angular.js/docs/content/guide/dev_guide.templates.databinding.ngdoc

@ngdoc overview
@name Developer Guide: Templates: Data Binding in Angular
@description

Data-binding in angular web apps is the automatic syncing of data between the model and view
components.  The way that angular 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

<img class="right" src="img/One_Way_Data_Binding.png"/>
Most templating systems bind data in only one direction: they merge template and model components
together into a view, as illustrated in the diagram. 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 Angular Templates

<img class="right" src="img/Two_Way_Data_Binding.png"/>
The way angular templates works is different, as illustrated in the diagram. They are different
because first the template (which is the uncompiled HTML along with any additional markup or
directives) is compiled on the browser, and second, the compilation step produces a live view. We
say live because any changes to the view are immediately reflected in the model, and any changes in
the model are propagated to the view. This makes the model always 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.


## Related Topics

* {@link scope Angular Scopes}
* {@link dev_guide.templates Angular Templates}