The specified type 'Observable<{ product: null; error: string; }>' cannot be assigned to type 'Observable<ProductResolved>'

I have the following IProduct and ProductResolved interfaces

export interface IProduct {
  id: number;
  productName: string;
  productCode: string;
  category: string;
  tags?: string[];
  releaseDate: string;
  price: number;
  description: string;
  starRating: number;
  imageUrl: string;
}

export interface ProductResolved {
 product: IProduct;
 error?: any;
}

I am attempting to use them in ProductResolver as shown below

    import { Injectable } from "@angular/core";
    import { ActivatedRouteSnapshot, Resolve, RouterStateSnapshot } from "@angular/router";
    import { Observable, of } from "rxjs";
    import { ProductResolved } from "./product";
    import { ProductService } from "./product.service";
    
    @Injectable({
        providedIn: 'root'
    })
    export class ProductResolver implements Resolve<ProductResolved> {
    
        constructor(private productService: ProductService) { }
    
        resolve(route: ActivatedRouteSnapshot, state: RouterStateSnapshot):Observable<ProductResolved> {
            const id = route.paramMap.get('id');
            if (isNaN(+!id)) {
                const message = `Product id was not a number: ${id}`;
                console.error(message);
                return of({product : null,error:message}); //error here
            }
            return of(); //just for example i have return of() here I will change it later
        } 
    }

when I return (in the code above)

return of({product : null,error:message}); //error here

an error is thrown stating

Type 'Observable<{ product: null; error: string; }>' is not assignable to type 'Observable'. Type '{ product: null; error: string; }' is not assignable to type 'ProductResolved'. Types of property 'product' are incompatible. Type 'null' is not assignable to type 'IProduct'.ts(2322)

Note :- the above code appears to work in typescript version "~3.5.3" but encounters an issue in typescript version "~4.3.2"

Question :-

  1. Are there any changes in the latest typescript version("~4.3.2") related to the code above?
  2. What are the best practices to handle the scenario mentioned above according to the new typescript standard? (if there are any relevant changes)

Answer №1

To address this issue, there are three possible solutions:

  • Adjust the property product to be optional: For more details, please refer to this link

    export interface ProductResolved {
          product?: IProduct;
          error?: any;
         }
    
  • Update the product property to accept either IProduct or null:

    export interface ProductResolved {
          product: IProduct | null;
          error?: any;
         }
    
  • Change the product property to any type:

    export interface ProductResolved {
          product: any;
          error?: any;
         }
    
  • Utilize type assertions such as:

    of({product: null, error: message} as IProduct)

For additional information, please visit this resource

Answer №2

Instead of using null, try using {}. If that doesn't solve the issue, consider providing a product: IProduct

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