Ensuring Type Safety in Typescript: Service Fails to Return Expected Class Object

Can you help me with this issue? Here is the code for the user service:

import { Injectable } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import { User } from './user';

@Injectable()
export class UserService {
   private _url = "https://jsonplaceholder.typicode.com/users";

   constructor(private _httpClient: HttpClient) {
   }

   getUser(id: string){
      return this._httpClient.get<User>(this._url + '/' + id);
   }
}

The getUser method should return an Observable of a User object. However, when I try to get the User object in my component, it retrieves the entire user object instead of the one defined in my user.ts file. Below is the code for user.ts:

export class Address {
   street: string;
   suite: string;
   city: string;
   zipcode: string;    
}

export class User {
   id: number;
   name: string;
   phone: string;
   email: string; 
   address = new Address();
}

When I call the service from the component, I am unable to retrieve the User object as expected. Here is the component code:

import { Component, OnInit } from '@angular/core';
import { UserService } from './user.service';
import { Router, ActivatedRoute } from '@angular/router';

@Component({
   selector: 'newuserform',
   templateUrl: './newuser-form.component.html',
   providers: [UserService]
})
export class NewUserFormComponent implements OnInit {
constructor(
   private _service: UserService, 
   private _router: Router,
   private _route: ActivatedRoute) { }

 ngOnInit(){
    let id = this._route.snapshot.paramMap.get('id');
    this._service.getUser(id)
       .subscribe(
          user => console.log(user);
       );
 }
 }

Answer №1

How to Ensure Type Safety in Typescript - Correcting Service Response

It is important to clarify that when using Typescript for type safety, you are essentially providing guidance to the compiler about the expected structure of your data. This does not mean there will be runtime checks to enforce this structure. The benefit lies in allowing the compiler to catch any deviations from your specified models during development, making debugging easier.

To ensure the properties you want are correctly set, you need to explicitly assign them within your code. This can be done by creating a function for this purpose or directly within functions like map. Consider using interfaces instead of classes if no specific methods are needed.

In cases where the response data differs from your model, you can manually map and transform it to match. Below are two examples demonstrating how this can be achieved:


getUser(id: string): Observable<User> {
  return this._httpClient.get<any>(this._url + "/" + id).pipe(
    map((user: any) => {
      return {
        id: user.id,
        name: user.name,
        phone: user.phone,
        email: user.email,
        address: {
          street: user.address.street,
          suite: user.address.suite,
          city: user.address.city,
          zipcode: user.address.zipcode
        } as Address
      } as User
    })
  );
}

In certain scenarios, unwanted properties can be removed from the response data as shown below:


getUser(id: string): Observable<User> {
  return this._httpClient.get<any>(this._url + "/" + id).pipe(
    map((user: any) => {
      delete user.company;
      delete user.address.geo;
      delete user.website;
      return user as User;
    })
  );
}

For practical examples, refer to the following STACKBLITZ link. Additionally, familiarize yourself with the documentation on classes and interfaces.

Answer №2

There are no restrictions on the server response. Give this a try:

Service:

getUserInfo(id: string): UserInfo {
    const info = this._httpClient.get(this._url + '/' + id);
    return {
        id: info.id,
        name: info.name,
        phone: info.phone,
        email: info.email,
        address: {
            street: info.address.street,
            suite: info.address.suite,
            city: info.address.city,
            zipcode: info.address.zipcode
        }
    };
}

Explanation: const info represents the entire object received from jsonplaceholder. The return statement adjusts the response to your requirements.

Ensure that you declare UserInfo and Address as interfaces rather than classes.

export interface Address {
   street: string;
   suite: string;
   city: string;
   zipcode: string;    
}

export interface UserInfo {
   id: number;
   name: string;
   phone: string;
   email: string; 
   address: Address;
}

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