Typescript Object is possibly 'undefined' after check

Question

Quick question - why Typescript still thinks it will be undefined?

When added code below to my application I have Typescript Error on line draft.splice(result.destination.index, 0, draggedItem); saying that result.destination Object is possibly undefined. This is partially true as it is typed like this: DragUpdate.destination?: DraggableLocation | undefined. However, I do a check at the very beginning of this function so at a given line it will never be undefined.

const onDragEnd = useCallback((result: DropResult) => {
        // do nothing if no destination
        if (!result.destination) return;
        setState((prevState: IOrderCard[]) =>
            produce(prevState, draft => {
                const draggedItem = draft[result.source.index];
                draft.splice(result.source.index, 1);
                draft.splice(result.destination.index, 0, draggedItem);
                return draft;
            })
        );
    }, []);

This is React app with hooks, using also immer and react-beautiful-dnd.

Answer 1

I can't comment to what Alexsey L. said but an even quicker fix is to just use ! when you know something will be defined.