playfabCatalogItemVSItemInstance.js

handlers.getItemPositionAndDamage = function(args) {
  
  var getCatalogItemsResponse = server.GetCatalogItems({});    //This returns the primary catalog for our game if we wanted a different version we must specify CatalogVersion in the argument
  var catalogItems = getCatalogItemResponse.Catalog;            //We must get the catalog field out of the response object

  var getUserInventoryResponse = server.GetUserInventory({     //This returns all the inventory items for the user who called ExecuteCloudScript()
      PlayFabId: currentPlayerId
    });
  var userInventory = getUserInventoryResponse.Inventory;      //We must get the Inventory field out of the response object

  var firstItemInstance = userInventory[0];                     //Gets the first item out of userInventory.

  var firstItemInstanceUnencodedCustomData = firstItemInstance.CustomData;                        //Gets the unencoded JSON object of this items ItemInstance.CustomData
  var firstItemInstanceInventoryPostion = firstItemInstanceUnencodedCustomData.InventoryPosition; //Get the InventoryPostion for this item

/*
  Below is probably the most important piece of code in this script it is how 
  we get the CatalogItem information with only an ItemInstance object. It uses 
  the find function of JavaScript arrays (Array.prototype.find()) to iterate 
  through all the individual catalogItems in catalogItems. it then will return 
  the CatalogItem object which has the same ItemId as our ItemInstance;
*/
  var catalogItemOfFirstItemInstance = catalogItems.find(function(catalogItem) { 
    return catalogItem.ItemId == firstItemInstance.ItemId;
    });

  var firstItemInstanceDescription = catalogItemOfFirstItemInstance.Description;  //Using the found catalogItem we can now get the items Description.
  
  var catalogItemEncodedCustomData = catalogItemOfFirstItemInstance.CustomData;   //To get to the items CustomData first we get the JSON encoded as a string
  var catalogItemUnencodedCustomData = JSON.parse(catalogItemEncodedCustomData);  //Then we use JSON.parse(encodedJSON) to parse our encoded JSON string into an unencoded JSON object
  var firstItemInstanceDamage = Number(catalogItemUnencodedCustomData.Damage);    //And lastly we use the JSON object o get the items damage we use Number(object) to cast it to a number

  return {
    InventoryPosition: firstItemInstanceInventoryPostion,
    Description: firstItemInstanceDescription,
    Damage: firstItemInstanceDamage
  };
};

Thanks for this example! It helps me a lot.