Detailed explanation of conditional comments in JScript_javascript skills

JScript can use the following statements to control the compilation of scripts based on the value of conditional compilation variables. You can either use the variables provided by JScript or define your own using the @set directive or the /define command line option.

@cc_on
Activate conditional compilation support.

@if
Conditionally execute a set of statements based on the value of an expression.

@set
Create variables using conditional compilation statements.

The @cc_on, @if, or @set statement activates conditional compilation. Some typical uses of conditional compilation include using new features in JScript, embedding debugging support into a script, and tracking code execution.

When writing scripts to be run by a web browser, always put conditional compilation code in comments. Therefore, hosts that do not support conditional compilation can ignore this code. Here is an example.

Copy code The code is as follows:

/*@cc_on @*/
/*@if (@_jscript_version >= 5)
document.write("JScript Version 5.0 or better.
");
@else @*/
document.write("You need a more recent script engine.
");
/*@end @*/

This example uses special comment delimiters, which are used only after the @cc_on statement activates conditional compilation. Scripting engines that do not support conditional compilation display a message suggesting that a new scripting engine needs to be used without generating an error. Engines that support conditional compilation compile the first or second document.write depending on the version of the engine. Note that version 7.x means JScript .NET. For more information, see Detecting Browser Capabilities.

Conditional compilation is also useful for server-side scripts and command-line programs. In these applications, conditional compilation can be used to compile other functions into a program for analysis in debug mode.

The following predefined variables are available for conditional compilation.

@_win32
True if running on a Win32 system and no /platform option is specified or the /platform:anycpu option is specified; NaN otherwise.

@_win16
True if running on a Win16 system; NaN otherwise.

@_mac
True if running on an Apple Macintosh system; NaN otherwise.

@_alpha
True if running on a DEC Alpha processor; NaN otherwise.

@_x86
True if running on an Intel processor and the /platform option is not specified or the /platform:anycpu option is specified; NaN otherwise.

@_mc680x0
True if running on a Motorola 680x0 processor; NaN otherwise.

@_PowerPC
True if running on a Motorola PowerPC processor; NaN otherwise.

@_jscript
is always true.

@_jscript_build
The build number of the JScript scripting engine.

@_jscript_version
A number representing the JScript version number in major.minor format.

@_debug
true if compiling in debug mode; false otherwise.

@_fast
true if compiling in fast mode; false otherwise.

Attention
JScript .NET reports a version number of 7.x. JScript 8.0 reports a version number of 8.x.

Before using conditional compilation variables, conditional compilation must be turned on. The @cc_on statement turns on conditional compilation. Conditional compilation variables are commonly used in scripts written for web browsers. Conditional compilation variables are rarely used in scripts written for ASP or ASP.NET pages or command-line programs because other methods can be used to determine compiler compatibility.

When writing scripts for use on web pages, always put conditional compilation code in comments. This way, hosts that don't support conditional compilation can ignore the code. Here is an example.

Copy code The code is as follows:

/*@cc_on
document.write("JScript version: " @_jscript_version ".
");
@if (@_win32)
Document.write("Running on 32-bit Windows.
");
@elif (@_win16)
Document.write("Running on 16-bit Windows.
");
@else
Document.write("Running on a different platform.
");
@end
@*/