writing functions in angularjs services

Here is fiddle with a basic example of how you can use (inject) services in controllers.

http://jsfiddle.net/uhmNR/1/

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


//Here is the service Users with its functions and attributes
//You can inject it in any controller, service is a singleton and its data persist between controllers
myApp.factory('Users', function () {

    var userName = "John Doe";

    return {
        getUserName: function () {
             return userName;                   
        },
        setUserName: function (newName) {
            userName = newName;
        }
    }
});

//An Util service with DaysInMonth method   
myApp.factory('Util', function () {

    return {
        daysInMonth: function (month,year) {

            return new Date(year, month+1,0).getDate();
        }
    };

});   

//Here I am injecting the User service andusing its methods   
myApp.controller('MyCtrl', ['$scope', 'Users', 'Util', function ($scope, Users, Util) {

    Users.setUserName('Robin Hood');

    $scope.name = Users.getUserName();

    //Using Util.daysInMonth()
    $scope.date = Util.daysInMonth(12,2012);
}]);

Hope It helps.

Expose the function as a service, then let the AngularJS injector do the rest. You can easily set a daysInMonth service as a static value in your module. See this in action at http://jsfiddle.net/hbulhoes/zdtnw/

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

// This is the declaration of the daysInMonth service. It's set as
// a value in the module, with the value being the very function
// you want to share among other services and controllers:
mod.value('daysInMonth', function(month, year) {
    return new Date(year, month+1,0).getDate();
});

// And this is an example controller that depends on the daysInMonth function.
function MyController($scope, daysInMonth){
    $scope.DaysInCurrentMonth = daysInMonth(12, 2012);
}

If you want a function to be available to all of your controllers, you might consider defining the method on $rootScope, instead of using a service:

myApp.run(function($rootScope) {
    $rootScope.daysInMonth = function(year, month) {
        return new Date(year, month+1,0).getDate();
    }
});

Then, due to prototypal scope inheritance, all of your controllers' scopes will have access to the method (without the need for dependency injection). You can call it in any controller like so:

function MyCtrl($scope) {
   console.log($scope.daysInMonth(12, 2012));
}​

JavaScript won't find function daysInMonth defined on $scope, so it will look in the parent scope (in this case the root scope) for the function, and it will find it.

You need to expose it. Wrap it in a return block:

var mod= angular.module('myapp', ['eventFilters', 'highlight', 'event', 'dayfilter', 'Services']).
config(['$routeProvider', function($routeProvider) {
$routeProvider.
  when('/', {templateUrl: 'template.html',   controller: Ctrl}).
  when('/temp', {templateUrl: 'temp.html',   controller: tempCtrl}).
  otherwise({redirectTo: '/invalid'});
}]);
mod.service ('sharedService', function() {
   return {
      daysInMonth: function(month,year) {
         return new Date(year, month+1,0).getDate();
      }
   };
});