Get token from the header?

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Angular5

Say I have the routing app-routing.module.ts

onst appRoutes: Routes = [
  { path: 'login/:id', canActivate: [AuthGuard], children: [] },
  { path: '', canActivateChild: [AuthGuard], children: [
    { path: '', redirectTo: '/courses', pathMatch: 'full' },
    { path: 'courses', component: CourseListComponent,  pathMatch: 'full'},
    { path: 'courses/:courseId', component: CourseDetailComponent, pathMatch: 'full' },
    { path: 'courses/:courseId/unit/:unitId', component: CoursePlayComponent,
      children: [
        { path: '', component: CourseListComponent },
        { path: 'lesson/:lessonId', component: CourseLessonComponent, data:{ type: 'lesson'} },
        { path: 'quiz/:quizId', component: CourseQuizComponent, data: {type: 'quiz'} }
      ]}
    ]},
  { path: '**', component: PageNotFoundComponent, pathMatch: 'full' }];

And auth.guard.ts

export class AuthGuard implements CanActivate , CanActivateChild {

    constructor(private authUserService: AuthUserService, private router: Router) {   }

    canActivate(route: ActivatedRouteSnapshot, state:
    RouterStateSnapshot): boolean |
    Observable<boolean> | Promise<boolean> {
      // save the id from route snapshot
      const id = +route.params.id;

      // if you try to logging with id
      if (id) {
        this.router.navigate(["/courses"]);
           return this.authUserService.login(id);
      }

      // if you already logged and just navigate between pages
      else if (this.authUserService.isLoggedIn())
         return true;

      else {
        this.router.navigate(["/page_not_found"]);
        return false;
      }
     }

      canActivateChild(route: ActivatedRouteSnapshot,state: RouterStateSnapshot): boolean |
      Observable<boolean> | Promise<boolean> {
         return this.canActivate(route, state);
       }

}

auth-user.service.ts

import { catchError, groupBy } from 'rxjs/operators';

import { UserService } from './user.service';
import { IUser } from './user';

@Injectable()
export class AuthUserService implements OnDestroy {

  private user: IUser;
  private errorMessage: string;

  constructor(private userService: UserService) { }

  // store the session and call http get
  login(id: number) {
  return this.userService.getUser(id).pipe(
    map((user) => {
      this.user = user;
      localStorage.setItem('user', JSON.stringify(this.user));
      localStorage.setItem('token', 'JWT');
      return true;
    }),
    catchError((error) => {
      this.errorMessage = <any>error;
      return of(false);
    })
  );
}

  isLoggedIn() {
   return !!localStorage.getItem('token');
  }

  ngOnDestroy() {
    localStorage.removeItem('user');
    localStorage.removeItem('token');
  }
}

In my application there are many pages. Everybody need to login, but it only need to login one time. That is enough. My question is that if I am in the other page, how to retrieve the token to check if I already login?

What I have tried:

login(email:string, password:string) {
    return this.httpClient.post<{access_token:  string}>('http://www.your-server.com/auth/login', {email, password}).pipe(tap(res => {
    localStorage.setItem('access_token', res.access_token);
}))
}