Decoupling Business Logic: A Fullstack Guide to Domain-Driven Design (DDD)

One of the biggest mistakes in modern development is letting your framework (React, NestJS, Flutter) dictate your business logic. When the "rules" of your application are scattered across UI components and Database controllers, your system becomes a "Big Ball of Mud."

Domain-Driven Design (DDD) is the architectural remedy. It allows us to build a "Brain" (The Domain) that is independent of the "Body" (The Framework).

Building on the scalable folder structures we discussed previously, we’re now diving into Domain-Driven Design (DDD). We are moving beyond simple organization to a modular architecture where each feature acts as a standalone unit, a vital step for anyone looking to implement Micro-Frontends or Microservices.

1. The Core Philosophy: The Bounded Context

In an enterprise system, the word "User" means different things to different departments.

• In Auth: A User is a set of credentials (email/password).
• In Billing: A User is a tax ID and a credit card.
• In Support: A User is a ticket history.

Instead of one massive User class that handles everything, DDD teaches us to create Bounded Contexts. We split the application into logical boundaries where terms are consistent.

2. The Layered Blueprint (Frontend & Backend)

Whether you are in a NestJS service or a React feature folder, the internal structure should follow these four layers:

A. The Domain Layer (The Core)

• Content: Entities, Value Objects, and Domain Services.
• Rule: Zero Dependencies. It should not know about React, NestJS, Axios, or TypeORM.
• Example: A calculateInvoiceTotal() function that only cares about math and business rules.

B. The Application Layer (The Orchestrator)

• Content: Use Cases, Ports, and Custom Hooks.
• Role: It tells the Domain what to do.
• Frontend: A React Hook like useCheckoutFlow().
• Backend: A NestJS Service that coordinates between the Database and the Domain.

C. The Infrastructure Layer (The External World)

• Content: Repositories, API Clients, Database Models.
• Role: Technical implementation. This is where you write your SQL queries or your fetch() calls.

D. The Presentation Layer (The Interface)

• Content: React Components / NestJS Controllers.
• Role: Translating user input (clicks or HTTP requests) into something the Application layer understands.

3. Why DDD is the Secret to Fullstack Harmony

When both your Frontend and Backend teams use DDD:

1. Shared Language: The DiscountCode logic in the React app matches the DiscountCode logic in the NestJS API.
2. Easier Migrations: If you want to move from Postgres to MongoDB (Infrastructure), your Business Logic (Domain) stays exactly the same.
3. Microservices & MFE Ready: Because your contexts are already isolated, splitting a Monolith into Microservices or Micro-Frontends becomes a "copy-paste" job rather than a rewrite.

4. Summary: The DDD Checklist

Layer	Responsibility	Framework Dependency?
Domain	Business Rules & Logic	No
Application	Workflow Orchestration	Minimal (Hooks/Services)
Infrastructure	Data & External Tools	Yes (DB/API)
Presentation	UI & Entry Points	Yes (React/Nest)

Conclusion

DDD is not about adding more files; it’s about putting logic where it belongs. By isolating your "Domain," you ensure that your business rules remain stable even as frontend frameworks and backend databases evolve.