What does getHandlerId() do and how to use it?

Some of the react-dnd examples use a getHandlerId() method.

For example in the simple example of a sortable list, the Card.tsx function:

Collects a handlerId from the monitor object within the useDrop method

collect(monitor) {
  return {
    handlerId: monitor.getHandlerId(),
  }
},

Returns that as an element of the "collected props"
```
const [{ handlerId }, drop] = useDrop<
```

Uses it to initialize an HTML attribute named data-handler-id

<div ref={ref} style={{ ...style, opacity }} data-handler-id={handlerId}>

What is this Id and why is it used?

What uses the data-handler-id attribute?

I'd expect to see getHandlerId() described in the API documentation as a method of the DropTargetMonitor (but it isn't).

Solution

I didn't dive deep into it but for me this information was enough to continue using it:

If you remove this data-handler-id, everything continue working but with some issues (item sometimes flickers, it doesn't go to another place as smoothly as it does with data-handler-id)
Here is an open issue https://github.com/react-dnd/react-dnd/issues/2621 about low performance, and this comment suggests to use handler id: https://github.com/react-dnd/react-dnd/issues/2621#issuecomment-847316022
As you can see in code https://github.com/react-dnd/react-dnd/search?q=handlerId&type=code, handler id is using for proper definition of drop item so it seems better to use it even if you don't have a lot of elements.