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nextjs-architecture

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Structure Next.js projects with Feature-Sliced Design layers, domain-grouped slices, and strict import hierarchy. Use when organizing features into FSD layers, enforcing slice boundaries, or keeping page.tsx thin.

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Architecture (Feature-Sliced Design)

Priority: P2 (MEDIUM)

Warning: FSD introduces boilerplate. Use it only if project expected to grow significantly (e.g., 20+ features). For smaller projects, simple module-based structure preferred.

Workflow: Create New Feature Slice

  1. Create feature folder — src/features/auth/login/ with ui/, model/, api/ segments.
  2. Add public API — Export via src/features/auth/login/index.ts.
  3. Wire into page — Import feature widget in app/login/page.tsx (thin page).
  4. Verify imports — Ensure no upward or cross-slice imports violate layer hierarchy.

Layer Hierarchy

App (app/) -> Widgets -> Features -> Entities -> Shared

See implementation examples for thin page example.

Strategy

  1. RSC Boundaries: Enforce strict serialization rules for props passed from Server to Client. See RSC Boundaries & Serialization.
  2. App Layer Thin: app/ directory (App Router) only for Routing.
  • Rule: page.tsx should only import Widgets/Features. No business logic (useEffect, fetch) directly in pages.
  1. Slices over Types: Group code by Business Domain (User, Product, Cart), not by File Type (Components, Hooks, Utils).
  • Bad: src/components/LoginForm.tsx, src/hooks/useLogin.ts
  • Good: src/features/auth/login/ containing both.
  1. Layer Hierarchy: Code can only import from layers below it.
  • App -> Widgets -> Features -> Entities -> Shared.
  1. Avoid Excessive Entities: not preemptively create Entities.
  • Rule: Start logic in Features or Pages. Move to Entities only when data/logic strictly reused across multiple differing features.
  • Rule: Simple CRUD belongs in shared/api, not entities.
  1. Standard Segments: Use standard segment names within slices.
  • ui (Components), model (State/actions), api (Data fetching), lib (Helpers), config (Constants).
  • Avoid: components, hooks, services as segment names.

Structure Reference

For specific directory layout and layer definitions, see reference documentation.

Architecture Checklist (Mandatory)

  • Layer Imports: any layer import from layer ABOVE it? (App > Widgets > Features > Entities > Shared)

  • Page Logic: page.tsx thin, containing only Widgets/Features and zero useEffect/fetch?

  • RSC Boundaries: Server Components isolated from Client Components with proper 'use client' boundaries?

  • Public API: all access to slice performed via top-level index.ts (public API)?

  • Cross-Slice: slices within same layer (e.g., two features) import from each other directly? (Prohibited)

  • Server Actions: Place them in model/ folder of Feature (e.g., features/auth/model/actions.ts).

  • Data Access (DAL): Place logic in model/ folder of Entity (e.g., entities/user/model/dal.ts).

  • UI Components: Base UI (shadcn) belongs in shared/ui. Feature-specific UI belongs in features/*/ui.

Anti-Patterns

  • No cross-slice imports: Slices in same layer must not import from each other directly.
  • No business logic in page.tsx: Pages import Widgets/Features only; zero useEffect/fetch.
  • No file-type folders: Group by domain (features/auth/), not type (components/, hooks/).
  • No premature Entity creation: Start in Features; move to Entities only on strict reuse.