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

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.

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.

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/

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

Public API Pattern

Every slice MUST expose a public API via

index.ts

. External code imports ONLY from this file.

typescript

// 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';

typescript

// ✅ 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:

typescript

// ❌
export * from './ui';

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

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

typescript

// 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.

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

TypeScript Configuration

json

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

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)

feature-slicing

NPX Install

Tags

SKILL.md Content

Feature-Sliced Design Architecture

THE IMPORT RULE (Critical)

Layer Hierarchy

Quick Decision Trees

"Where does this code go?"

"Feature or Entity?"

"Which segment?"

Directory Structure

Public API Pattern

Cross-Entity References (@x Notation)

Anti-Patterns

TypeScript Configuration

Reference Documentation

Resources

Official Sources

Community