There is a typo in the below statement. The property item is incorrect, it is items.

Original code

function drop(e, board, item) {
   ...
   const currentIndex = currentBoard.item.indexOf(currentItem)
   ...
}

Corrected Code

function drop(e, board, item) {
   ...
   const currentIndex = currentBoard.items.indexOf(currentItem)
   ...
}

Test run by fixing the typo

a) On load of the app

b) After the drag and drop of a onto b

c) After the drag and drop of a onto c