Use try/throw/catch plus the Spidermonkey Error.fileName extension to get

the startup path.

xtrn/tw2/filename.js

@@ -2,5 +2,5 @@ function fname(name)
 {
 	if(name.search(/^(?:[A-Z]:)?[\/\\]/)!=-1)
 		return(name);
-	return("../xtrn/tw2/"+name);
+	return(startup_path+name);
 }

xtrn/tw2/tw2.js

+var startup_path='.';
+try { throw new Error() } catch(e) { startup_path=e.fileName }
+startup_path=startup_path.replace(/\/[^\/]*$/,'');
+startup_path=backslash(startup_path);
+
 load("recordfile.js");
 load("lockfile.js");
-load("../xtrn/tw2/filename.js");
+load(startup_path+"filename.js");
 load(fname("gamesettings.js"));
 var Settings=new GameSettings();
 

xtrn/tw2/twint500.js

-load("../xtrn/tw2/filename.js");
+var startup_path='.';
+try { throw new Error() } catch(e) { startup_path=e.fileName }
+startup_path=startup_path.replace(/\/[^\/]*$/,'');
+startup_path=backslash(startup_path);
+
+load(startup_path+"filename.js");
 load(fname("gamesettings.js"));
 load(fname("sector_map.js"));
 load(fname("ports_map.js"));