Angular Admin Guard 问题无法测试“void”类型的表达式的真实性

Question

I have

public getUser(userId: string) {
return this.http.get<{
  _id: string;
  registrationStep: number;
  userType: string;
  isAdmin: boolean;
}>(BACKEND_URL+"getUserData/" + userId);

}

in my auth service. I want to read boolean value from database and set Admin guard to true.

I'm calling the function getIsAdmin from AdminGuard

getIsAdmin() {
this.getUser(this.getUserId()).subscribe(result => {
  return result.isAdmin;
});

}

And this is my Admin Guard code

    import {
  CanActivate,
  ActivatedRouteSnapshot,
  RouterStateSnapshot,
  Router
} from "@angular/router";
import { Injectable } from "@angular/core";
import { Observable } from "rxjs";

import { AuthService } from "./auth.service";

@Injectable()
export class AdminGuard implements CanActivate {
  constructor(private authService: AuthService, private router: Router) {}

  canActivate(
    route: ActivatedRouteSnapshot,
    state: RouterStateSnapshot
  ): boolean | Observable<boolean> | Promise<boolean> {
    const isAdmin = this.authService.getIsAdmin();
    if (!isAdmin) {
      this.router.navigate(['auth/admin']);
    }
    return isAdmin;
  }
}

I get An expression of type 'void' cannot be tested for truthiness. Thank you for your help!

Answer 1

As others are pointing out you DO NOT need to subscribe. canActivate accepts an Observable<boolean> as a return type.

If you subscribe, the canActivate function will have already returned and you won't get the value of the subscription back.

The following implementation should suffice:

// authService
getIsAdmin(): Observable<boolean> {
    return this.getUser(this.getUserId()).pipe(
        map(user => user.isAdmin)
    );
}

And for the canActivate you want to do a custom redirect upon receiving false so you could:

canActivate(…): Observable<boolean> {
    const isAdmin$ = this.authService.getIsAdmin();
    return isAdmin$.pipe(
        tap(isAdmin => {
            if(!isAdmin) {
                this.router.navigate(['auth/admin']);
            }
        })
    );
}

Answer 2

return an observable from the method "getIsAdmin", then subscribe it in canActivate method and the assign the value to isAdmin variable