Node.js fs write text file asynchronously

var fs = require('fs');

console.log('Going to write into existing file');
fs.writeFile('output.txt', 'Noi dung moi ghi vao file !', function(err){
  if (err) {/*from   w  w w.  j  a  v a 2 s  .c  om*/
    return console.error(err);
  }
  console.log('Data written successfully !');
  console.log('Lers read newly witten data');
  fs.readFile('output.txt', function (err, data){
    if (err) {
      return console.error(err);
    }
    console.log('Asynchronous Reading file: ' + data.toString());
  });
});