React Router v7

# React Router v7 Rules

## Route Modules

- Use route modules as the boundary for route UI, loader data, actions, metadata, and error boundaries.
- Keep route modules small; move shared UI to components and reusable data access to services.
- Prefer file-based routing in framework mode when the project is configured for it.
- Use nested routes for shared layouts and progressive disclosure.
- Export route-specific `ErrorBoundary` components for recoverable route failures.

## Data Loading

- Use loaders for route data that should be available before render.
- Keep loaders deterministic and side-effect free.
- Validate params and search params at the loader boundary.
- Return typed data and consume it through route hooks rather than duplicating fetch logic in components.
- Use deferred or streaming patterns only when they improve perceived performance.

## Mutations

- Use actions for route mutations and form submissions.
- Prefer `Form`, `useFetcher`, and `useSubmit` for progressive enhancement.
- Revalidate affected loader data after mutations.
- Handle validation errors as typed action data instead of generic exceptions.
- Keep server-only secrets and privileged operations out of client actions.

## Navigation and State

- Store shareable state in URL params or search params.
- Keep ephemeral UI state local to components.
- Use pending navigation state to show optimistic or loading UI.
- Avoid global state for data that belongs to route loaders.

## TypeScript and Testing

- Type loader and action return values.
- Add tests for route loaders, actions, validation failures, and error boundaries.
- Use integration tests for critical form and navigation flows.
- Mock network and persistence at the route-service boundary.

## Common Mistakes

- Do not duplicate loader fetches in `useEffect`.
- Do not mutate data in loaders.
- Do not hide route errors behind a single generic app-level catch-all.
- Do not put auth checks only in components when loader data is protected.