Throw error when custom action creator does not have `type` value

[chawes13]

It'd be great to have a guard for passing in an action creator that doesn't have a truthy type string value.

I encountered this issue when migrating over to RTK. We built some custom action creators that we use in combination with a custom api middleware (we're not ready to switch to RTQ just yet - baby steps 😅!). While these have the toString() override, the builder addCase method is not using that and it seems like this override is getting deprecated in v2.

The following led to a fairly subtle bug, since it fails silently.

extraReducers: (builder) => {
  builder.addCase(apiActions.fetchTodo, todoHandler) // fetchTodo.type === undefined
}

Since I'm working in JS, this issue is easier to run into. Although, I believe a guard may still be helpful to prevent accidental assignments of an empty string to the type attribute in the implementation of a custom action creator in TS.

redux-toolkit/packages/toolkit/src/mapBuilders.ts

Lines 158 to 168 in 24fc343

	const type =
	typeof typeOrActionCreator === 'string'
	? typeOrActionCreator
	: typeOrActionCreator.type
	if (type in actionsMap) {
	throw new Error(
	'addCase cannot be called with two reducers for the same action type'
	)
	}
	actionsMap[type] = reducer
	return builder

[EskiMojo14]

I'm sure we'd be open to a PR that throws an error if the type variable is undefined :)