Then, in src/index.js, import the sign method from the jsonwebtoken package and pass it down to the engine through the handlers. Then, update the engine function to return a signed JWT when a user is found with those credentials. Note that we are using the private key, stored at process.env.PRIVATE_KEY, to sign the token:

function loginUser(req, db, validator, ValidationError, sign) {
  ...
  return client.search( ... )
    .then((res) => {
      if (res.hits.total > 0) {
        const payload = { sub: res.hits.hits[0]._id };
        const options = { algorithm: 'RS512' };
        const token = sign(payload, process.env.PRIVATE_KEY, options);
        return token;
      }
      return Promise.reject(new Error('Not Found'));
    });
}

Now, run our tests again and they should all pass.