events.js:183
      throw er; // Unhandled 'error' event
      ^
CastError: Cast to ObjectId failed for value "john" at path "_id" for model "User"

module.exports = function(passport) {
  let opts = {};
  opts.jwtFromRequest = ExtractJwt.fromAuthHeaderAsBearerToken();
  opts.secretOrKey = config.secret;
  passport.use(new JwtStrategy(opts, (jwt_payload, done) => {
    User.getUserById(jwt_payload._id, (err, user) => {
      if(err) {
        return done(err, false);
      } if (user) {
        return done(null, user);
      } else {
        return done(null, false);
      }
    });
  }));
}

router.post('/authenticate', (req, res, next) => {
  const username = req.body.username;
  const password = req.body.password;

  User.getUserById(username, (err, user) => {
    if(err) throw err;
    if(!user) {
      return res.json({success: false, msg: 'User not found'});
    }

    User.comparePassword(password, user.password, (err, isMatch) => {
      if(err) throw err;
      if(isMatch) {
        const token = jwt.sign(user, config.secret, {
          expiresIn: 604800 // 1 week
        });

        res.json({
          success: true,
          token: 'JWT ' + token,
          user: {
            id: user._id,
            name: user.name,
            username: user.username,
            email: user.email
          }
        });
      } else {
        return res.json({success: false, msg: 'Incorrect password'});
      }
    });

  });
});

module.exports.comparePassword = function(candidatePassword, hash, callback){
  bcrypt.compare(candidatePassword, hash, (err, isMatch) => {
    if(err) throw err;
    callback(null, isMatch);
  });
}