How NestJS Changed the Way I Build Backends

A few weeks ago, I was given an interview task to build a backend. I didn’t get the role — they wanted someone with more experience — but that assignment completely changed the way I build backend systems.

Until then, I mostly used Express.js. It’s flexible and fast, but as projects grow, the code usually turns into a pile of routes, middlewares, and functions tightly coupled together. Scaling that kind of codebase is painful.

During that task, I discovered NestJS — a framework built on top of Express. On the surface, it might look like a wrapper. But once I built my community-backend project with it, I realized NestJS is really about architecture.

Express.js vs NestJS: A Quick Example

Express.js (typical route)

// routes/user.js
const express = require("express");
const router = express.Router();
const db = require("../db");

router.post("/users", async (req, res) => {
  const { name, email } = req.body;
  if (!email) return res.status(400).send("Email is required");

  const user = await db.user.create({ name, email });
  res.json(user);
});

module.exports = router;

Here, validation, DB logic, and request handling all live in one place. Works fine for small apps — but becomes unmanageable in large systems.

NestJS (modular structure)

// user.controller.ts
@Controller('users')
export class UserController {
  constructor(private readonly userService: UserService) {}

  @Post()
  createUser(@Body() dto: CreateUserDto) {
    return this.userService.create(dto);
  }
}

// user.service.ts
@Injectable()
export class UserService {
  constructor(private readonly prisma: PrismaService) {}

  async create(dto: CreateUserDto) {
    return this.prisma.user.create({ data: dto });
  }
}

// create-user.dto.ts
export class CreateUserDto {
  @IsString()
  name: string;

  @IsEmail()
  email: string;
}

Here, the controller is clean, the service contains the business logic, and the DTO handles validation automatically. This separation of concerns is what makes NestJS shine.

Technical Advantages I Saw in NestJS

Modules as Building Blocks
Each feature (auth, user, posts) lives in its own module. That makes the codebase predictable and scalable.
Dependency Injection (DI)
Instead of manually wiring dependencies, NestJS injects them. For example, UserService automatically gets PrismaService. This makes testing much easier.
DTOs + Validation Pipes
Using class-validator and class-transformer, NestJS enforces type safety and validation at the boundary. No more manual if (!req.body.email) checks.
Guards, Interceptors, Filters
- Guards → handle authentication & authorization
- Interceptors → transform responses, add logging
- Filters → handle exceptions consistently
These make cross-cutting concerns first-class citizens.
Testability
Since each service is isolated, writing unit tests becomes straightforward. You can mock dependencies easily.

Lessons from My Community Backend

In my community-backend repo, these ideas play out:

The Auth Module uses guards and a JWT strategy for route protection.
The User Module keeps controller logic minimal -- all heavy lifting is in services.
Validation is handled by DTOs, not by scattered if statements.
Errors are caught by global exception filters, so the API returns consistent error responses.

The result? A backend that feels structured, testable, and scalable.

Closing Thoughts

I didn’t land that interview, but learning NestJS was the real win. It showed me that backend frameworks can be more than “just wrappers.” They can enforce architecture, modularity, and discipline — things that really matter in large systems.

If you’ve been working with Express.js and struggling with messy, tightly coupled code, NestJS is worth a try. It changed how I write backends — and it might change yours too.

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