code: function promisifyThis(fn, self, ...args) {
    return new Promise((resolve, reject) => {
      args = args.slice();
      args.push(function(err, result) {
        if (err) {
          reject(err); 
        } else {
          resolve(result); 
        }
      });
      try {
        fn.apply(self, args);
      } catch (ex) {
        reject(ex);
      }
    });
  }
docs: /*
  function promisify(fn, ...args) {
  return new Promise((resolve, reject) => {
  args = args.slice();
  args.push(function(err, result) {
  if (err)
  reject(err);
  else
  resolve(result);
  });
  try {
  fn.apply(null, args);
  } catch(ex) {
  reject(ex);
  }
  });
  }
  module.exports.promisify = promisify;