error-first callback

It is really only two rules for defining an error-first callback:
1. The first argument of the callback is reserved for an error object.
2. The second argument of the callback is reserved for any successful response data.

fs.readFile('/foo.txt', function(err, data) {
  // TODO: Error Handling
   if(err) {
    console.log('Unknown Error');
    return;
  }
  console.log(data);
});