Why does this variable return undefined instead of an object in my Angular 13 app?

Question

I am working on an e-commerce app who's front-end is made in Angular 13.

I use a service to handle the products coming from an API. I run into a problem while ring to display the product details.

See Stackblitz demo HERE.

In app\services\product.service.ts I have:

import { Injectable } from '@angular/core';
import { Observable } from 'rxjs';
import { HttpClient } from '@angular/common/http';
import { Product, ProductResponse } from '../models/product';

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

  products: Product[] = [];

  apiURL: string = 'https://dummyjson.com';

  constructor(private http: HttpClient) {}
  
  // Product List       
  public getProducts(): Observable<ProductResponse>{
    return this.http.get<ProductResponse>(`${this.apiURL}/products`);
  }

  // Product Details (single product)
  public getProductDetails(id: Number): Observable<ProductResponse>{
    return this.http.get<ProductResponse>(`${this.apiURL}/products/${id}`);
  }
}

In app\app.module.ts:

import { NgModule } from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';
import { CommonModule } from '@angular/common';
import { HttpClientModule } from '@angular/common/http';
import { AppComponent } from './app.component';
import { Routes, RouterModule } from '@angular/router';
import { NavbarComponent } from './components/navbar/navbar.component';
import { FooterComponent } from './components/footer/footer.component';
import { SidebarComponent } from './components/sidebar/sidebar.component';
import { HomeComponent } from './components/home/home.component';
import { ProductItemComponent } from './components/product-item/product-item.component';
import { ProductsListComponent } from './components/products-list/products-list.component';
import { ProductsDetailsComponent } from './components/products-details/products-details.component';

const routes: Routes = [
  { path: '', component:  HomeComponent},
  { path: 'products', component:  ProductsListComponent},
  { path: 'products/show/:id', component:  ProductsDetailsComponent},
];

@NgModule({
  declarations: [
    AppComponent,
    NavbarComponent,
    FooterComponent,
    SidebarComponent,
    ProductsListComponent,
    ProductItemComponent ,
    ProductsDetailsComponent
  ],
  imports: [
    CommonModule,
    BrowserModule,
    HttpClientModule,
    RouterModule.forRoot(routes),
  ],
  providers: [],
  bootstrap: [AppComponent]
})
export class AppModule { }

In app\models\product.ts:

export class Product {
    id?: number;
    title?: string;
    description?: string;
    price?: number;
    discountPercentage?: number;
    rating?: number;
    stock?: number;
    brand?: string;
    category?: string;
    thumbnail?: string;
}

export interface ProductResponse {
    products: Product[];
    total: number;
    skip: number;
    limit: number;
}

In app\components\products-details\products-details.component.ts I have:

import { Component, OnInit, InputDecorator, Input } from '@angular/core';
import { Router, ActivatedRoute } from '@angular/router';
import { Product, ProductResponse } from '../../models/product';
import { ProductService } from '../../services/product.service';

@Component({
  selector: 'app-products-details',
  templateUrl: './products-details.component.html',
  styleUrls: ['./products-details.component.css']
})
export class ProductsDetailsComponent implements OnInit {

  @Input() product!: Product;
  productResponse: any;

  constructor(private ProductService: ProductService, private Router: Router, private ActivatedRoute:ActivatedRoute) { }

  ngOnInit(): void {
    const id = Number(this.ActivatedRoute.snapshot.paramMap.get('id'));
    this.ProductService.getProductDetails(id).subscribe((response) => (this.productResponse = response));

  }
}

In app\components\products-details\products-details.component.html I have:

<h1>{{ product.title }}</h1>

The problem

When I access a product details route (for instance, http://localhost:4200/products/show/1), the page displays an empty <h1> tag and the Chrome console shows Cannot read properties of undefined (reading 'title').

Where is my mistake?

Answer 1

Unlike the https://dummyjson.com/products endpoint, the https://dummyjson.com/products/{id} returns a plain Product object, so:

// product.service.ts
public getProductDetails(id: Number): Observable<Product>{
  return this.http.get<Product>(`${this.apiURL}/products/${id}`);
}

// products-details.component.ts
// the @Input decorator is wrong — the data is not passed to the component from outside
// but instead fetched inside of the component
@Input() product!: Product;
// productResponse: any — this field is unused and should be removed

ngOnInit(): void {
  ...
  this.ProductService.getProductDetails(id).subscribe((product) => (this.product = product));
}