Feature-Sliced Design: The Structural Cure for Frontend Spaghetti Code

Frontend developers eventually hit a wall: the application grows, components become entangled, and a simple change in one module breaks logic in another. This is the scaling problem. To solve this, we need an architecture that enforces loose coupling and high cohesion by default.

This article analyzes Feature-Sliced Design (FSD), an architectural methodology that is rapidly gaining traction for solving these exact pain points. We will dissect its hierarchy, compare it with classical approaches, and understand why it might be the best choice for complex projects.

The Core Hierarchy: Layers, Slices, and Segments

To understand FSD, we must distinguish three key decomposition concepts: Layer, Slice, and Segment.

1. Layers (The First Level)

Layers are the top-level directories. They are standardized and hierarchical. The critical rule of FSD is the unidirectional data flow: a layer can only import functionality from layers strictly below it.

Here is the breakdown of responsibilities, from top to bottom:

• App: The entry point. Contains setup logic, providers, global styles, and type declarations. It initializes the application.
• Processes: (Deprecated/Optional) Handles complex workflows spanning multiple pages (e.g., a multi-step checkout). Modern FSD often handles this within Features or Pages.
• Pages: Compositional layer. It assembles widgets, features, and entities to form a full route/view for the user.
• Widgets: Independent, complete UI blocks (e.g., a Header, a Feed, or a Sidebar) that combine entities and features.
• Features: User scenarios that carry business value (e.g., "Add to Cart", "Like Post", "User Registration").
• Entities: Business logic coupled to domain entities (e.g., User, Product, Order). This layer is for data display and manipulation logic, not direct user actions.
• Shared: The foundation. Reusable code not bound to specific business logic (UI Kit, API clients, helpers, general configurations).

  

The Interaction Rule: Features usually contain interactivity (actions), while Entities contain data and presentation. Features can use Entities, but Entities cannot use Features.

2. Slices (The Second Level)

Inside each layer, we have Slices. A slice partitions the code by the business domain. Slices are not standardized; they are named based on your project's needs.

For example, in the entities layer, you might have slices like user, post, comment. In the features layer, you might have auth-by-email, send-comment.

Crucial Rule: Slices within the same layer cannot import each other (High Cohesion, Low Coupling). They must be isolated.

3. Segments (The Third Level)

A slice is composed of Segments. These are technical divisions within a business unit:

• ui: React/Vue components.
• model: State management (Redux, Zustand, atoms) and business logic.
• api: Backend requests and DTOs.
• lib: Helpers specific to this slice.
• config: Configuration constants.

The Power of Public API

One of the strongest technical advantages of FSD is the concept of the Public API. Each slice/segment must have an index.ts or index.js file.

This file acts as a gateway. It explicitly exports only what the outside world is allowed to use. Internal helper functions or private components remain encapsulated.

// features/auth-by-email/index.ts

// Only these are accessible to the rest of the app
export { LoginForm } from './ui/LoginForm';
export { loginReducer } from './model/slice';

// Internal helper functions inside ./lib/validation.ts
// are NOT exported, keeping the namespace clean.

This mechanism simplifies refactoring. If you change the internal implementation but keep the Public API contract, the rest of the application remains unaffected.

Architectural Logic: Abstraction and Isolation

FSD applies fundamental software engineering principles—Encapsulation, Polymorphism, and Abstraction—directly to the directory structure.

• Lower layers (Shared, Entities): Highly abstract, reusable, stable.
• Higher layers (Pages, App): Business-specific, concrete, volatile.

  

Comparison with Other Architectures

FSD vs. Classic Architecture

Classic architecture often groups files by technical type (e.g., /components, /hooks, /services). While simple to start, this structure fails in large projects because it creates implicit connections. A single component folder becomes a dumping ground for unrelated business logic.

FSD vs. Simple Modular Architecture

Simple modular architectures group by feature but often lack strict dependency rules. Module A imports Module B, which imports Module A, leading to circular dependencies and tight coupling. FSD prevents this through its strict hierarchy.

Pros and Cons Summary

Conclusion

Feature-Sliced Design is not just a folder structure; it is a set of rules that force you to think about code value and scope. While it requires an initial investment in learning, the payoff in long-term maintainability for enterprise-level or complex applications is immense.