Type for the return from a message handler.

void: if the return type is void the handle will remain open boolean: if the return value is true, the handle will be closed and further callbacks will stop. If the value is false the handle will remain open.

The return type can also be a Promise to void or boolean, and the same applies. delete() on the handle will be deferred until the Promise is resolved. If the promise is rejected, the handle is always closed.