TS2322：输入&#39;Promise <Hero | undefined> &#39;不可分配给&#39;Promise <Hero> “

Question

I'm learning angular4 from angular tutorial . Below is a function to get a hero from service's function:

@Injectable()
export class HeroService {
    getHeroes(): Promise<Hero[]> {

        return new Promise(resolve => {
            // Simulate server latency with 2 second delay
            setTimeout(() => resolve(HEROES), 2000);
        });
    }

    getHero(id: number): Promise<Hero> {

        if (id < 0) {
            throw new Error('Hero is not found.');
        }
        return this.getHeroes()
            .then(heroes => heroes.find(hero => hero.id === id));
    }
}

On execution it throws error:

TS2322: Type 'Promise<Hero | undefined>' is not assignable to type 'Promise<Hero>'.
Type 'Hero | undefined' is not assignable to type 'Hero'.
Type 'undefined' is not assignable to type 'Hero'.

Anybody else also faced this issue ? Please help. Thank you.

@Component({
    selector: 'hero-detail',
    templateUrl: './hero-detail.component.html'
})
export class HeroDetailComponent implements OnInit {
    @Input() hero: Hero;

    constructor(private heroService: HeroService, private route: ActivatedRoute, private location: Location) { }

    ngOnInit(): void {
        this.route.paramMap
            .switchMap((params: ParamMap) => this.heroService.getHero(+(params.get('id') || -1)))
            .subscribe(hero => this.hero = hero);
    }

    goBack(): void {
        this.location.back();
    }
}

Answer 1

The reason typescript throws this error is because Array.find will return undefined when all elements don't matched the condition hero.id === id .

Refer doc about Array.find :

The find() method returns the value of the first element in the array that satisfies the provided testing function. Otherwise undefined is returned.

---

To avoid the error, you should change your function's return type to Promise<Hero | undefined> Promise<Hero | undefined> .

getHero(id: number): Promise<Hero | undefined> {    // <----mention here for return type
  return this.getHeroes()
             .then(heroes => heroes.find(hero => hero.id === id));
}