Using Angular 2 to convert and display data as a particular object type in

Question

Using Angular 2 to convert and display data as a particular object type in

I have recently developed a basic application using the Angular2 tutorial as my guide.

Initially, I established a straightforward "Book" model:

 /**
 * Definition of book model
 */
export class Book {
    public data;

    /**
     * Constructor for Book class
     * @param id
     * @param title
     * @param pages
     */
    constructor(
        public id,
        public title:string,
        public pages:Array
    ){
        alert('it works'); // just a sanity check
    }
}

Within my service, I retrieve a book in the following manner:

return this._http.get('getBook/1')
        .map(function(res){
            return <Book> res.json();
        })

My initial assumption was that this code snippet would convert the JSON data received into a Book object.

However, the output is merely an object with the type "Object."

To resolve this issue, I manually create a new Book object and provide the necessary parameters to its constructor, as shown below:

return new Book(res.id, res.title, res.pages);

Is this approach optimal? Have I overlooked any better alternatives?

angular typescript rxjs

Answer 1

Answer №1

Indeed, when casting an object to a type in TypeScript, it's important to note that it doesn't actually create an instance of the specified type. Instead, it serves as a tool for type checking purposes.

If you require an actual instance of Book, you would need to implement something along these lines:

return this._http.get('getBook/1')
    .map(function(res){
        var data = res.json();
        return new Book(data.id, data.title, data.pages);
    })

In response to your inquiry, if your type only consists of fields (perhaps defined through an interface), casting may suffice. However, if there are methods associated with the type that you intend to utilize later on, creating an instance of the Book type explicitly is necessary (as demonstrated above) instead of relying solely on casting. Otherwise, any attempt to access these methods will result in them being undefined within your object...

For further insights, feel free to explore this related question:

How do I cast a JSON object to a typescript class

Answer 2

Indeed, when casting an object to a type in TypeScript, it's important to note that it doesn't actually create an instance of the specified type. Instead, it serves as a tool for type checking purposes.

If you require an actual instance of Book, you would need to implement something along these lines:

return this._http.get('getBook/1')
    .map(function(res){
        var data = res.json();
        return new Book(data.id, data.title, data.pages);
    })

In response to your inquiry, if your type only consists of fields (perhaps defined through an interface), casting may suffice. However, if there are methods associated with the type that you intend to utilize later on, creating an instance of the Book type explicitly is necessary (as demonstrated above) instead of relying solely on casting. Otherwise, any attempt to access these methods will result in them being undefined within your object...

For further insights, feel free to explore this related question:

How do I cast a JSON object to a typescript class

Answer 3

Answer №2

Consider creating an interface called Book instead of a class named book:

export interface Book {
    public id;
    public title:string;
    public pages:Array;
}

In your service:

//retrieve one record
return this._http.get('getBook/1')
        .map(function(res){
            return <Book> res.json();
        });

//retrieve multiple records
return this._http.get('getBooks')
        .map(function(res){
            return <Book> res.json();
        });

Answer 4

Consider creating an interface called Book instead of a class named book:

export interface Book {
    public id;
    public title:string;
    public pages:Array;
}

In your service:

//retrieve one record
return this._http.get('getBook/1')
        .map(function(res){
            return <Book> res.json();
        });

//retrieve multiple records
return this._http.get('getBooks')
        .map(function(res){
            return <Book> res.json();
        });

Answer 5

Answer №3

It is recommended to retrieve data from a GET response using

Observable<Model>

(as per the Angular documentation https://angular.io/guide/http) And then:

// imports

import {HttpClient} from "@angular/common/http";

// in constructor parameter list

private http: HttpClient

// service method

getBook(): Observable<Book> {return this.http.get<Book>({url}, {options});}

Answer 6

It is recommended to retrieve data from a GET response using

Observable<Model>

(as per the Angular documentation https://angular.io/guide/http) And then:

// imports

import {HttpClient} from "@angular/common/http";

// in constructor parameter list

private http: HttpClient

// service method

getBook(): Observable<Book> {return this.http.get<Book>({url}, {options});}

Answer 7

Answer №4

Referencing this Angular documentation

In order to specify the type of response object, it is recommended to create an interface with the necessary properties. Using an interface instead of a class is preferred because the response is a simple object that cannot be directly converted into an instance of a class.

It's important to always use interfaces when receiving responses as all that's required is mapping fields. Trying to use a class may result in errors, especially if there are instance functions involved.

product.getPrice is not a function

For example:

interface ProductInterface {
  id: number;
  name: string;
  price: number;
}

class Product implements ProductInterface {
  id: number;
  name: string;
  price: number;
  
  constructor(productData: ProductInterface) {
    this.id = product.id;
    this.name = product.name;
    this.price = product.price;
  }
  
  public getPrice(): string {
    return this.price + " INR";
  }
}

class ProducService {
  ...
  ...


  getProduct(): Observable<Product> {
    return this.http.get<Product>('assets/product.json').pipe(map(data => new Product(data))); 
  }

  getProductWithoutMappingToClass(): Observable<Product> {
    return this.http.get<Product>('assets/product.json');
  } // Throws runtime error ctx_r1.product.getPrice is not a function
}

Answer 8

Referencing this Angular documentation

In order to specify the type of response object, it is recommended to create an interface with the necessary properties. Using an interface instead of a class is preferred because the response is a simple object that cannot be directly converted into an instance of a class.

It's important to always use interfaces when receiving responses as all that's required is mapping fields. Trying to use a class may result in errors, especially if there are instance functions involved.

product.getPrice is not a function

For example:

interface ProductInterface {
  id: number;
  name: string;
  price: number;
}

class Product implements ProductInterface {
  id: number;
  name: string;
  price: number;
  
  constructor(productData: ProductInterface) {
    this.id = product.id;
    this.name = product.name;
    this.price = product.price;
  }
  
  public getPrice(): string {
    return this.price + " INR";
  }
}

class ProducService {
  ...
  ...


  getProduct(): Observable<Product> {
    return this.http.get<Product>('assets/product.json').pipe(map(data => new Product(data))); 
  }

  getProductWithoutMappingToClass(): Observable<Product> {
    return this.http.get<Product>('assets/product.json');
  } // Throws runtime error ctx_r1.product.getPrice is not a function
}

Using Angular 2 to convert and display data as a particular object type in

Answer №1

Answer №2

Answer №3

Answer №4

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