Substitution on BATCH (JScript)

@if (1 == 1) /*
@echo off
cscript //nologo //e:JScript "%~f0" %*
@goto :eof
*/
@end
var arg = WScript.Arguments;
if (arg.length != 1) WScript.Quit(1);
var fso = new ActiveXObject("Scripting.FileSystemObject");
var fin = fso.OpenTextFile(arg(0), 1);
var out = WScript.StdOut;
//var out = fso.CreateTextFile(arg(0) + ".new", true);
var log = WScript.StdErr;
//var log = fso.CreateTextFile(arg(0) + ".log", true);
var no = 0;
while (!fin.AtEndOfStream) {
  ++no;
  var line = fin.ReadLine();
  if (no <= 4) {
    out.Write(line + "\r\n");
    continue;
  }
  var separate = line.split("\t");
  if (separate[4] != "") {
    out.Write(line + "\r\n");
    continue;
  }
  separate[4] = "999";
  out.Write(separate.join("\t") + "\r\n");
  log.Write("line: " + no + ", changed.\r\n");
}
WScript.Quit(0);