Data-binding in AngularJS apps is the automatic synchronization of data between the model and view components. The way that AngularJS implements data-binding lets you treat the model as the single-source-of-truth in your application. The view is a projection of the model at all times. When the model changes, the view reflects the change, and vice versa.

Data Binding in Classical Template Systems

Most templating systems bind data in only one direction: they merge template and model components together into a view. After the merge occurs, changes to the model or related sections of the view are NOT automatically reflected in the view. Worse, any changes that the user makes to the view are not reflected in the model. This means that the developer has to write code that constantly syncs the view with the model and the model with the view.

Data Binding in AngularJS Templates

AngularJS templates work differently. First the template (which is the uncompiled HTML along with any additional markup or directives) is compiled on the browser. The compilation step produces a live view. Any changes to the view are immediately reflected in the model, and any changes in the model are propagated to the view. The model is the single-source-of-truth for the application state, greatly simplifying the programming model for the developer. You can think of the view as simply an instant projection of your model.

Because the view is just a projection of the model, the controller is completely separated from the view and unaware of it. This makes testing a snap because it is easy to test your controller in isolation without the view and the related DOM/browser dependency.

Data binding in AngularJS is the synchronization between the model and the view.

Data Model
AngularJS applications usually have a data model. The data model is a collection of data available for the application.

Example
var app = angular.module(‘myApp’, []);
app.controller(‘myCtrl’, function($scope) {
$scope.firstname = “John”;
$scope.lastname = “Doe”;
});

HTML View
The HTML container where the AngularJS application is displayed, is called the view.

The view has access to the model, and there are several ways of displaying model data in the view.

You can use the ng-bind directive, which will bind the innerHTML of the element to the specified model property:

Example
<p ng-bind=”firstname”></p>

You can also use double braces {{ }} to display content from the model:

Example
<p>First name: {{firstname}}</p>

Or you can use the ng-model directive on HTML controls to bind the model to the view.

The ng-model Directive
Use the ng-model directive to bind data from the model to the view on HTML controls (input, select, textarea)

Example
<input ng-model=”firstname”>
The ng-model directive provides a two-way binding between the model and the view.

Two-way Binding
Data binding in AngularJS is the synchronization between the model and the view.

When data in the model changes, the view reflects the change, and when data in the view changes, the model is updated as well. This happens immediately and automatically, which makes sure that the model and the view is updated at all times.

Example

<script>
var app = angular.module(‘myApp’, []);
app.controller(‘myCtrl’, function($scope) {
$scope.firstname = “John”;
$scope.lastname = “Doe”;
});
</script>

AngularJS Controller
Applications in AngularJS are controlled by controllers. Read about controllers in the AngularJS Controllers chapter.

Because of the immediate synchronization of the model and the view, the controller can be completely separated from the view, and simply concentrate on the model data. Thanks to the data binding in AngularJS, the view will reflect any changes made in the controller.

Example

<script>
var app = angular.module(‘myApp’, []);
app.controller(‘myCtrl’, function($scope) {
$scope.firstname = “John”;
$scope.changeName = function() {
$scope.firstname = “Nelly”;
}
});
</script>