Flash Success Messages in Inertia

Let's show some success messages when submitting the form.

It is very common to show some success message when redirecting back.

app/Http/Controllers/PostController.php:

class PostController extends Controller
{
    // ...
 
    public function store(StorePostRequest $request): RedirectResponse
    {
        Post::create($request->validated());
 
        return redirect()->route('posts.index')
            ->with('message', 'Post created successfully.'); 
    }
}

But how do we catch this message in Inertia? We must pass that variable as...