Full override of the reaction function on a message.
This function must return a function that takes as a parameter the `reactionType` as a string.
Please refer to [the guide on customizing message actions](../../../../guides/message_actions_customization.mdx) for details.

| Type |
| - |
| function \| null |

| Parameter | Description |
| - | - |
| message	| message the action is called on |