gistfile1.js

class RowNotFound extends Error {
  constructor() {
    const message = `Row not found`;
    super(message);
    this.message = message;
    this.name = 'RowNotFound';
  }
}

function checkRowExists(row) {
  if (!row) return Promise.reject(new RowNotFound())
  return row;
}

function update(_id) {
  return User
    .findOneAndUpdate({ _id }, { $set: { updated: true } }, { new: true })
    .exec()
    .then(checkRowExists)
    .then(sendToView)
}

// If you reject the promise with an error instead of a string, you can see the stacktrace (to see what triggered it) and check what kind of error it is
// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/reject
// http://www.devthought.com/2011/12/22/a-string-is-not-an-error/