title: Push object into new array with interfaces in Angular2
body: <p>Im retrieving a list of objects from a service in Angular2.</p>
  <p>On click I want to put the object from my ngFor onto a new array (with the same interface).</p>
  <p>I'm able to display the list with items just fine. I'm also able to display the item I selected in the console, but I'm not able to push this object into a new array with the same interface.</p>
  <p><strong>My interface:</strong></p>
  <pre><code>export interface Item {
      id: string,
      name: string,
      icon_url: string,
      exterior: string,
      type: string,
      tag: string,
      addedClass: string,
      selected: boolean
  }
  </code></pre>
  <p><strong>My component:</strong></p>
  <pre><code>import {Component, OnInit, DoCheck} from 'angular2/core';
  import {Item} from './../interfaces/interfaces.ts';
  import {ItemService} from './../services/item.service.ts';
  
  
  @Component({
      template: `
          &lt;h2&gt;Add item&lt;/h2&gt;
          &lt;div class=&quot;itemlist&quot;&gt;
              &lt;ul class=&quot;items&quot;&gt;
                  &lt;li 
                      *ngFor=&quot;#item of items&quot;
                      [class.selected]=&quot;item === selectedItem&quot;
                      (click)=&quot;onSelect(item)&quot;
                  &gt;
                      &lt;span class=&quot;item&quot;&gt;
                          {{item.name}}
                      &lt;/span&gt;
                  &lt;/li&gt;
              &lt;/ul&gt;
          &lt;/div&gt;
      `
  })
  
  export class AddTradeComponent implements OnInit {
  
      public items: Item[];
      public selectedItems: Item[];
  
  
      constructor(private _itemService: ItemService) { }
  
      getUserItems() {
          this._itemService.getUserItems().then(userItems =&gt; this.items = userItems);
      
      }
  
      ngOnInit() {
          this.getUserItems();
      }
  
      onSelect(item) {
          console.log('items ', item);
          this.selectedItems.push(item);
      }
  
  }
  </code></pre>
  <p><strong>Errors that I'm getting:</strong></p>
  <blockquote>
  <p>Error during evaluation of &quot;click&quot;</p>
  <p>TypeError: Cannot read property 'push' of undefined</p>
  </blockquote>
  <p>Logging the object works fine, I just cannot push it onto this.selectedItems.</p>