http://jsfiddle.net/rommsen/QY8w2/

onlinesstatus.js

'use strict';

var onlineStatusApp = angular.module('onlineStatusApp', []);

onlineStatusApp.factory('onlineStatus', ["$window", "$rootScope", function ($window, $rootScope) {
    var onlineStatus = {};

    onlineStatus.onLine = $window.navigator.onLine;

    onlineStatus.isOnline = function() {
        return onlineStatus.onLine;
    }

    $window.addEventListener("online", function () {
        onlineStatus.onLine = true;
        $rootScope.$digest();
    }, true);

    $window.addEventListener("offline", function () {
        onlineStatus.onLine = false;
        $rootScope.$digest();
    }, true);

    return onlineStatus;
}]);
    
onlineStatusApp.controller( 'OnlineStatusCtrl', function OnlineStatusCtrl( $scope, onlineStatus ) {
    $scope.onlineStatus = onlineStatus;
    
    $scope.$watch('onlineStatus.isOnline()', function(online) {
        $scope.online_status_string = online ? 'online' : 'offline';
    });
});