feature-slicing

Feature-Sliced Design Architecture

Frontend architecture methodology with strict layer hierarchy and import rules for scalable, maintainable applications. FSD organizes code by business domain rather than technical role.

Official Docs: feature-sliced.design | GitHub: feature-sliced

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THE IMPORT RULE (Critical)

Modules can ONLY import from layers strictly below them. Never sideways or upward.

app → pages → widgets → features → entities → shared
 ↓      ↓        ↓          ↓          ↓         ✓
 ✓      ✓        ✓          ✓          ✓      (external only)

Violation	Example	Fix
Cross-slice (same layer)	features/auth → features/user	Extract to entities/ or shared/
Upward import	entities/user → features/auth	Move shared code down
Shared importing up	shared/ → entities/	Shared has NO internal deps

Exception: app/ and shared/ have no slices, so internal cross-imports are allowed within them.

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Layer Hierarchy

Layer	Purpose	Has Slices	Required
app/	Initialization, routing, providers, global styles	No	Yes
pages/	Route-based screens (one slice per route)	Yes	Yes
widgets/	Complex reusable UI blocks (header, sidebar)	Yes	No
features/	User interactions with business value (login, checkout)	Yes	No
entities/	Business domain models (user, product, order)	Yes	No
shared/	Project-agnostic infrastructure (UI kit, API client, utils)	No	Yes

Minimal setup: app/, pages/, shared/ — add other layers as complexity grows.

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Quick Decision Trees

"Where does this code go?"

Code Placement:
├─ App-wide config, providers, routing    → app/
├─ Full page / route component            → pages/
├─ Complex reusable UI block              → widgets/
├─ User action with business value        → features/
├─ Business domain object (data model)    → entities/
└─ Reusable, domain-agnostic code         → shared/

"Feature or Entity?"

Entity (noun)	Feature (verb)
user — user data model	auth — login/logout actions
product — product info	add-to-cart — adding to cart
comment — comment data	write-comment — creating comments
order — order record	checkout — completing purchase

Rule: Entities represent THINGS with identity. Features represent ACTIONS with side effects.

"Which segment?"

Segments (within a slice):
├─ ui/      → React components, styles
├─ api/     → Backend calls, data fetching, DTOs
├─ model/   → Types, schemas, stores, business logic
├─ lib/     → Slice-specific utilities
└─ config/  → Feature flags, constants

Naming: Use purpose-driven names (api/, model/) not essence-based (hooks/, types/).

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Directory Structure

src/
├── app/                    # App layer (no slices)
│   ├── providers/          # React context, QueryClient, theme
│   ├── routes/             # Router configuration
│   └── styles/             # Global CSS, theme tokens
├── pages/                  # Page slices
│   └── {page-name}/
│       ├── ui/             # Page components
│       ├── api/            # Loaders, server actions
│       ├── model/          # Page-specific state
│       └── index.ts        # Public API
├── widgets/                # Widget slices
│   └── {widget-name}/
│       ├── ui/             # Composed UI
│       └── index.ts
├── features/               # Feature slices
│   └── {feature-name}/
│       ├── ui/             # Feature UI
│       ├── api/            # Feature API calls
│       ├── model/          # State, schemas
│       └── index.ts
├── entities/               # Entity slices
│   └── {entity-name}/
│       ├── ui/             # Entity UI (Card, Avatar)
│       ├── api/            # CRUD operations
│       ├── model/          # Types, mappers, validation
│       └── index.ts
└── shared/                 # Shared layer (no slices)
    ├── ui/                 # Design system components
    ├── api/                # API client, interceptors
    ├── lib/                # Utilities (dates, validation)
    ├── config/             # Environment, constants
    ├── routes/             # Route path constants
    └── i18n/               # Translations

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Public API Pattern

Every slice MUST expose a public API via index.ts. External code imports ONLY from this file.

// entities/user/index.ts
export { UserCard } from './ui/UserCard';
export { UserAvatar } from './ui/UserAvatar';
export { getUser, updateUser } from './api/userApi';
export type { User, UserRole } from './model/types';
export { userSchema } from './model/schema';

// ✅ Correct
import { UserCard, type User } from '@/entities/user';

// ❌ Wrong
import { UserCard } from '@/entities/user/ui/UserCard';

Avoid wildcard exports — they expose internals and harm tree-shaking:

// ❌
export * from './ui';

// ✅
export { UserCard } from './ui/UserCard';

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Cross-Entity References (@x Notation)

When entities legitimately reference each other, use the @x notation:

entities/
├── product/
│   ├── @x/
│   │   └── order.ts    # API specifically for order entity
│   └── index.ts
└── order/
    └── model/types.ts  # Imports from product/@x/order

// entities/product/@x/order.ts
export type { ProductId } from '../model/types';

// entities/order/model/types.ts
import type { ProductId } from '@/entities/product/@x/order';

Guidelines: Keep cross-imports minimal. Consider merging entities if references are extensive.

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Anti-Patterns

Anti-Pattern	Problem	Fix
Cross-slice import	features/a → features/b	Extract shared logic down
Generic segments	components/, hooks/	Use ui/, lib/, model/
Wildcard exports	export * from './button'	Explicit named exports
Business logic in shared	Domain logic in shared/lib	Move to entities/
Single-use widgets	Widget used by one page	Keep in page slice
Skipping public API	Import from internal paths	Always use index.ts
Making everything a feature	All interactions as features	Only reused actions

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TypeScript Configuration

{
  "compilerOptions": {
    "baseUrl": ".",
    "paths": {
      "@/*": ["./src/*"]
    }
  }
}

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Reference Documentation

File	Purpose
references/LAYERS.md	Complete layer specifications, flowcharts
references/PUBLIC-API.md	Export patterns, @x notation, tree-shaking
references/IMPLEMENTATION.md	Code patterns: entities, features, React Query
references/NEXTJS.md	App Router integration, page re-exports
references/MIGRATION.md	Incremental migration strategy
references/CHEATSHEET.md	Quick reference, import matrix

Resources

Official Sources

• Official Documentation: https://feature-sliced.design
• GitHub Organization: https://github.com/feature-sliced
• Official Examples: https://github.com/feature-sliced/examples
• Specification: https://feature-sliced.design/docs/reference

Community

• Awesome FSD: https://github.com/feature-sliced/awesome (curated articles, videos, tools)