Javascript: why this line is required for functionality?

This function is to return to the main page. But I have one question: When I delete the line requestpath += options.index; why does it give me an error 'encountered error while processing GET of “/“' with response code 500? without that line wouldn't it be localhost:3000/ which would return me to index. I'm guessing it is something with fs.exist function at the end.

var return_index = function(request, response, requestpath)  {

    var exists_callback = function(file_exists) {
      if (file_exists) {
        return serve_file(request, response, requestpath);
      } else {
        return respond(request, response, 404);
      }
  }

  if (requestpath.substr(-1) !== '/') {
    requestpath += "/";
  }
  requestpath += options.index;
  return fs.exists(requestpath, exists_callback);
}

Any help is appreciated

Edit: option is:

var options = {
  host: 'localhost',
  port: 8080,
  index: 'index.html',
  docroot: '.'
};

so option.index will give 'index.html'