Push object into new array with interfaces in Angular2 - arrays

Question

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

Answer 1

You didn't initialize your arrays. Either initialize the arrays when you declare your member variables:
public items: Item[] = [];
public selectedItems: Item[] = [];
Or initialize them in your constructor:
constructor() {
this.items = [];
this.selectedItems = [];
}