gecko-dev/dom/ipc/tests/file_app.sjs

var gBasePath = "tests/dom/ipc/tests/";

function handleRequest(request, response) {
  var query = getQuery(request);

  var testToken = '';
  if ('testToken' in query) {
    testToken = query.testToken;
  }

  var template = '';
  if ('template' in query) {
    template = query.template;
  }
  var template = gBasePath + template;
  response.setHeader("Content-Type", "application/x-web-app-manifest+json", false);
  response.write(readTemplate(template).replace(/TESTTOKEN/g, testToken));
}

// Copy-pasted incantations. There ought to be a better way to synchronously read
// a file into a string, but I guess we're trying to discourage that.
function readTemplate(path) {
  var file = Components.classes["@mozilla.org/file/directory_service;1"].
                        getService(Components.interfaces.nsIProperties).
                        get("CurWorkD", Components.interfaces.nsILocalFile);
  var fis  = Components.classes['@mozilla.org/network/file-input-stream;1'].
                        createInstance(Components.interfaces.nsIFileInputStream);
  var cis = Components.classes["@mozilla.org/intl/converter-input-stream;1"].
                       createInstance(Components.interfaces.nsIConverterInputStream);
  var split = path.split("/");
  for(var i = 0; i < split.length; ++i) {
    file.append(split[i]);
  }
  fis.init(file, -1, -1, false);
  cis.init(fis, "UTF-8", 0, 0);

  var data = "";
  let str = {};
  let read = 0;
  do {
    read = cis.readString(0xffffffff, str); // read as much as we can and put it in str.value
    data += str.value;
  } while (read != 0);
  cis.close();
  return data;
}

function getQuery(request) {
  var query = {};
  request.queryString.split('&').forEach(function (val) {
    var [name, value] = val.split('=');
    query[name] = unescape(value);
  });
  return query;
}