Argument 'fn' is not a function got string

The problem was in using the 'wrong' syntax to create the service
instead of using:

messageServices.factory('MessageService', 
    ['MessageData','localStorageService', 'UserService'], 
    function(MessageData, localStorageService, UserService){
        return new MessageService(MessageData, localStorageService, UserService);
    }
);

I had to use:

messageServices.factory('MessageService', 
    ['MessageData','localStorageService', 'UserService', 
    function(MessageData, localStorageService, UserService){
        return new MessageService(MessageData, localStorageService, UserService);
    }
]);

I closed the array with parameters to soon, and since I'm still learning I didn't see it directly, anyhow I hope I can help others who stumble upon this.

Today I got the same kind of error doing that silly mistake:

(function(){

  angular
    .module('mymodule')
    .factory('myFactory', 'myFactory');   // <-- silly mistake 

  myFactory.$inject = ['myDeps'];
  function myFactory(myDeps){
    ...
  }

}());

instead of that:

(function(){

  angular
    .module('mymodule')
    .factory('myFactory', myFactory);   // <-- right way to write it    

  myFactory.$inject = ['myDeps'];
  function myFactory(myDeps){
    ...
  }

}());

In fact the string "myFactory" was brought into the injector who was waiting for a function and not a string. That explained the [ng:areq] error.