core.js：5873错误TypeError：无法读取未定义的属性“ subscribe”

Question

我正尝试为Angular / NodeJS应用程序编写一个sigup方法，如下所示，但是当我尝试检查要注册的用户是否存在时，我的相关方法不起作用。

users.js：

router.get('/findUser', cors.corsWithOptions, (req,res,next) => {
  User.find(req.body.user)
  .then((user) => {
      res.statusCode = 200;
      res.setHeader('Content-Type', 'application/json');
      return res.json({status: 'User founded...', success: true});
  }, (err) => next(err))
  .catch((err) => next(err));
});

router.post('/signup', cors.corsWithOptions, (req, res, next) => {
  User.register(new User({username: req.body.username}), 
    req.body.password, req.body.admin, (err, user) => {
    if(err) {
      res.statusCode = 500;
      res.setHeader('Content-Type', 'application/json');
      res.json({err: err});
    }
    else {
      user.password = req.body.password;
      if (req.body.firstname)
        user.firstname = req.body.firstname;
      if (req.body.lastname)
        user.lastname = req.body.lastname;
      if (req.body.admin)
        user.admin = req.body.admin;        
      user.save((err, user) => {
        if (err) {
          res.statusCode = 500;
          res.setHeader('Content-Type', 'application/json');
          res.json({err: err});
          return next(err);
        }
        passport.authenticate('local')(req, res, () => {
          res.statusCode = 200;
          res.setHeader('Content-Type', 'application/json');
          res.json({success: true, status: 'Registration Successful!'});
        });
      });
    }
  });
});

auth.service.ts：

   checkUserExistence(user : any) : any {
    this.http.get<UserResponse>(baseURL + 'users/findUser')
    .subscribe(res => {
      if(res.success){
        console.log('user founded!');
        return true;
      }
    },
    err => {
      console.log('A problem happened: ', err);
      this.destroyUserCredentials();
      return false;
    });
  }


   signUp(user: any): Observable<any> {
    if (this.checkUserExistence(user) === false) {
      return this.http.post<RegResponse>(baseURL + 'users/signup',
      {'username': user.username, 'password': user.password})
      .pipe( map(res => {
          this.storeUserCredentials({username: user.username});
          return {'success': true, 'username': user.username };
      }),
       catchError(error => this.processHTTPMsgService.handleError(error)));

    }
    else{
      console.log('This user already exists!')
    }

  }

在chrome控制台中，我看到它将用户保存在db上，并且还出现以下错误：

User:  {username: "testuser", password: "123", remember: false}
auth.service.ts:124 This user already exists!
core.js:5873 ERROR TypeError: Cannot read property 'subscribe' of undefined
    at SignupComponent.onSubmit (signup.component.ts:23)
    at SignupComponent_Template_form_ngSubmit_7_listener (signup.component.html:13)
    at executeListenerWithErrorHandling (core.js:21685)
    at wrapListenerIn_markDirtyAndPreventDefault (core.js:21727)
    at SafeSubscriber.schedulerFn [as _next] (core.js:36900)
    at SafeSubscriber.__tryOrUnsub (Subscriber.js:183)
    at SafeSubscriber.next (Subscriber.js:122)
    at Subscriber._next (Subscriber.js:72)
    at Subscriber.next (Subscriber.js:49)
    at EventEmitter.next (Subject.js:39)
defaultErrorLogger @ core.js:5873
handleError @ core.js:5926
handleError @ core.js:13548
executeListenerWithErrorHandling @ core.js:21688
wrapListenerIn_markDirtyAndPreventDefault @ core.js:21727
schedulerFn @ core.js:36900
__tryOrUnsub @ Subscriber.js:183
next @ Subscriber.js:122
_next @ Subscriber.js:72
next @ Subscriber.js:49
next @ Subject.js:39
emit @ core.js:36819
onSubmit @ forms.js:6253
NgForm_submit_HostBindingHandler @ forms.js:6294
executeListenerWithErrorHandling @ core.js:21685
wrapListenerIn_markDirtyAndPreventDefault @ core.js:21727
(anonymous) @ platform-browser.js:934
invokeTask @ zone-evergreen.js:400
onInvokeTask @ core.js:41235
invokeTask @ zone-evergreen.js:399
runTask @ zone-evergreen.js:168
invokeTask @ zone-evergreen.js:481
invokeTask @ zone-evergreen.js:1596
globalZoneAwareCallback @ zone-evergreen.js:1622
auth.service.ts:50 user founded!

我尝试多次修改此代码，但无法解决问题！由于我是一名新学习者，请帮助我知道为什么会发生这种情况以及如何解决？