解决ts(2729)报错：Property 'userStatus' is used before its initialization

TS2729报错的本质是你声明了userStatus属性为string类型但未设置初始值，就直接在初始化userStatusChanges这个BehaviorSubject时将this.userStatus当作初始参数传入，此时userStatus还未完成赋值，符合「属性在初始化前被使用」的判定规则。

方案1：给userStatus设置初始值 #

最通用的解决方式，根据你的业务逻辑给userStatus设置一个合理的初始值即可：

@Injectable({
  providedIn: 'root'
})
export class FirebaseService {

  constructor(private ngZone: NgZone, private afAuth: AngularFireAuth, private firestore: AngularFirestore , private router: Router) { }

  public currentUser: any;
  // 新增初始值，可根据业务替换为'online'/'offline'/'guest'等自定义标识
  public userStatus: string = 'logged-out';
  public userStatusChanges: BehaviorSubject<string> = new BehaviorSubject<string>(this.userStatus);
  

  setUserStatus(userStatus: any): void {
    this.userStatus = userStatus;
    this.userStatusChanges.next(userStatus);
  }
}

方案2：将userStatus设为可选类型 #

如果业务逻辑允许userStatus初始为未定义状态，可以修改属性类型兼容undefined：

@Injectable({
  providedIn: 'root'
})
export class FirebaseService {

  constructor(private ngZone: NgZone, private afAuth: AngularFireAuth, private firestore: AngularFirestore , private router: Router) { }

  public currentUser: any;
  // 加?标识为可选属性，初始值默认为undefined
  public userStatus?: string;
  // 同步修改BehaviorSubject的泛型类型
  public userStatusChanges: BehaviorSubject<string | undefined> = new BehaviorSubject<string | undefined>(this.userStatus);
  

  setUserStatus(userStatus: any): void {
    this.userStatus = userStatus;
    this.userStatusChanges.next(userStatus);
  }
}

方案3：把BehaviorSubject初始化放到构造函数中 #

你也可以调整初始化顺序，先给userStatus赋值后再初始化Subject：

@Injectable({
  providedIn: 'root'
})
export class FirebaseService {

  public currentUser: any;
  public userStatus: string;
  // 先声明属性暂不初始化
  public userStatusChanges: BehaviorSubject<string>;

  constructor(private ngZone: NgZone, private afAuth: AngularFireAuth, private firestore: AngularFirestore , private router: Router) { 
    // 先给userStatus赋值
    this.userStatus = 'logged-out';
    // 再初始化BehaviorSubject
    this.userStatusChanges = new BehaviorSubject<string>(this.userStatus);
  }

  setUserStatus(userStatus: any): void {
    this.userStatus = userStatus;
    this.userStatusChanges.next(userStatus);
  }
}

内容的提问来源于stack exchange，提问作者adib zikri