In Redux Toolkit, the prepare callback allows you to customize action’s payload before it reaches the reducer. It is used inside a reducer definition within createSlice() and returns an object containing payload, and optionally, meta and error fields.

When to Use the prepare Callback

1
To preprocess or format arguments before passing them as the action payload.
2
To include additional fields such as timestamps or IDs along with the payload.
3
To maintain clean reducer logic by separating payload creation from state updates.

Example: Using prepare to Add Metadata

Here, the prepare callback formats the payload with an auto-generated ID and timestamp before the action reaches the reducer, keeping the reducer focused solely on updating the state.