属性未定义Angular2和TypeScript

我得到的错误是一个未定义的属性,但我认为它应该在Observable / Subscribe调用onComplete的prepareCredentials函数时初始化.

以下是错误,

TypeError: Cannot read property 'clientId' of undefined
    at SpotifyComponent.prepareCredentials (spotify.component.ts:58)
    at SafeSubscriber.eval [as _complete] (spotify.component.ts:38)
    at SafeSubscriber.__tryOrUnsub (Subscriber.ts:240)
    at SafeSubscriber.complete (Subscriber.ts:226)
    at Subscriber._complete (Subscriber.ts:142)
    at Subscriber.complete (Subscriber.ts:120)
    at MapSubscriber.Subscriber._complete (Subscriber.ts:142)
    at MapSubscriber.Subscriber.complete (Subscriber.ts:120)
    at XMLHttpRequest.onLoad (xhr_backend.ts:67)
    at ZoneDelegate.invokeTask (zone.js:356)

零件,

import { Component,OnInit } from '@angular/core';
import { Http,Response } from '@angular/http';

import { Observable } from 'rxjs/Observable';
import 'rxjs/add/operator/map';
import 'rxjs/add/operator/catch';

import { SpotifyService } from './spotify.service';

@Component({
  moduleId: module.id,selector: 'app-spotify',templateUrl: 'spotify.component.html',styleUrls: ['spotify.component.css'],providers: [SpotifyService]
})

export class SpotifyComponent implements OnInit {
  private credentialsData: {
    clientId: string,clientSecret: string
  };

  constructor(
    private http: Http,private spotifyService: SpotifyService
  ) { }

  ngOnInit() {
    if (this.spotifyService) {
      this.http.get('../app/data/credentials.json')
        .map(this.handleResponse)
        .subscribe(
          this.setupCredentials,this.handleError,() => { this.prepareCredentials(); }
        );
    }
  }

  private setupCredentials(subData) {
    console.log('Setting up credentials...');
    this.credentialsData = {
      clientId: <string>subData.clientId,clientSecret: <string>subData.clientSecret
    };
    console.log('credentials: ' +
        JSON.stringify(this.credentialsData));
    console.log('credentials clientId: ' +  this.credentialsData.clientId);
    console.log('credentials clientSecret: ' + this.credentialsData.clientSecret);
  }

  private prepareCredentials() {
    console.log('Preparing credentials...');
    this.spotifyService.prepare(
      this.credentialsData.clientId,this.credentialsData.clientSecret,'','http://localhost:4200/spotify');

  }

  private handleResponse(res: Response) {
    console.log(JSON.stringify(res.json()));
    return res.json().spotify;
  }

  private handleError(error: any) {
    let errMsg = (error.message) ? error.message :
      error.status ? `${error.status} - ${error.statusText}` : 'Server     error';
    console.error(errMsg); // log to console instead
    return Observable.throw(errMsg);
  }

}

和服务,

import { Injectable } from '@angular/core';

@Injectable()
export class SpotifyService {
  private clientId: string;
  private clientSecret: string;
  private scopes: string;
  private callbackUrl: string;

  constructor() { }

  wakeUpTest(): string {
    console.log('SpotifyService is awake and initiated.');
    return 'SpotifyService is awake and initiated.';
  }

  prepare(clientId: string,clientSecret: string,scopes: string,callbackUrl: string): void {
        console.log(clientId);
  }

  getAuthCode(): void {
    let authUrl: string = 'https://accounts.spotify.com/authorize' +
      '?response_type=code' +
      '&client_id=' + this.clientId +
      '&scope=' + encodeURIComponent(this.scopes) +
      '&redirect_uri=' + encodeURIComponent(this.callbackUrl);
  }

}

提前感谢任何帮助或指示,因为这对我来说相对较新.