"Type 'Object' is not assignable to type" with new HttpClient / HttpGetModule

Question

Following Google's official Angular 4.3.2 doc here, I was able to do a simple get request from a local json file. I wanted to practice hitting a real endpoint from JSON placeholder site, but I'm having trouble figuring out what to put in the .subscribe() operator. I made an IUser interface to capture the fields of the payload, but the line with .subscribe(data => {this.users = data}) throws the error Type 'Object' is not assignable to type 'IUser[]'. What's the proper way to handle this? Seems pretty basic but I'm a noob.

My code is below:

import { Component, OnInit } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import { IUsers } from './users';

@Component({
  selector: 'pm-http',
  templateUrl: './http.component.html',
  styleUrls: ['./http.component.css']
})
export class HttpComponent implements OnInit {
  productUrl = 'https://jsonplaceholder.typicode.com/users';
  users: IUsers[];
  constructor(private _http: HttpClient) { }

  ngOnInit(): void {    
    this._http.get(this.productUrl).subscribe(data => {this.users = data});
  }

}

Answer 1

You actually have a few options here, but use generics to cast it to the type you're expecting.

   // Notice the Generic of IUsers[] casting the Type for resulting "data"
   this.http.get<IUsers[]>(this.productUrl).subscribe(data => ...

   // or in the subscribe
   .subscribe((data: IUsers[]) => ...

Also I'd recommend using async pipes in your template that auto subscribe / unsubscribe, especially if you don't need any fancy logic, and you're just mapping the value.

users: Observable<IUsers[]>; // different type now

this.users = this.http.get<IUsers[]>(this.productUrl);

// template:
*ngFor="let user of users | async"

Answer 2

I'm on the Angular doc team and one open todo item is to change these docs to show the "best practice" way to access Http ... which is through a service.

Here is an example:

import { Injectable } from '@angular/core';
import { HttpClient, HttpErrorResponse } from '@angular/common/http';
import { Observable } from 'rxjs/Observable';
import 'rxjs/add/observable/throw';
import 'rxjs/add/operator/catch';
import 'rxjs/add/operator/do';
import 'rxjs/add/operator/map';

import { IProduct } from './product';

@Injectable()
export class ProductService {
    private _productUrl = './api/products/products.json';

    constructor(private _http: HttpClient) { }

    getProducts(): Observable<IProduct[]> {
        return this._http.get<IProduct[]>(this._productUrl)
            .do(data => console.log('All: ' + JSON.stringify(data)))
            .catch(this.handleError);
    }

    private handleError(err: HttpErrorResponse) {
        // in a real world app, we may send the server to some remote logging infrastructure
        // instead of just logging it to the console
        let errorMessage = '';
        if (err.error instanceof Error) {
            // A client-side or network error occurred. Handle it accordingly.
            errorMessage = `An error occurred: ${err.error.message}`;
        } else {
            // The backend returned an unsuccessful response code.
            // The response body may contain clues as to what went wrong,
            errorMessage = `Server returned code: ${err.status}, error message is: ${err.message}`;
        }
        console.error(errorMessage);
        return Observable.throw(errorMessage);
    }
}

The component would then look like this:

ngOnInit(): void {
    this._productService.getProducts()
            .subscribe(products => this.products = products,
                       error => this.errorMessage = <any>error);
}