"if debug" in JavaScript?

Question 1

No.

#if/#endif are preprocessor directives in C# (and other languages) that tells the compiler to conditionally include/exclude a section of code when compiling.

JavaScript is a script language that is not precompiled, and therefore it would not make much sense to have preprocessor directives like these.

Question 2

A bit late, but I needed the same and could not give up until a viable solution.

I have a kind of "main" javascript file, where I have a line like:

Site.DEBUG = false;

Then in the code I can check for this constant. Now I needed to solve that at build time, some automation would set this for me according to project configuration. Here I've found fnr.exe command-line tool for find and replace in files. It's a quite good piece of utility, would be worth to check out anyway. So at this point I've created a folder in the project directory called BuildScripts, I've copied the fnr.exe file into it, and created a batch file like this.

switch_client_debug.bat

REM Params: path to folder, filename, change-DEBUG-from-this, to-this
fnr.exe --cl --dir "%1" --fileMask "%2" --caseSensitive --showEncoding --find "DEBUG = %3" --replace "DEBUG = %4"

Then I defined the corresponding pre-build events at the web project like this:

cd $(ProjectDir)BuildScripts
call switch_client_debug.bat $(ProjectDir)ts site.ts false true

and its pair at Release config:

cd $(ProjectDir)BuildScripts
call switch_client_debug.bat $(ProjectDir)ts site.ts true false

Now everything works like a charm and I can have logging, tracing, special logic for Debug and for Release configuration in Javascript.

Question 3

Only for IE there is the conditional compilation:

/*@cc_on
@set @version = @_jscript_version
@if (@_win32)
document.write("You are running 32 bit IE " + @version);
@elif (@win_16)
document.write("You are running 16 bit IE " + @version);
@else @*/
document.write("You are running another browser or an old IE.");
/*@end @*/

nice article here

Question 4

Late in the party, but here is what I do on Visual Studio

// #region Uncomment when in debug mode
// localStorage.clear();
// #endregion