In the ever-evolving world of backend development, maintainability and modularity are no longer luxuries but they’re necessities. If you’ve ever had to trace through tangled, messy spaghetti code or just stood on the sidelines of feature addition because “something might break,” you know exactly what we’re talking about.
That’s where NestJS comes in. With its powerful architecture inspired by Angular, NestJS offers a robust and scalable foundation to build backend applications that are clean, predictable, and adaptable on a road to growth.
Through this blog post, we are sharing actionable lessons to build modular and maintainable NestJS backends, all based on real-world experience and some hard lessons learned.
1. Start with a Scalable Project Structure
NestJS encourages structure by default. But don’t stop there and plan your architecture around features, not layers.
Bad:
/controllers
/services
/entities
Better:
/users
└── users.controller.ts
└── users.service.ts
└── users.module.ts
/products
└── products.controller.ts
└── products.service.ts
└── products.module.ts
This feature-first structure allows you to encapsulate logic and makes it easier for developers to reason about each part of the app.
2. Break Down Your App Into Modules
Think of modules as the building blocks of your application. They help you encapsulate related functionality and promote reusability.
A good rule of thumb: If a piece of logic serves a specific domain or feature, give it its own module.
Also, don’t be afraid to create shared modules for things like authentication, logging, and utilities. Just make sure to keep them clean and focused shared does not mean “dumping ground.”
3. Use Dependency Injection Thoughtfully
NestJS’s dependency injection system is incredibly powerful. Use it to:
- Inject services and repositories instead of manually instantiating them.
- Write more testable and decoupled code.
- Avoid circular dependencies by keeping module boundaries clean.
Avoid injecting more than you need. If a service has too many responsibilities, it might be time to split it up.
4. Stick to the SOLID Principles
These aren’t just buzzwords but they’re guiding principles for writing clean and scalable code.
- Single Responsibility: Keep classes focused on one thing.
- Open/Closed: Make your code extensible without modifying existing logic.
- Liskov Substitution: Favor interfaces or abstract classes where possible.
- Interface Segregation: Don’t force classes to depend on things they don’t use.
- Dependency Inversion: Rely on abstractions, not concrete classes.
These principles align beautifully with NestJS’s architecture.
5. Write Tests That Matter
Don’t aim for 100% coverage; aim for meaningful coverage.
- Write unit tests for services and utility functions.
- Write e2e tests for full API behavior using @nestjs/testing.
- Use mocks and fakes to isolate logic and make your tests faster and more reliable.
When your backend grows, good tests are the safety net that lets you move fast without breaking things.
6. Embrace Middleware, Pipes, and Guards
These are NestJS’s secret weapons for writing DRY and clean code:
- Middleware: Pre-process requests globally or for specific routes (e.g., logging, request transformations).
- Pipes: Validate and transform incoming data (e.g., DTO validation).
- Guards: Handle access control and authorization in a clean, reusable way.
By leveraging these tools, you reduce clutter in your controllers and services.
7. Use DTOs and Validation
Never trust incoming data blindly. Use Data Transfer Objects (DTOs) with decorators from class-validator and class-transformer.
export class CreateUserDto {
@IsEmail()
email: string;
@MinLength(6)
password: string;
}
This ensures clean boundaries between your API layer and business logic, and improves both security and reliability.
8. Don’t Mix Concerns
It’s tempting to put business logic in controllers but resist the urge.
- Controllers: Handle routing and responses.
- Services: Handle business logic.
- Repositories/ORM layers: Handle data access.
Keeping these roles separate makes your app easier to reason about and scale.
9. Log Smartly and Monitor Early
Use NestJS’s built-in logger or integrate with tools like Winston, Datadog, or Sentry.
You can’t fix what you can’t see.
Set up structured logging and error reporting from day one. When things go wrong in production (and they will), you’ll be glad you did.
10. Documentation Is a Feature, Not an Afterthought
Use tools like Swagger (via @nestjs/swagger) to automatically generate API docs.
@ApiTags(‘users’)
@Controller(‘users’)
export class UsersController {
// …
}
Good documentation helps your future self, your teammates, and anyone who uses your API. It’s a gift that keeps on giving.
Modularity and maintainability are not only for the big software teams but they are for any team that wants to build software that will survive the changing times.
NestJS gives you the tools. Like any working tool, your usage determines all. Think of your codebase as a garden: tend to it kindly, keep things trimmed and tidy, and do not wait for it to go wild.
Build small. Build smart. Build for tomorrow and not just today. If you have a NestJS project running in the field or in your mind and need some help in how to structure or refactor it in the big picture, Capital Compute is here to help! We love working on backends that are clean, modular, and scale with your products. We are interested in hearing about your ideas. We’d love to hear about what you’re building Get in touch with us and let’s explore how we can support your journey to a scalable, maintainable backend together.
