title: Angular2 / Firebase - EXCEPTION: TypeError: Cannot read property 'push' of undefined
body: <p>I'm trying to create an Angular2 and Firebase Web application (CRUD). I have an error when I try to add a new business user.</p>
  
  <p>appComponent.ts file :</p>
  
  <pre><code>    import { Component, OnInit} from '@angular/core';
  import { AngularFire, FirebaseListObservable } from 'angularfire2';
  import {FirebaseService} from './services/firebase.service';
  import 'rxjs/add/operator/map';
  @Component({
    selector: 'app-root',
    templateUrl: 'app.component.html',
    styleUrls: ['app.component.css'],
    providers:[FirebaseService]
  })
  
  export class AppComponent implements OnInit {
     title = 'app works';
     BusinessUsers : Business[];
     categories : Category[];
     appState: string;
     activeKey: string;
  
     constructor(private _firebaseService:FirebaseService) {
  
     }
     ngOnInit(){
       this.appState = 'default';
       this._firebaseService.getBusinessUsers().subscribe(BusinessUsers =&gt; {
         this.BusinessUsers = BusinessUsers
       });
  
       this._firebaseService.getCategories().subscribe(categories =&gt; {
         this.categories = categories
       });
  
     }
  
     changeState(state, key = null){
      if(key){
        this.activeKey = key;
      }
      this.appState = state;
    }
  
  
  
  
    addBusiness(
      website: URL,
    ){
      var created_at = new Date().toString();
  
      var newBusiness = {
  
       website: website,
       created_at:created_at
    }
      this._firebaseService.addBusiness(newBusiness);
  
      this.changeState('default');
    }
  }
  
  export interface Business {
      $Key:string;
      address: string;
      company?: string;
      zipcode: number;
      suite: Date;
      email: string;
      name: string;
      website: URL;
  
  }
  
  export interface Category {
      $Key:string;
      value: string;
  }
  </code></pre>
  
  <p>In <code>firebase.service.ts</code> file:</p>
  
  <pre><code>    import {Injectable} from '@angular/core';
  import { AngularFire, FirebaseListObservable } from 'angularfire2';
  
  
  @Injectable()
  export class FirebaseService{
      BusinessUsers : FirebaseListObservable&lt;Business[]&gt;;
      categories : FirebaseListObservable&lt;Category[]&gt;;
  
      constructor(private af:AngularFire) {
  
      }
  
      getBusinessUsers(){
          this.BusinessUsers = this.af.database.list('/BusinessUsers') as FirebaseListObservable&lt;Business[]&gt;;
          return this.BusinessUsers;
      }
      getCategories(){
          this.categories = this.af.database.list('/Categories') as FirebaseListObservable&lt;Category[]&gt;;
          return this.categories;
      }
  
      addBusiness(newBusiness): Promise&lt;any&gt;{
        return this.businessUsers.push(newBusiness);
      }
  }
  
  
  export interface Business {
      $Key:string;
      address: string;
      company?: string;
      zipcode: number;
      suite: Date;
      email: string;
      name: string;
      website: URL;
  
  }
  
  export interface Category {
      $Key:string;
      value: string;
  }
  </code></pre>
  
  <p>This is the image of the error when I write a text and click add:</p>
  
  <p><img src="https://i.stack.imgur.com/5caET.png" alt="enter image description here"></p>
  
  <p>I have a Firebase Database.</p>