nestjs

NestJS

NestJS is a progressive Node.js framework for building scalable enterprise server-side applications using TypeScript, dependency injection, and modular architecture.

When to Use

• Enterprise Node.js Backend Microservices: Architecture inspired by Angular with modules, providers, and dependency injection.
• Domain-Driven Modular Applications: Structuring large codebases with clear domain separation and clean layering.
• Multi-Transport Microservices: Seamlessly switching between HTTP, gRPC, RabbitMQ, Kafka, and Redis transports.
• Type-Safe REST & GraphQL Backends: Combining TypeORM/Prisma, class-validator, and auto-generated Swagger documentation.

Quick Start

// cats.controller.ts
@Controller("cats")
export class CatsController {
  constructor(private catsService: CatsService) {}

@Get() async findAll(): Promise<Cat[]> { return this.catsService.findAll(); } }

## Core Concepts

### Modular Architecture & Dependency Injection

Structuring modules, controllers, and injectable services:

```typescript
// users.service.ts
import { Injectable, NotFoundException } from "@nestjs/common";

export interface User {
  id: string;
  email: string;
}

@Injectable()
export class UsersService {
  private users: User[] = [{ id: "1", email: "admin@example.com" }];

  async findById(id: string): Promise<User> {
    const user = this.users.find((u) => u.id === id);
    if (!user) {
      throw new NotFoundException(`User with ID ${id} not found`);
    }
    return user;
  }
}

// users.controller.ts
import { Controller, Get, Param } from "@nestjs/common";

@Controller("users")
export class UsersController {
  constructor(private readonly usersService: UsersService) {}

  @Get(":id")
  async getUser(@Param("id") id: string) {
    return this.usersService.findById(id);
  }
}

Request Validation with Pipes & Class-Validator

Validating incoming JSON payloads before controller execution:

// create-user.dto.ts
import { IsEmail, IsNotEmpty, MinLength } from "class-validator";

export class CreateUserDto {
  @IsEmail({}, { message: "Invalid email address" })
  email: string;

  @IsNotEmpty()
  @MinLength(8, { message: "Password must be at least 8 characters" })
  password: string;
}

// main.ts
import { ValidationPipe } from "@nestjs/common";
import { NestFactory } from "@nestjs/core";
import { AppModule } from "./app.module";

async function bootstrap() {
  const app = await NestFactory.create(AppModule);
  app.useGlobalPipes(
    new ValidationPipe({
      whitelist: true, // Strip unvalidated properties
      forbidNonWhitelisted: true,
      transform: true,
    }),
  );
  await app.listen(3000);
}
bootstrap();

Execution Guards & Role-Based Access Control (RBAC)

Protecting endpoints with custom authentication guards:

import {
  Injectable,
  CanActivate,
  ExecutionContext,
  UnauthorizedException,
} from "@nestjs/common";
import { Request } from "express";

@Injectable()
export class AuthGuard implements CanActivate {
  canActivate(context: ExecutionContext): boolean {
    const request = context.switchToHttp().getRequest<Request>();
    const authHeader = request.headers.authorization;

    if (!authHeader || !authHeader.startsWith("Bearer ")) {
      throw new UnauthorizedException("Valid authorization token required");
    }

    // Attach verified user payload
    request["user"] = { id: "usr_101", role: "admin" };
    return true;
  }
}

Common Patterns

Role-Based Access Control (RBAC) Guard with Reflector

Problem: Duplicating role checks across individual controller route handlers.

Solution: Use custom SetMetadata decorator and global RolesGuard:

import { Injectable, CanActivate, ExecutionContext } from "@nestjs/common";
import { Reflector } from "@nestjs/core";

@Injectable()
export class RolesGuard implements CanActivate {
  constructor(private reflector: Reflector) {}

  canActivate(context: ExecutionContext): boolean {
    const requiredRoles = this.reflector.get<string[]>(
      "roles",
      context.getHandler(),
    );
    if (!requiredRoles) return true;
    const request = context.switchToHttp().getRequest();
    const user = request.user;
    return requiredRoles.some((role) => user?.roles?.includes(role));
  }
}

Best Practices

Do:

• Enable whitelist: true and forbidNonWhitelisted: true in global ValidationPipe to prevent mass-assignment attacks.
• Split applications into discrete domain modules (UserModule, OrderModule, AuthModule) with explicit exports.
• Use Fastify adapter (@nestjs/platform-fastify) for performance-sensitive microservices.
• Handle configuration with @nestjs/config and validate environment variables with Joi or Zod.

Don't:

• Put business logic inside controllers; keep controllers strictly focused on request handling.
• Use circular dependencies between modules; use forwardRef() only as an absolute last resort.
• Catch exceptions silently without rethrowing or logging through Nest's Logger service.

Troubleshooting

Error	Cause	Solution
Nest can't resolve dependencies of the ...	Required service or repository not listed in module providers or imports.	Add the missing service to module providers or export it from its host module.
Cannot find module '...' or its corresponding type declarations	Path alias in tsconfig.json not recognized by Nest CLI runner.	Verify baseUrl and paths mapping in both tsconfig.json and nest-cli.json.
Circular dependency detected in ...	Two services directly injecting each other in constructor.	Use forwardRef(() => OtherService) in both @Inject() annotations.

References

• NestJS Documentation