Detect unsaved changes and alert user using angularjs

Below is the code so far

    <!doctype html>
<html ng-app>
<head>
    <script src="http://code.angularjs.org/1.1.2/angular.min.js"></script>
    <script type="text/javascript">
    function Ctrl($scope) {
        var initial = {text: 'initial value'};
        $scope.myModel = angular.copy(initial);
        $scope.revert = function() {
            $scope.myModel = angular.copy(initial);
            $scope.myForm.$setPristine();
        }
    }
    </script>
</head>
<body>
    <form name="myForm" ng-controller="Ctrl">
        myModel.text: <input name="input" ng-model="myModel.text">
        <p>myModel.text = {{myModel.text}}</p>
        <p>$pristine = {{myForm.$pristine}}</p>
        <p>$dirty = {{myForm.$dirty}}</p>
        <button ng-click="revert()">Set pristine</button>
    </form>
</body>
</html>

How to alert on browser close or url redirect in case there is some unsaved data, so that user can decide whether to continue?

Something like this should do it:

<!doctype html>
<html ng-app="myApp">
<head>
    <script src="http://code.angularjs.org/1.1.2/angular.min.js"></script>
    <script type="text/javascript">
    function Ctrl($scope) {
        var initial = {text: 'initial value'};
        $scope.myModel = angular.copy(initial);
        $scope.revert = function() {
            $scope.myModel = angular.copy(initial);
            $scope.myForm.$setPristine();
        }
    }

    angular.module("myApp", []).directive('confirmOnExit', function() {
        return {
            link: function($scope, elem, attrs) {
                window.onbeforeunload = function(){
                    if ($scope.myForm.$dirty) {
                        return "The form is dirty, do you want to stay on the page?";
                    }
                }
                $scope.$on('$locationChangeStart', function(event, next, current) {
                    if ($scope.myForm.$dirty) {
                        if(!confirm("The form is dirty, do you want to stay on the page?")) {
                            event.preventDefault();
                        }
                    }
                });
            }
        };
    });
    </script>
</head>
<body>
    <form name="myForm" ng-controller="Ctrl" confirm-on-exit>
        myModel.text: <input name="input" ng-model="myModel.text">
        <p>myModel.text = {{myModel.text}}</p>
        <p>$pristine = {{myForm.$pristine}}</p>
        <p>$dirty = {{myForm.$dirty}}</p>
        <button ng-click="revert()">Set pristine</button>
    </form>
</body>
</html>

Note that the listener for $locationChangeStart isn't triggered in this example since AngularJS doesn't handle any routing in such a simple example, but it should work in an actual Angular application.

I've extended the @Anders answer to clean up listeners (unbind listers) when directive is destroyed (ex: when route changes), and added some syntactic sugar to generalise the usage.

confirmOnExit Directive:

/**
 * @name confirmOnExit
 * 
 * @description
 * Prompts user while he navigating away from the current route (or, as long as this directive 
 * is not destroyed) if any unsaved form changes present.
 * 
 * @element Attribute
 * @scope
 * @param confirmOnExit Scope function which will be called on window refresh/close or AngularS $route change to
 *                          decide whether to display the prompt or not.
 * @param confirmMessageWindow Custom message to display before browser refresh or closed.
 * @param confirmMessageRoute Custom message to display before navigating to other route.
 * @param confirmMessage Custom message to display when above specific message is not set.
 * 
 * @example
 * Usage:
 * Example Controller: (using controllerAs syntax in this example)
 * 
 *      angular.module('AppModule', []).controller('pageCtrl', [function () {
 *          this.isDirty = function () {
 *              // do your logic and return 'true' to display the prompt, or 'false' otherwise.
 *              return true;
 *          };
 *      }]);
 * 
 * Template:
 * 
 *      <div confirm-on-exit="pageCtrl.isDirty()" 
 *          confirm-message-window="All your changes will be lost."
 *          confirm-message-route="All your changes will be lost. Are you sure you want to do this?">
 * 
 * @see
 * http://stackoverflow.com/a/28905954/340290
 * 
 * @author Manikanta G
 */
ngxDirectivesModule.directive('confirmOnExit', function() {
    return {
        scope: {
            confirmOnExit: '&',
            confirmMessageWindow: '@',
            confirmMessageRoute: '@',
            confirmMessage: '@'
        },
        link: function($scope, elem, attrs) {
            window.onbeforeunload = function(){
                if ($scope.confirmOnExit()) {
                    return $scope.confirmMessageWindow || $scope.confirmMessage;
                }
            }
            var $locationChangeStartUnbind = $scope.$on('$locationChangeStart', function(event, next, current) {
                if ($scope.confirmOnExit()) {
                    if(! confirm($scope.confirmMessageRoute || $scope.confirmMessage)) {
                        event.preventDefault();
                    }
                }
            });

            $scope.$on('$destroy', function() {
                window.onbeforeunload = null;
                $locationChangeStartUnbind();
            });
        }
    };
});

Usage: Example Controller: (using controllerAs syntax in this example)

angular.module('AppModule', []).controller('pageCtrl', [function () {
    this.isDirty = function () {
        // do your logic and return 'true' to display the prompt, or 'false' otherwise.

        return true;
    };
}]);

Template:

<div confirm-on-exit="pageCtrl.isDirty()" 
    confirm-message-window="All your changes will be lost." 
    confirm-message-route="All your changes will be lost. Are you sure you want to do this?">

Anders's answer works fine, However, for people who uses Angular ui-router, you should use '$stateChangeStart' instead of '$locationChangeStart'.

Detect unsaved changes and alert user using angularjs

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