This appears to be caused by the fact that fs module is mutated by promisify('fs'). Internally, fs.writeFile uses some other fs methods in a callback fashion, so it expects fs to be un-modified.

Repro:

const promisify = require('promisify-node');
const fs = promisify('fs');
fs.writeFile('./testfile.txt', 'Hello World')
  .then(() => {
    console.log('File saved!');
  })
  .catch((error) => {
    console.error('Error doing something:', error);
  });

Neither 'File saved!' or 'Error doing something' ever gets logged. The file gets created but no data is written to it. Tested on OS X 10.11 using Node v5.