This PR adds support for Flash Data. This allows you to send data to the frontend that isn't persisted in the browser history state. This makes it ideal for one-time notifications like toasts, highlights, or other transient UI states that shouldn't reappear when navigating back/forward.

You may use Inertia::flash() to flash data from the backend.

public function store(StoreUserRequest $request)
{
    $user = User::create($request->validated());

    Inertia::flash('toast', [
        'type' => 'success',
        'message' => 'User created successfully!',
    ]);

    return back();
}

You may chain back() on flash().

return Inertia::flash('newUserId', $user->id)->back();

Or chain flash() on render() and vice versa.

return Inertia::render('Projects/Index', [
    'projects' => $projects,
])->flash('highlight', $project->id);

// or...

return Inertia::flash()->render();

Flash Data is automatically reflashed on redirects, so it reaches the client after the redirect.