Angular2 Service Failing to Return Expected Value

It's frustrating that my services are not functioning properly. Despite spending the last two days scouring Stack Overflow for solutions, I haven't been able to find a solution that matches my specific issue.

Here is a snippet of my Service.ts code:


import { Injectable } from '@angular/core';
import {  Http, Response } from '@angular/http';
import { Observable } from 'rxjs/Observable';
import 'rxjs/add/operator/catch';
import 'rxjs/add/operator/map';
import { CarObject } from './make';

@Injectable()
export class EdmundsService {
  private stylesurl = 'REDACTED';

 constructor(private http: Http) { }

 getCars(): Observable<CarObject[]> {
   return this.http.get(this.stylesurl)
   .map(this.extractData)
   .catch(this.handleError);
  }

  private extractData(res: Response) {
   let body = res.json();
   return body.data || { };
  }
  private handleError (error: Response | any) {
   // Remote logging infrastructure could be utilized in a real-world application
   let errMsg: string;
   if (error instanceof Response) {
      const body = error.json() || '';
      const err = body.error || JSON.stringify(body);
      errMsg = `${error.status} - ${error.statusText || ''} ${err}`;
     } else {
      errMsg = error.message ? error.message : error.toString();
    }
    console.error(errMsg);
    return Observable.throw(errMsg);
  }

}

These are the 'models' being used:


class Style {
  id: number;
  name: string;
  make: Make;
  model: Model;
  year: Year;
  submodel: Submodel;
  trim: string;
  states: string[];
  engine: Engine;
  transmission: Transmission;
  options: Options[];
  colors: Color[];
  drivenWheels: string;
  numOfDoors: string;
  squishVins: string[];
  categories: Categories;
  MPG: MPG;
  manufacturerOptionCode: string;
 }

export class CarObject {
styles: Style[];
stylesCount: number;
}

This is my component setup:


import { CarObject } from './make';
import { EdmundsService } from './edmunds-search-result.service';

@Component({REDACTED
providers: [EdmundsService] })


export class EdmundsSearchResultComponent implements OnInit {
  cars: CarObject[];
  errorMessage: string;



  constructor(private _edmundsService: EdmundsService) { }

   getCars(): void {
     this._edmundsService.getCars()
     .subscribe(
      cars => this.cars = cars,
      error =>  this.errorMessage = <any>error);
   }


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

}

Component HTML: {{ cars.stylesCount | async }}

Sample API Response: http://pastebin.com/0LyZuPGW

Error Output:


EXCEPTION: Error in ./EdmundsSearchResultComponent class 
EdmundsSearchResultComponent - inline template:0:0 caused by: 
Cannot read property 'stylesCount' of undefined
  1. The structure of CarObject was specifically designed to match the API Response, so removing the array brackets ( [] ) may be acceptable.
  2. Despite closely following the Tour Of Heroes HTTP/Services tutorial, I'm puzzled as to why the object data isn't displaying on my template.

My goal is to retrieve data through an HTTP request using the variable 'styleurl,' which seems to be working based on my observation in the Chrome dev tools 'Network' tab. I aim to have my CarObject consume the JSON response and make it accessible to my component/template.

Answer №1

Your component has a reserved car property, but it is not initialized, so it remains as undefined.

When your HTML renders, the promise has not been fulfilled yet, so your car still holds the value of undefined when you try to access a property from it.

Here are a few solutions:

Initialize it:

cars: CarObject = new CarObject(); // or <CarObject>{}

Use the elvis operator in your template:

 {{ cars?.stylesCount }}

Use ngIf:

 <div *ngIf="cars">{{ cars.styleCount }}</div>

There are other potential ways to address this issue as well.

Regarding your use of the async pipe, there may be errors there too based on how you are trying to implement it. See below for more information.


Additionally, I recommend familiarizing yourself with TypeScript types and best practices for Angular and TypeScript, especially regarding models, interfaces, and Observables over Promises. While there are some issues in your code, they may not be directly related to the current problem.

I hope this helps!


Update:

About your use of the async pipe:

The async pipe subscribes to an Observable or Promise and returns the latest value it has emitted.

You are using it with an array of CarObjects, which, by the way, should not be an array. Please refer to the documentation for the async pipe for proper usage guidelines.

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