How to append to a file in Node

Question

I am trying to append a string to a log file. However writeFile will erase the content each time before writing the string.

fs.writeFile('log.txt', 'Hello Node', function (err) {
  if (err) throw err;
  console.log('It\'s saved!');
}); // => message.txt erased, contains only 'Hello Node'

Any idea how to do this the easy way?