The parameter in Angular cannot be assigned a type of 'null'

@Injectable({ providedIn: 'root' })
export class AuthenticationService {
    private currentUserSubject: BehaviorSubject<User>;
    public currentUser: Observable<User>;

    constructor(private http: HttpClient) {
        this.currentUserSubject = new BehaviorSubject<User>(JSON.parse(localStorage.getItem('currentUser')));
        this.currentUser = this.currentUserSubject.asObservable();
    }

    public get currentUserValue(): User {
        return this.currentUserSubject.value;
    }

    login(username: string, password: string) {
        return this.http.post<any>(`${environment.apiUrl}/users/authenticate`, { username, password })
            .pipe(map(user => {
                // store user details and jwt token in local storage to keep user logged in between page refreshes
                localStorage.setItem('currentUser', JSON.stringify(user));
                this.currentUserSubject.next(user);
                return user;
            }));
    }

    logout() {
        // handle logging out the user
        this.removeCurrentUserFromLocalStorageAndBehaviorSubject();
    }

    private removeCurrentUserFromLocalStorageAndBehaviorSubject() {
        localStorage.removeItem('currentUser');
        this.currentUserSubject.next(null);
    }
}

Can someone help me understand why I am encountering an error on the last line (this.currentUserSubject.next(null))? The error message states: Argument of type 'null' is not assignable to parameter of type 'User'. Is this due to the latest TypeScript version restrictions? What could be an alternative solution?

Answer №1

Typescript has implemented stricter rules.

To work around this, you have two options: disable strict mode or modify your observable to allow for null values:

private currentUserSubject: BehaviorSubject<User | null>;

Answer №2

Determine the necessity of

this.currentUserSubject.next(null);
in the codebase

Answer №3

It is highly recommended to always use <User| null> as the best practice.

For example, you can declare a private currentUserSource = new ReplaySubject<User | null>(1);

Then, you can call that function like this: this.currentUserSource.next(null); or simply

declare private currentUserSource = new ReplaySubject(1);

And then use this.currentUserSource.next(); () which can be interpreted as null.

Answer №4

Here is how you can utilize it:

Simply implement it as follows:

or

Assign the currentUserSubject to a new BehaviorSubject<User | null>(JSON.parse(localStorage.getItem('currentUser')));

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