promisify.js

const fs = require('fs');
const readFile = promisify(fs.readFile);

readFile('./hello', 'utf8').then(console.log);
// outputs
'world';

function promisify(fn) {
   return (...a) => {
      return new Promise((resolve, reject) => {
        fn(...a, (e, a) => {
          if (e)
            throw e;

          resolve(a);
        });
      });
    }
}