Angular: watch radio buttons

http://angularjs.org/

HTML

<div class="col-sm-4">
                    <div class="row btn-group btn-group-toggle" data-toggle="buttons" id="operator">
                        <label class="col-sm-6" for="operator" translate>Operator</label>
                       
                            <input type="radio" name="operater"  ng-model="extra.operatorId"  ng-value=""  checked>All
                     
                        <label class="btn btn-secondary col-sm-3">
                            <input type="radio" name="operater" ng-model="extra.operatorId"  ng-value="operator">{{operator}}
                        </label>
                    </div>
                </div>

JavaScript

var myApp = angular.module('myApp',[]);
    
    myApp.controller('MyCtrl', function($scope) {
        $scope.extra = {};
        $scope.extra.operatorId = '0';
        
        $scope.$watch('extra.operatorId', function(value) {
           alert(value);
        });
    })