Custom JS class error - function is not a member of the class

I'm trying to make a very basic Momento JS class, but when I call the functions within the class I get the error message storeState and applyState are not members of Momento.

Here's the code for my class

class Memento {

 var objectRotation : Quaternion;
 var objectPostion : Vector3;

 function storeState(player_Object : Transform)
 {
     objectRotation = player_Object.rotation;
     objectPostion = player_Object.position;
 }

 function applyState(player_Object : Transform)
 {

     player_Object.rotation = objectRotation;
     player_Object.position = objectPostion;
 }

}

I then call the functions Momento.storeState(transform object) or Momento.applyState(transform object), but then get the above errors - 'storeState' is not a member of 'Momento' and 'applyState' is not a member of 'Momento'

Any advice is appreciated.