* function formatter(value) {
* if (value) {
* return value.toUpperCase();
* }
* }
* ngModel.$formatters.push(formatter);
*
* @property {Object} $error An object hash with all errors as keys.
*
* @property {boolean} $pristine True if user has not interacted with the control yet.
* @property {boolean} $dirty True if user has already interacted with the control.
* @property {boolean} $valid True if there is no error.
* @property {boolean} $invalid True if at least one error on the control.
*
* @description
*
* `NgModelController` provides API for the `ng-model` directive. The controller contains
* services for data-binding, validation, CSS updates, and value formatting and parsing. It
* purposefully does not contain any logic which deals with DOM rendering or listening to
* DOM events. Such DOM related logic should be provided by other directives which make use of
* `NgModelController` for data-binding.
*
* ## Custom Control Example
* This example shows how to use `NgModelController` with a custom control to achieve
* data-binding. Notice how different directives (`contenteditable`, `ng-model`, and `required`)
* collaborate together to achieve the desired result.
*
* Note that `contenteditable` is an HTML5 attribute, which tells the browser to let the element
* contents be edited in place by the user. This will not work on older browsers.
*
*