Feature Slicing

Feature-Sliced Design Architecture

Overview

Feature-Sliced Design (FSD) is an architectural methodology for scaffolding frontend applications with rules and conventions for organizing code to remain understandable and stable amid changing business requirements.

Official Documentation: https://feature-sliced.design/llms.txt

Core Principles

1. Layers - 7 standardized horizontal levels (top to bottom):

   • app/ → routing, entrypoints, global styles, providers
   • processes/ → (deprecated) complex cross-page scenarios
   • pages/ → full pages or nested routing sections
   • widgets/ → self-contained UI blocks delivering complete use cases
   • features/ → reused product functionality with business value
   • entities/ → business domain objects (user, product, order)
   • shared/ → reusable, project-detached functionality

2. Import Rule - Modules can only import from layers strictly below them. Never import sideways or upward.

3. Slices - Business-domain partitions within layers (e.g., user, product, cart). Cannot reference other slices at the same layer.

4. Segments - Purpose-based groupings within slices:

   • ui/ → components, formatters, styles
   • api/ → backend interactions, data types
   • model/ → schemas, stores, business logic
   • lib/ → slice-specific utilities
   • config/ → feature flags, configuration

5. Public API - Each slice exposes functionality via index.ts barrel file.

Quick Reference Structure

src/
├── app/              # Layer: Application initialization
│   ├── providers/    # Context providers, store setup
│   ├── routes/       # Router configuration
│   └── styles/       # Global styles
├── pages/            # Layer: Route-based screens
│   └── {page-name}/
│       ├── ui/
│       ├── api/
│       └── index.ts  # Public API
├── widgets/          # Layer: Complex reusable blocks
│   └── {widget-name}/
│       ├── ui/
│       └── index.ts
├── features/         # Layer: User interactions
│   └── {feature-name}/
│       ├── ui/
│       ├── api/
│       ├── model/
│       └── index.ts
├── entities/         # Layer: Business entities
│   └── {entity-name}/
│       ├── ui/
│       ├── api/
│       ├── model/
│       └── index.ts
└── shared/           # Layer: Shared infrastructure
    ├── ui/           # UI kit, design system
    ├── api/          # API client, request functions
    ├── lib/          # Utilities (dates, validation)
    ├── config/       # Environment, constants
    └── i18n/         # Internationalization

When Implementing FSD

1. Creating a new feature: Place in features/ if reused across pages, otherwise keep in pages/
2. Creating a new entity: Place in entities/ with ui/, api/, model/ segments
3. Creating shared utilities: Place in shared/lib/ or shared/ui/
4. Integrating with Next.js: Place App Router in src/app/ (no root app/), which serves as both routing and FSD app layer

Reference Documentation

For detailed implementation guidance, consult these reference files:

• Layer Details - Complete layer specifications and guidelines
• Public API Patterns - Export patterns, barrel files, cross-imports
• Implementation Patterns - Code examples, entity/feature patterns
• Next.js Integration - App Router and Pages Router setup
• Migration Guide - Migrating existing projects to FSD
• Cheatsheet - Quick decision guide and common patterns

Key Anti-Patterns to Avoid

• Importing from higher layers (breaks unidirectional flow)
• Cross-slice imports at the same layer (use lower layers instead)
• Generic segment names like components/, hooks/, types/
• Wildcard exports (export * from) in public APIs
• Storing business logic in shared/ layer