Angular2 Sorting Pipe with Object Array
<h1> How to make a sorting pipe in angular2 with an array of objects</h1>

<h2>Original Problem:</h2>

<p>I have a TODOs list, (Todo[ ]) and I want to sort it every time I make some changes. I want that the completed todo are displayed at the bottom of the list. The Todo object has a property named .completed that stores a boolean value, it will tell us if the todo is completed or not.</p>

<h2>Creating the Pipe:</h2>

<p>In Angular2 the "OrderBy" pipe does not exist. So we have to build it:</p>

<pre class="lang-js prettyprint-override"><code>import { Pipe, PipeTransform } from "angular2/core";
//Todo is the interface for our todo object
import {Todo} from './todo';

@Pipe({
  name: "sort",
  //set to false so it will always update, read below the code.
  pure: false
})
export class TodosSortPipe implements PipeTransform {
  transform(array: Todo[], args: any): Todo[] {
    //watch the console to see how many times you pipe is called
    console.log("calling pipe");
    /* javascript is async, so could be that the pipe is called before
    that the todos list is created, in this case we do nothing
    returning the array as it is */
    if (isBlank(array)) return null;
    array.sort((a, b) =&gt; {
      if (a.completed &lt; b.completed) {
        return -1;
      //.completed because we want to sort the list by completed property
      } else if (a.completed &gt; b.completed) {
        return 1;
      } else {
        return 0;
      }
    });
    return array;
  }
}
</code></pre>

<p>If you didn't understand the sort method check MDN: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/sort" rel="nofollow">https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/sort</a></p>

<p>The Pipe is done, let's move to the Component.</p>

<h2>Creating the Component:</h2>

<p>The AppComponent class, creates an array of Todo, called Todos, getting the objects from a mock with a service.</p>

<pre class="lang-js prettyprint-override"><code>import {Component, OnInit} from 'angular2/core';
import {Todo} from './todo';
import {TodoService} from './todo.service';
import {TodosSortPipe} from './sort-pipe.component'

@Component({
    //name of the html element
    selector: 'my-app',
    //view of the selector,  " ` " is alt + 9
    templateUrl: "./app/todo-list.component.html",
    providers: [TodoService],
    pipes: [ TodosSortPipe ]
})

export class AppComponent implements OnInit{

    public todos: Todo[];
    public edited = false;
    public changes = 0;
    //creating an istance of todoService
    constructor(private _todoService: TodoService) { };

    //getting the todos list from the service
    getTodos() {
        this._todoService.getTodos().then(todos =&gt; this.todos = todos);
    }

   (...)
   editTodo(todo: Todo): void {
        //slice is very important, read below the code
        this.todos = this.todos.slice();
        this.saveTodos();
    }
}
</code></pre>

<h2>Template implementation</h2>

<p>This is the pipe calling:</p>

<pre class="lang-html prettyprint-override"><code>&lt;li *ngFor="#todo of todos | sort; #i=index"&gt;
 (...)
&lt;/li&gt;
</code></pre>

<h2>Demo: </h2>

<p>For the full example with all the code: <a href="https://plnkr.co/edit/VICRMVNhqdqK9V4rJZYm?p=preview" rel="nofollow">https://plnkr.co/edit/VICRMVNhqdqK9V4rJZYm?p=preview</a>
Watch it on github: <a href="https://github.com/AndreaMiotto/Angular2-TodoApp" rel="nofollow">https://github.com/AndreaMiotto/Angular2-TodoApp</a></p>

<h2> Pipe Unpure </h2>

<p>Pipes only change by default when your Pipe input parameters change and not when your data changes. 
Setting Pure to false, you will make it "unpure" so you pipe will always update.</p>