A tutorial on ensuring Angular loads data prior to attempting to load a module

Question

A tutorial on ensuring Angular loads data prior to attempting to load a module

Just starting my Angular journey... Here's some code snippet:

  ngOnInit(): void {
    this.getProduct();
  }

  getProduct(): void {
    const id = +this.route.snapshot.paramMap.get('id');
    this.product = this.products.getProduct(id);
    console.log(this.product);
  }

When I first open this, I get "TypeError: Cannot read property 'src' of undefined" The console.log shows "undefined", but when I navigate home and then come back, it works perfectly... Logs the correct item and displays the right image

public getProduct(id: number): Product {
   // console.log('returning product');
   return (this.products.find(product => product.id === id));
 }

The module that returns the list of products was previously loaded from a .json file

  constructor( private http: HttpClient ) {
    this.getJSON().subscribe(data => {
      this.products = data.products;
     });
  }

This module is used to create a grid of products, so to avoid loading the .json file twice, I imported it and reused it.

If anyone can explain why it doesn't work the first time after loading the page (using ng serve) and why it works fine every subsequent time, I would greatly appreciate it.

Edit, here is the template and the entire component.ts as requested:

<div>
<img src="{{product.src}}" alt="">
</div>

import { Component, OnInit } from '@angular/core';
import { ActivatedRoute } from '@angular/router';
import { Location } from '@angular/common';

import { Product } from '../classes/product.interface';
import { ShoppingGridComponent } from '../shopping-grid/shopping-grid.component';

@Component({
  selector: 'app-product',
  templateUrl: './product.component.html',
  styleUrls: ['./product.component.css']
})
export class ProductComponent implements OnInit {
  product: Product;
  constructor(
    private route: ActivatedRoute,
    private location: Location,
    private products: ShoppingGridComponent
  ) {}
  ngOnInit(): void {
    this.getProduct();
  }

  getProduct(): void {
    const id = +this.route.snapshot.paramMap.get('id');
    console.log(id);
    this.product = this.products.getProduct(id);
    console.log(this.product);
  }
}

angular typescript

Answer 1

Answer №1

The purpose of utilizing Resolvers is to preload data prior to component initialization.

Below is an illustration of a resolver:

@Injectable({ providedIn: 'root' })
export class ProductsResolver implements Resolve<Product> {
  constructor(private http: HttpService) {}

  resolve(
    route: ActivatedRouteSnapshot,
    state: RouterStateSnapshot
  ): Observable<any>|Promise<any>|any {
    return this.http.get('api/v1/products/' + route.paramMap.get('id'));
  }
}

You can then assign it to your routes:

const routes = [
    {
        path: 'products/:id'
        component: ProductComponent,
        resolve: {
            product: ProductsResolver
            //additional resolvers can also be included here!
        }
    }
]

Finally, you can retrieve the data in your component:

constructor(private route: ActivatedRoute) {}

ngOnInit() {
  this.product = this.route.snapshot.data.product;
}

Answer 2

The purpose of utilizing Resolvers is to preload data prior to component initialization.

Below is an illustration of a resolver:

@Injectable({ providedIn: 'root' })
export class ProductsResolver implements Resolve<Product> {
  constructor(private http: HttpService) {}

  resolve(
    route: ActivatedRouteSnapshot,
    state: RouterStateSnapshot
  ): Observable<any>|Promise<any>|any {
    return this.http.get('api/v1/products/' + route.paramMap.get('id'));
  }
}

You can then assign it to your routes:

const routes = [
    {
        path: 'products/:id'
        component: ProductComponent,
        resolve: {
            product: ProductsResolver
            //additional resolvers can also be included here!
        }
    }
]

Finally, you can retrieve the data in your component:

constructor(private route: ActivatedRoute) {}

ngOnInit() {
  this.product = this.route.snapshot.data.product;
}

Answer 3

Answer №2

When dealing with subscriptions, it's important to understand that asynchronous data is at play. If you want to learn more about how they work, check out this resource.

With asynchronous data, the information isn't immediately returned like a normal function call. Instead, you subscribe to receive the data and it's up to the observable to decide when you get it.

In the code snippet provided, the getProduct() function assumes that the this.products variable is already defined, but that may not always be the case.

If you're subscribing to the observable in the service, I recommend moving the subscription logic to the component instead.

You can achieve this by consolidating everything, including the subscription, within the OnInit hook in the following manner:

Component


products: Product[] = [];

ngOnInit(): void {
  this.getJSON().subscribe(data => {
    this.products = data.products;
    
    this.getProduct();
  });
}

getProduct(): void {
  const id = +this.route.snapshot.paramMap.get('id');
  this.product = this.products.getProduct(id);
  console.log(this.product);
}

Answer 4

When dealing with subscriptions, it's important to understand that asynchronous data is at play. If you want to learn more about how they work, check out this resource.

With asynchronous data, the information isn't immediately returned like a normal function call. Instead, you subscribe to receive the data and it's up to the observable to decide when you get it.

In the code snippet provided, the getProduct() function assumes that the this.products variable is already defined, but that may not always be the case.

If you're subscribing to the observable in the service, I recommend moving the subscription logic to the component instead.

You can achieve this by consolidating everything, including the subscription, within the OnInit hook in the following manner:

Component


products: Product[] = [];

ngOnInit(): void {
  this.getJSON().subscribe(data => {
    this.products = data.products;
    
    this.getProduct();
  });
}

getProduct(): void {
  const id = +this.route.snapshot.paramMap.get('id');
  this.product = this.products.getProduct(id);
  console.log(this.product);
}

Answer 5

Answer №3

To tackle this issue, consider implementing resolvers. Here's a helpful resource on how to utilize them effectively: https://codeburst.io/understanding-resolvers-in-angular-736e9db71267

Answer 6

To tackle this issue, consider implementing resolvers. Here's a helpful resource on how to utilize them effectively: https://codeburst.io/understanding-resolvers-in-angular-736e9db71267

Answer 7

Answer №4

To prevent errors, incorporate the optional chaining operator (also known as the "question mark") in your template code:

<div>
  <img src="{{product?.src}}" alt="">
</div>

Commonly referred to as the "elvis operator," this technique ensures that no errors will occur if the product variable is null or undefined.

This method assumes that eventually, a value will be assigned to product. It is especially useful when you are retrieving product information from a database, and during the retrieval process, product remains undefined.

[UPDATE]: With the latest modifications to the original question, consider the following updated code snippet:

ngOnInit() {
  this.getJSON().pipe(map(data => data.products)).subscribe(products => {
    this.products = products;
    this.product = this.products.find(product => product.id === id);
  });
}

Answer 8

To prevent errors, incorporate the optional chaining operator (also known as the "question mark") in your template code:

<div>
  <img src="{{product?.src}}" alt="">
</div>

Commonly referred to as the "elvis operator," this technique ensures that no errors will occur if the product variable is null or undefined.

This method assumes that eventually, a value will be assigned to product. It is especially useful when you are retrieving product information from a database, and during the retrieval process, product remains undefined.

[UPDATE]: With the latest modifications to the original question, consider the following updated code snippet:

ngOnInit() {
  this.getJSON().pipe(map(data => data.products)).subscribe(products => {
    this.products = products;
    this.product = this.products.find(product => product.id === id);
  });
}

A tutorial on ensuring Angular loads data prior to attempting to load a module

Answer №1

Answer №2

Answer №3

Answer №4

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