refactoring-nodejs-router-with-hof.js

function closeWithStatus (response, status, headers) {
  const data = headers || {};
  function end () {
    response.writeHead(status, data);
    response.end();
  }
  return end;
}

function closeWithContent (response, status, headers) {
  const data = headers || {};
  function endWith (content) {
    response.writeHead(status, data);
    response.write(content);
    response.end();
  }
  return endWith;
}

function higherOrderNodeCallback (onSuccess, onError) {
  return function nodeCallback (err, content) {
    if (err) {
      onError(err);
    } else {
      onSuccess(content);
    }
  };
}

router.get(url, function (req, res) {
  const headers = { "Content-Type": "application/octet-stream" };
  const onSuccess = closeWithContent(res, 200, headers);
  const onError = closeWithStatus(res, 404);
  const callback = higherOrderNodeCallback(onSuccess, onError);

  readFile(filePath, callback);
});