returns true if running on a DEC Alpha processor, otherwise NaN.
@_x86	Returns true if running on an Intel Processor, otherwise NaN.
@_mc680x0	Returns true if running on a Motorola 680x0 processor, otherwise NaN.
@_PowerPC	Returns true if running on a Motorola PowerPC processor, otherwise NaN.
@_jscript	Always returns true.
@_jscript_build	The build number of the JScript scripting engine.
@_jscript_version	A number representing the JScript version number in major.minor format. IE4 supports JScript 3.x IE5.x supports JScript 5.5 or less IE6 supports JScript 5.6 The version number reported for JScript .NET is 7.x.
@_debug	Returns true if compiled in debug mode, otherwise false.
@_fast	Returns true if compiled in fast mode, otherwise false.

In most cases, you probably will be limited to just using @_win and @jscript_build:

/*@cc_on
@if (@_win32)
  document.write("OS is 32-bit. Browser is IE.");
@else
  document.write("OS is NOT 32-bit. Browser is IE.");
@end
@*/

You can also define your own variables to use within the conditional compilation block, with the syntax being:

@set @varname = term

Numeric and Boolean variables are supported for conditional compilation, though strings are not. For example:

@set @myvar1 = 35
@set @myvar3 = @_jscript_version

The standard set of operators are supported in conditional compilation logic:

You can test if a user defined variable has been defined by testing for NaN:

@if (@newVar != @newVar)
//this variable isn't defined.

This works since NaN is the only value not equal to itself.

References:

Web Developers Pit