question

The callback value Inventory of the GetUserInventory method is an array, even if there is only one data inside. I modified your code, you can use the following code and refer to the api document for testing.

function inv()
{
    // save these values locally to ease use
    // save title id            
    // build http request object for LoginWithCustomId                   
    var GetUserInventoryRequest = {};            
    OutputStatus("Logging in...");            
    PlayFabClientSDK.GetUserInventory(GetUserInventoryRequest,
        (response, error) => 
        {                
            if(error)                
            {                  
                OutputError(error);
            }                 
            else                
            {                  

                // display account details                  
                //var result = response.data;                  
                //var inv = result.Inventory;                 
                //var status = inv.ItemId;                  
                //OutputStatus(status); 
                

                var result = response.data;             
                var inv = result.Inventory;
                for(var i=0;i<inv.length;i++)
                {
                    var status=inv[i].ItemId;
                    OutputStatus(status);
                }

            }            
        });          
}