Build a Tiny NestJS Todo App: The Minimal MVC Approach

export class RegisterItemDto {
  sku: string;
  name: string;
  count: number;
}

import { Injectable } from '@nestjs/common';
import { RegisterItemDto } from './dto/register-item.dto';

export interface StockRecord {
  sku: string;
  name: string;
  count: number;
}

@Injectable()
export class InventoryService {
  private ledger: StockRecord[] = [];

  getLedger(): StockRecord[] {
    return this.ledger;
  }

  register(payload: RegisterItemDto): StockRecord {
    const record: StockRecord = {
      sku: payload.sku,
      name: payload.name,
      count: payload.count,
    };
    this.ledger.push(record);
    return record;
  }

  remove(sku: string): boolean {
    const initialSize = this.ledger.length;
    this.ledger = this.ledger.filter(item => item.sku !== sku);
    return this.ledger.length < initialSize;
  }
}

import { Controller, Get, Post, Delete, Body, Param, HttpCode, HttpStatus } from '@nestjs/common';
import { InventoryService } from './inventory.service';
import { RegisterItemDto } from './dto/register-item.dto';

@Controller('inventory')
export class InventoryController {
  constructor(private readonly registry: InventoryService) {}

  @Get()
  listAll()

: StockRecord[] { return this.registry.getLedger(); }

@Post() add(@Body() payload: RegisterItemDto): StockRecord { return this.registry.register(payload); }

@Delete(':sku') @HttpCode(HttpStatus.OK) purge(@Param('sku') sku: string): { result: string } { const success = this.registry.remove(sku); if (!success) { return { result: 'not_found' }; } return { result: 'removed' }; } }

**Rationale:**
*   **Constructor Injection:** `private readonly registry: InventoryService` injects the service. NestJS resolves this dependency automatically based on the type.
*   **Decorators:** `@Controller('inventory')` sets the base path. `@Get()`, `@Post()`, and `@Delete()` map to HTTP verbs.
*   **Parameter Extraction:** `@Body()` deserializes the JSON payload into `RegisterItemDto`. `@Param('sku')` extracts the URL parameter.
*   **Status Codes:** `@HttpCode(HttpStatus.OK)` explicitly sets the response status, overriding default behaviors for clarity.

#### 4. Wire the Module

Modules define the scope of providers and controllers. They tell NestJS which components belong together and manage the DI container's visibility.

**File:** `src/inventory/inventory.module.ts`

```typescript
import { Module } from '@nestjs/common';
import { InventoryController } from './inventory.controller';
import { InventoryService } from './inventory.service';

@Module({
  controllers: [InventoryController],
  providers: [InventoryService],
})
export class InventoryModule {}

Rationale: Even for a small feature, creating a dedicated module establishes a scalable pattern. In larger applications, this module would be imported by AppModule. This structure supports lazy loading and feature isolation.

5. Bootstrap the Application

The entry point initializes the NestJS application factory and starts the HTTP server.

File: src/main.ts

import { NestFactory } from '@nestjs/core';
import { InventoryModule } from './inventory/inventory.module';

async function bootstrap() {
  const app = await NestFactory.create(InventoryModule);
  await app.listen(3000);
  console.log('Inventory API active on port 3000');
}
bootstrap();

Rationale: NestFactory.create compiles the module graph and resolves all dependencies. The application listens on port 3000, ready to accept requests.

Pitfall Guide

1. Controller Logic Leakage

• Explanation: Developers often place business logic, data transformation, or validation directly in the Controller.
• Impact: Controllers become difficult to test and violate the Single Responsibility Principle. Logic cannot be reused by other services or CLI commands.
• Fix: Keep Controllers thin. They should only extract request data, call service methods, and format responses. Move all logic to the Service.

2. Missing @Injectable() Decorator

• Explanation: Forgetting to add @Injectable() to a Service class prevents NestJS from managing it in the DI container.
• Impact: Runtime error: Nest can't resolve dependencies. The framework cannot instantiate the service.
• Fix: Always decorate service classes with @Injectable(). This signals to the compiler that the class participates in DI.

3. Module Registration Omission

• Explanation: Creating a Controller or Service but failing to list it in the controllers or providers array of a @Module.
• Impact: The endpoint returns 404, or the service is undefined. NestJS only instantiates components declared in modules.
• Fix: Verify that every Controller and Service is registered in a module. Use feature modules to group related components.

4. Loose Type Definitions

• Explanation: Using any types in DTOs or Service methods, or relying on implicit types.
• Impact: Loss of compile-time safety. Runtime errors occur when unexpected data shapes arrive. IDE autocomplete fails.
• Fix: Define strict interfaces for domain models and classes for DTOs. Enable strict: true in tsconfig.json.

5. Parameter Parsing Errors

• Explanation: URL parameters arrive as strings. Passing them directly to methods expecting numbers or specific formats causes type mismatches.
• Impact: Logic failures, such as comparing a string ID to a number ID, resulting in missed records.
• Fix: Use parseInt or parseFloat for numeric params. For production, implement a ValidationPipe with transform: true to automatically cast types.

6. Ephemeral State Assumption

• Explanation: Treating in-memory arrays as persistent storage during development.
• Impact: Data loss on server restart. Developers may write code that assumes data survives across requests, leading to bugs when switching to a database.
• Fix: Acknowledge that in-memory storage is volatile. Design services to be swappable. Write tests that seed data explicitly rather than relying on previous request state.

7. Singleton State Contamination

• Explanation: Storing request-specific data in service properties instead of passing it as arguments.
• Impact: Race conditions in concurrent requests. Data from one user leaks to another.
• Fix: Services should be stateless regarding request context. Pass all request data as method arguments. Use request-scoped providers only when absolutely necessary.

Production Bundle

Action Checklist

• Verify DI Wiring: Ensure all services have @Injectable() and are listed in module providers.
• Validate DTOs: Confirm DTOs match the expected API contract and enable validation pipes for production.
• Check Module Scope: Review @Module decorators to ensure controllers and providers are correctly registered.
• Test Error Paths: Verify behavior for missing resources, invalid payloads, and concurrent requests.
• Review Separation: Audit controllers to ensure no business logic exists outside services.
• Configure CORS: Add CORS settings if the API will be consumed by frontend applications.
• Set Global Prefix: Use app.setGlobalPrefix('api/v1') to version and namespace endpoints.

Decision Matrix

Scenario	Recommended Approach	Why	Cost Impact
Prototype / MVP	In-Memory Service	Zero infrastructure cost. Fast iteration. Validates API design immediately.	Low
Production / Multi-User	Database Service	Persistence, concurrency, and data integrity. Required for reliability.	Medium (DB ops)
Simple Data Shape	Interface DTO	Lightweight. No runtime overhead. Sufficient for internal APIs.	Low
External API / Validation	Class DTO	Runtime type checking. Enables class-validator. Better error messages.	Low
Small App	Root Module	Simpler structure. Less boilerplate. Easier for small teams.	Low
Large App	Feature Modules	Encapsulation. Lazy loading. Clear boundaries. Scalable organization.	Medium (Complexity)

Configuration Template

Use this main.ts template to bootstrap a production-ready application with global validation and error handling.

import { NestFactory } from '@nestjs/core';
import { ValidationPipe } from '@nestjs/common';
import { InventoryModule } from './inventory/inventory.module';

async function bootstrap() {
  const app = await NestFactory.create(InventoryModule);

  // Global Validation Pipe
  app.useGlobalPipes(new ValidationPipe({
    whitelist: true,
    forbidNonWhitelisted: true,
    transform: true,
  }));

  // Global Prefix
  app.setGlobalPrefix('api/v1');

  // Enable CORS
  app.enableCors();

  await app.listen(3000);
  console.log('Application is running on: http://localhost:3000');
}
bootstrap();

Quick Start Guide

1. Install CLI: Run npm i -g @nestjs/cli.
2. Create Project: Execute nest new inventory-api and select your package manager.
3. Generate Components:
   • nest g module inventory
   • nest g controller inventory
   • nest g service inventory
4. Implement Code: Copy the DTO, Service, Controller, and Module code from the Core Solution into the generated files.
5. Run: Execute npm run start:dev to start the development server with hot reload.