Angular : TypeError: Cannot read property 'length' of null when subscribe()

Question

I'm getting this response when making an HTTP request in service:

TypeError: Cannot read property 'length' of null
    at eval (http.js:123)
    at Array.forEach (<anonymous>)
    at HttpHeaders.lazyInit (http.js:117)
    at HttpHeaders.init (http.js:265)
    at HttpHeaders.forEach (http.js:368)
    at Observable.eval [as _subscribe] (http.js:2172)
    at Observable.subscribe (Observable.js:162)
    at eval (subscribeToObservable.js:16)
    at subscribeToResult (subscribeToResult.js:6)
    at MergeMapSubscriber._innerSub (mergeMap.js:127)
ALERT!!!!! 

I subscribe the HTTP request when loading my component:

export class TasksComponent implements OnInit {
  currentUser:any;
  username:string=null;
  constructor(private usersService:UsersService) {}
  ngOnInit() {
    this.username=localStorage.getItem('currentUsername');
    console.log(this.username);
    this.usersService.getUserByUsername(this.username)
      .subscribe(data=>{
        console.log(data);
        this.currentUser=data;
      },err=>{
        console.log(err);
        console.log("ALERT!!!!! ");
      })
  }
}

UsersService:

//for getting a user by its username
getUserByUsername(username:string){
  if(this.jwtToken==null) this.authenticationService.loadToken();
    return this.http.get(this.host+"/userByUsername?username="+username
          , {headers:new HttpHeaders({'Authorization':this.jwtToken})}
    );
}

How I stored the username in the localStorage so I can use it to find the user with all his properties :

@Injectable()
export class AuthenticationService {
  private host:string="http://localhost:8080";
  constructor(private http:HttpClient){}
  login(user){
    localStorage.setItem('currentUsername', user.username);
    return this.http.post(this.host+"/login",user, {observe:'response'});
  }
}

My localStorage after a Log In

Knowing that the method is working in the back-end like in this picture what do you think the problem is? is it a service injection problem or is it about dependencies, or another thing?

EDIT The loadToken function:

loadToken(){
    this.jwtToken=localStorage.getItem('token');
    console.log(this.jwtToken);
    let jwtHelper=new JwtHelper();
    this.roles=jwtHelper.decodeToken(this.jwtToken).roles;
    return this.jwtToken;
}

Answer 1

since you don't see the request in the network-panel of your browser, it seems that you don't send the request. This is probably because you don't set the token in this line:

if(this.jwtToken==null) this.authenticationService.loadToken();

the error:

TypeError: Cannot read property 'length' of null
    at eval (http.js:123)
    at Array.forEach (<anonymous>)
    at HttpHeaders.lazyInit (http.js:117)
    at HttpHeaders.init (http.js:265)
    at HttpHeaders.forEach (http.js:368)

indicates, that your Header (Authorization) is null and therefore can't be read.

try to change the line to:

if(this.jwtToken==null) this.jwtToken = this.authenticationService.loadToken();

now you should see your request in the network panel

or maybe you just want to check the token on the authenticationService itself:

if(this.authenticationService.jwtToken==null) this.authenticationService.loadToken();
    return this.http.get(this.host+"/userByUsername?username="+username
      , {headers:new HttpHeaders({'Authorization':this.authenticationService.jwtToken})}
);