// Node.js
router.get('/users/all', authenticate, (req, res) => {
    User.find({some query}).select('firstName lastName email').then((users) => {
        res.send(users);
    });


//Angular Service
return this.http.get(getUserDataUrl, opts)
      .pipe(
        map(response => {
          // here is the problem
          console.log(response.json());
          return response.json()
        }),
        catchError(this.handleErrors)
      );

// Node.js
router.get('/users/all', authenticate, (req, res) => {
User.findOne({some query}).select('firstName lastName email').then((user) => {
    res.send(user); // there was a typo I changed it too. look in the question not user it was users.
});