Check the application here. You can get the complete code here.

These are the three features that make Ember a joy to use:

1. Bindings
2. Computed properties
3. Auto-updating templates

Use bindings to keep properties between two different objects in sync. You just declare a binding once, and Ember will make sure changes get propagated in either direction.

Here's how you create a binding between two objects:

MyApp.president = Em.Object.create({
  name: "Barack Obama"
});

MyApp.country = Em.Object.create({
  // Ending a property with 'Binding' tells Ember to
  // create a binding to the presidentName property.
  presidentNameBinding: 'MyApp.president.name'
});

// Later, after Ember has resolved bindings...
MyApp.country.get('presidentName');
// "Barack Obama"

Bindings allow you to architect your application using the MVC (Model-View-Controller) pattern, then rest easy knowing that data will always flow correctly from layer to layer.

Computed properties allow you to treat a function like a property:

MyApp.president = Em.Object.create({
  firstName: "Barack",
  lastName: "Obama",

  fullName: function() {
    return this.get('firstName') + ' ' + this.get('lastName');

  // Call this flag to mark the function as a property
  }.property()
});

MyApp.president.get('fullName');
// "Barack Obama"

Computed properties are useful because they can work with bindings, just like any other property.

Many computed properties have dependencies on other properties. For example, in the above example, the fullName property depends on firstName and lastName to determine its value. You can tell Ember about these dependencies like this:

MyApp.president = Em.Object.create({
  firstName: "Barack",
  lastName: "Obama",

  fullName: function() {
    return this.get('firstName') + ' ' + this.get('lastName');

  // Tell Ember that this computed property depends on firstName
  // and lastName
  }.property('firstName', 'lastName')
});

Make sure you list these dependencies so Ember knows when to update bindings that connect to a computed property.

Auto-updating templates:

Ember uses Handlebars, a semantic templating library. To take data from your JavaScript application and put it into the DOM, create a script tag and put it into your HTML, wherever you'd like the value to appear:

<script type="text/x-handlebars">
  The President of the United States is {{MyApp.president.fullName}}.
</script>

Hopefully you can see how all three of these powerful tools work together: start with some primitive properties, then start building up more sophisticated properties and their dependencies using computed properties. Once you've described the data, you only have to say how it gets displayed once, and Ember takes care of the rest. It doesn't matter how the underlying data changes, whether from an XHR request or the user performing an action; your user interface always stays up-to-date. This eliminates entire categories of edge cases that developers struggle with every day.

Download the lastest Ember.js Starter Kit if you haven’t already. Unzip it and open the directory in your favorite text editor.

Every Ember app should have an instance of Em.Application. This object will serve as the globally-accessible namespace for all of the other classes and instances in your app.

Open the JavaScript file located at js/app.js. Replace the default content with the following:

window.Todos = Em.Application.create();

This code creates a namespace for your application called Todos.

In this tutorial, we want to create a list for managing todos. Users should be able to create a new todo with a specific task, then check it off once it’s done.

Let’s define our model as a new subclass of Em.Object in app.js:

Todos.Todo = Em.Object.extend({
  title:  null,
  isDone: false
});

We’ve now defined a Todo class with two properties: title and isDone.

Note: The Ember rules for capitalization are pretty straight-forward. If it's a class or a namespace, it's uppercase (Todo); if it's an instance, it's lowercase (myTodo).

In MVC frameworks like Ember, the controller layer bridges the model layer, which is only concerned with a pure-data representation of objects, and the view layer, which is only concerned with representing those objects.

Now that we know what our data looks like, let’s create a controller to manage it. Since we want to maintain an ordered list of todos, we’ll use an instance of Em.ArrayController:

Todos.todosController = Em.ArrayController.create({
  // Initialize the array controller with an empty array.
  content: []
});

Behind the scenes, Em.ArrayController acts as a proxy onto its content Array property. Ember will propagate any modifications made to the ArrayController to the content property.

Now we have an array controller with no content. Let’s add a method to create a new todo:

Todos.todosController = Em.ArrayController.create({
  // Initialize the array controller with an empty array
  content: [],

  // Creates a new todo with the passed title, then adds it to the array
  createTodo: function(title) {
    var todo = Todos.Todo.create({ title: title });

    this.pushObject(todo);
  }
});

We’ve provided a simple stylesheet to give the application some style. Download the CSS file and add it to your project’s css directory. Then include the next line to the <head> tag in index.html:

<link rel="stylesheet" href="css/todos.css">

We’ve got our model and controller set up, so let’s move on to the fun part: creating the interface for our users. The first step is to create a text field into which the user types the name of their todo. Ember uses Handlebars templates to quickly define the application’s interface. While Handlebars makes it easy to markup HTML quickly, you’ll see that it also has been extended to automatically take advantage of Ember's bindings.

To start building our views, let’s open index.html.

First, add an <h1> tag with the name of the application:

<h1>Todos</h1>

You’ll also see that the starter kit includes a default Handlebars template. Remove it and replace it with the following Handlebars template, which emits a text field for our users to type in:

<script type="text/x-handlebars">
  {{view Em.TextField id="new-todo"
      placeholder="What needs to be done?"}}
</script>

Here we’ve specified a text field, with a unique id attribute (so it can be styled via CSS), as well as a placeholder attribute that will be displayed in modern HTML5 browsers.

**For more information about using Handlebars, visit the Handlebars website. To learn more about using Handlebars with Ember, make sure to check out the Using Handlebars Templates guide.

Now that we’ve got model, view, and controller represented, it’s time to open the app in our browser and see how it looks.

Double click on your index.html file to open it in your browser. You should see your application load. Once you’ve verified that the application is up and running, it’s time to tell Ember how to handle events for your <input> tag.

When the user types in the field and presses the return key, we will create a new Todo and have it inserted into the content of the array controller.

In Ember, view objects are responsible for updating the DOM and handling events. Among other things, this allows us to buffer changes to the DOM for maximum performance and to support generic cross-platform event handling. Whenever you want to display dynamic content or handle events, you will use a view object.

Insert this code in js/app.js file:

Todos.CreateTodoView = Em.TextField.extend({
  insertNewline: function() {
    var value = this.get('value');

    if (value) {
      Todos.todosController.createTodo(value);
      this.set('value', '');
    }
  }
});

Since CreateTodoView will handle events for a text field, we create a subclass of Em.TextField, which provides several conveniences for working with these input controls. For example, you can access the value property and respond to higher level events, such as insertNewline, when the user presses enter.

Now that we have defined our view, let’s update the template to use our new view subclass. Switch back to index.html and update the view helper to say Todos.CreateTodoView instead of Em.TextField.

<script type="text/x-handlebars">
  {{view Todos.CreateTodoView id="new-todo"
      placeholder="What needs to be done?"}}
</script>

Now that we have UI to create new todos, let’s create the code to display them. We’ll use the Handlebars #collection helper to display a list of items. #collection will create an instance of Em.CollectionView that renders every item in its underlying Array using the enclosed HTML.

<script type="text/x-handlebars">
  {{view Todos.CreateTodoView id="new-todo"
      placeholder="What needs to be done?"}}

  {{#collection contentBinding="Todos.todosController"
      tagName="ul"}}
    {{view.content.title}}
  {{/collection}}
</script>

Notice that we’ve also told the collection to bind its content property to our todosController. For every item in the array controller, the collection view will create a new child view that renders the {{view.content.title}} template.

You set up bindings by creating a property whose name ends in Binding. In this case, we bind Todos.todosController to the collection view’s content property. When one end of a binding changes, Ember will automatically update the other end.

This is a good time to open index.html again (or reload if you still have it loaded in your browser). It should look the same as before. Type a todo into the text field and hit return.

Look at that! As soon as we create a new todo and insert it into the ArrayController the view updates automatically.

You’ve now seen a little bit of the power of Ember. By using Ember’s bindings to describe the relationship between your data and your views, you were able to change the data layer and let Ember do the hard work of updating the view layer for you.

This is actually a core concept in Ember, not just something that demos well. Ember’s binding system is designed with the view system in mind, which makes it easy to work directly with your data and not need to worry about manually keeping your view layer in sync. You will see this concept over and over again in the rest of this tutorial and in other guides.

More about setting up Bindings here

We now have the ability to add todos, but no way to mark them as done. Before the frustration of a never-ending todo list gets the better of us, let’s add the ability to mark todos complete.

The first thing we need to do is add a checkbox to each todo list item. As was mentioned earlier, if we want to handle events, such as user input, we need a view. In this case, we are adding a checkbox and want to be notified whenever the value of the checkbox is changed by the user. Let’s update the Handlebars template in index.html to look like the following:

<script type="text/x-handlebars">
  {{view Todos.CreateTodoView id="new-todo"
      placeholder="What needs to be done?"}}

  {{#collection contentBinding="Todos.todosController" tagName="ul"}}
    {{view Em.Checkbox titleBinding="view.content.title"
        valueBinding="view.content.isDone"}}
  {{/collection}}
</script>

Let’s take a second to talk about the bindings we just set up. For every item in its underlying array, Em.CollectionView will create a new item view whose content property contains the object the view should represent. In our case, there will be a child view for each todo. Since the checkbox is a child view of each item view, when we bind to view.content.title, we’re saying to bind to the title property of the Todo object represented by the item view.

Under the hood, Ember binds an event handler to the change event of the checkbox and updates value when the event occurs. This may change as needed for browser compatibility, but working with a Ember property insulates you from those concerns.

Before you reload the browser to see the new checkbox todo list items, the stylesheet we provided includes a CSS class meant to give completed todos a unique look. Let’s bind the class of each item to the object’s isDone property.

We’ll use itemClassBinding property on the collection helper to set up this binding:

{{#collection contentBinding="Todos.todosController"
    tagName="ul" itemClassBinding="view.content.isDone"}}
  {{view Em.Checkbox titleBinding="view.content.title"
      valueBinding="view.content.isDone"}}
{{/collection}}

This property sets up a binding on each of the item views. Each item will get the class is-done if its associated content object’s isDone property is true. Ember will automatically dasherize property names into class names.

All views have a number of properties, including id, class, and classBinding. The collection helper allows you to prefix any of those properties with item, which will then apply to each of the child item views. For instance, if you use the itemClass property on a collection, each item will get that class name.

Now reload your application in the browser and try it out. As soon as you click a todo’s checkbox, the text will become crossed out. Keep in mind you didn’t need to update any views when marking the Todo as done; bindings did it for you. If a different part of the app changes the Todo item’s isDone property, your list item would automatically update without any more work on your part.

We can now create todos and mark them as being complete. While 76% of all statistics are made up, let’s see if we can display more accurate information from the data we have. At the top of our list, let’s display the number of todos remaining. Open index.html and insert this new view before {{#collection}} helper:

{{#view Todos.StatsView id="stats"}}
  {{remainingString}} remaining
{{/view}}

Handlebars expressions, like {{remainingString}}, allows us to automatically update the DOM when a property on our view changes. In this case, we’re saying that Todos.StatsView should always display the most up-to-date value to the view’s remainingString property. As with the other bindings, this will be updated for us automatically whenever the value changes.

Let’s go ahead and implement that view in app.js now:

Todos.StatsView = Em.View.extend({
  remainingBinding: 'Todos.todosController.remaining',

  remainingString: function() {
    var remaining = this.get('remaining');

    return remaining + (remaining === 1 ? " item" : " items");
  }.property('remaining')
});

remainingString contains a pluralized string, based on the number of remaining todos. Here is another core piece of Ember in action, this one called a computed property. Computed properties are properties whose value is determined by running a function. For example, if remaining equals 1, remainingString will contain the string "1 item", and if remaining is greater than 1, remainingString will contain the string "X items".

We say that remainingString depends on remaining because it requires another value to generate its own value. We list these dependent keys inside the property() declaration.

We’ve also bound the view’s remaining property to the todosController‘s remaining property. This means that, because of the dependency chain we’ve described, if Todos.todosController.remaining changes, remainingString will automatically update.

When we have information that views need but is based on aggregate information about our models, the array controller is a good place to put it. Let’s add a new computed property to todosController in app.js:

Todos.todosController = Em.ArrayController.create({
  // Initialize the array controller with an empty array.
  content: [],

  // Creates a new todo with the passed title, then adds it to the array.
  createTodo: function(title) {
    var todo = Todos.Todo.create({ title: title });

    this.pushObject(todo);
  },

  remaining: function() {
    return this.filterProperty('isDone', false).get('length');
  }.property('@each.isDone')
});

Here, we specify our dependent key using @each. This allows us to depend on properties of the array’s items. In this case, we want to update the remaining property any time isDone changes on a Todo.

The @each property also updates when an item is added or removed.

It’s important to declare dependent keys because Ember uses this information to know when to update bindings. In our case, our Todos.StatsView updates any time todosController’s remaining property changes.

As we build up our application, we create declarative links between objects. These links describe how application state flows from the model layer to the HTML in the presentation layer.

As we populate our list with todos, we may want to periodically clear out those we’ve completed. As you have learned, we will want to make that change to the todosController, and let Ember automatically propagate those changes to the DOM.

Let’s add a new clearCompletedTodos method to Todos.todosController.

Todos.todosController = Em.ArrayController.create({
  // Initialize the array controller with an empty array.
  content: [],

  // Creates a new todo with the passed title, then adds it to the array.
  createTodo: function(title) {
    var todo = Todos.Todo.create({ title: title });

    this.pushObject(todo);
  },

  remaining: function() {
    return this.filterProperty('isDone', false).get('length');
  }.property('@each.isDone'),

  clearCompletedTodos: function() {
    this.filterProperty('isDone', true).forEach(this.removeObject, this);
  }
});

Next, let’s add a button to our template. Open index.html and add a Em.Button control inside the HTML for StatsView as shown here:

{{#view Todos.StatsView id="stats"}}
  {{#view Em.Button classBinding="isActive"
      target="Todos.todosController"
      action="clearCompletedTodos"}}
    Clear Completed Todos
  {{/view}}

  {{remainingString}} remaining.
{{/view}}

We’ve defined an instance of Em.Button, which calls a method on an object when it is clicked. In this case we’ve told the button to call the clearCompletedTodos method (its action) on the Todos.todosController object (its target).

We’ve also told it to add an is-active class to the view when it is clicked or tapped. Every Em.Button has an isActive property that will be true when it is in the process of being clicked. This allows us to display a visual cue to the user that they have hit the right target.

Go back to your browser and try it out. Add some todos, then mark them done and clear them. Because we previously bound the visual list to the todosController, making a change through new means has the expected effect.

Let’s say you’ve decided to actually get all of your work done. Wouldn’t it be nice to have a way to easily mark every todo as complete?

It turns out that, due to our application’s declarative nature, all the hard work is already done. We just need a way to mark every Todo complete.

Let’s first create a new computed property on our controller that describes whether or not every todo is done. It might look something like this:

Todos.todosController = Em.ArrayController.create({
  // Initialize the array controller with an empty array.
  content: [],

  // Creates a new todo with the passed title, then adds it to the array.
  createTodo: function(title) {
    var todo = Todos.Todo.create({ title: title });

    this.pushObject(todo);
  },

  remaining: function() {
    return this.filterProperty('isDone', false).get('length');
  }.property('@each.isDone'),

  clearCompletedTodos: function() {
    this.filterProperty('isDone', true).forEach(this.removeObject, this);
  },

  allAreDone: function() {
    return this.get('length') && this.everyProperty('isDone', true);
  }.property('@each.isDone')
});

**Ember has many enumerable helpers. everyProperty(‘isDone’, true) returns true if every item in the array has an isDone property that evaluates to true. You can find out more in the Ember Enumerable API guide.

Next, open index.html, we’ll create a checkbox view to mark all items complete and bind its value to the controller’s property:

{{view Em.Checkbox class="mark-all-done"
    title="Mark All as Done"
    valueBinding="Todos.todosController.allAreDone"}}

{{#collection contentBinding="Todos.todosController"
...

If you were to reload your browser and use the app now, you’d notice that clicking all of the checkboxes of your todos will cause the “Mark All as Done” checkbox to become checked. However, it doesn’t work in the other direction: clicking “Mark All as Done” has no effect.

So far, our computed properties have described how to calculate a value from dependent properties. However, in this case, we want to accept a new value, and update dependent properties to reflect it. Lets update our allAreDone computed property to also accept a value.

allAreDone: function(key, value) {
  if (value !== undefined) {
    this.setEach('isDone', value);
    return value;
  } else {
    return !!this.get('length') && this.everyProperty('isDone', true);
  }
}.property('@each.isDone')

When you set a computed property, your computed property function is called with the property key as the first parameter and the new value as the second. To determine if Ember is asking for the value or trying to set it, we examine the value parameter. If it is defined, we iterate through each Todo and set its isDone property to the new value.

Because bindings are bi-directional, Ember will set allAreDone to true when the user clicks the “Mark All as Done” checkbox. Conversely, unchecking it will make Ember set allAreDone to false, unchecking all todos.

Reload the app and add some todos. Click “Mark All as Done”. Wow! Each of the individual check boxes gets checked off. If you uncheck one of them, the “Mark All as Done” checkbox un-checks itself.

When you use Ember, as you scale your UI, you never need to wonder whether a new feature will work consistently with parts of the UI you already implemented. Since you build your view layer to simply reflect the state of your models, you can make changes however you want and see them update automatically.

## Resources:

• Ember.js docs: http://emberjs.com/
• Github Repository: https://github.com/emberjs/ember.js
• Wiki: https://github.com/emberjs/ember.js/wiki
• IRC Channel: #emberjs