NestJS Development Rules Skill

NestJS Development Rules

Overview

This skill provides procedural rules for working with NestJS in this project. It covers component generation, testing patterns, module structure, and deployment configuration. For GraphQL-specific patterns (resolvers, types, auth decorators), use the nestjs-graphql skill.

Component Generation

Always use NestJS CLI to create components rather than manually creating files.

Module Generation

bunx nest g module <name> --no-spec

Service Generation

bunx nest g service <name> --no-spec

Resolver Generation (GraphQL)

bunx nest g resolver <name> --no-spec

Controller Generation (REST)

bunx nest g controller <name> --no-spec

Why `--no-spec`

The --no-spec flag is used because this project follows TDD (Test-Driven Development). Tests are written first with a custom test structure before implementation, not auto-generated by the CLI.

Database Migrations

Never Create Migration Files Manually

Never create or modify a TypeORM migration file directly. Instead, use migration:generate from package.json:

bun run migration:generate

Exception: Direct modification is allowed only for changes that don't break syncing with entity files (e.g., adding indexes, comments, or non-structural changes).

Why Auto-Generation

TypeORM compares your entity definitions against the current database schema to generate migrations. Manual creation can:

Introduce drift between entities and migrations
Miss important schema changes
Create migrations that don't match entity metadata

Test-Driven Development Pattern

Write Tests First

Create test files before implementation:

// src/feature/feature.service.test.ts
import { Test, TestingModule } from "@nestjs/testing";
import { FeatureService } from "./feature.service";

describe("FeatureService", () => {
  const service: FeatureService;

  beforeEach(async () => {
    const module: TestingModule = await Test.createTestingModule({
      providers: [FeatureService],
    }).compile();

    service = module.get<FeatureService>(FeatureService);
  });

  describe("methodName", () => {
    it("should do expected behavior", () => {
      expect(service.methodName()).toBe("expected");
    });
  });
});

Test File Naming

Unit tests: *.test.ts
Integration tests: *.integration.test.ts

Running Tests

# Unit tests only
bun run test:unit

# Integration tests only
bun run test:integration

# All tests
bun run test

Module Structure

Feature Module Pattern

src/
├── <feature>/
│   ├── <feature>.module.ts        # Module definition
│   ├── <feature>.service.ts       # Business logic
│   ├── <feature>.service.test.ts  # Service unit tests
│   ├── <feature>.resolver.ts      # GraphQL resolver (if applicable)
│   ├── <feature>.resolver.test.ts # Resolver unit tests
│   ├── <feature>.controller.ts    # REST controller (if applicable)
│   ├── <feature>.controller.test.ts
│   ├── dto/                       # Data transfer objects
│   │   ├── create-<feature>.input.ts
│   │   └── update-<feature>.input.ts
│   └── entities/                  # Entity definitions
│       └── <feature>.entity.ts

Module Registration

import { Module } from "@nestjs/common";
import { FeatureModule } from "./feature/feature.module";

@Module({
  imports: [
    // ... other imports
    FeatureModule,
  ],
})
export class AppModule {}

Lambda Handler Pattern

Entry Point Structure

// src/main.ts
import { NestFactory } from "@nestjs/core";
import { configure as serverlessExpress } from "@vendia/serverless-express";
import { AppModule } from "./app.module";

type ServerlessHandler = ReturnType<typeof serverlessExpress>;

/**
 * Creates a lazy-initialized server getter using closure pattern
 * @description Encapsulates mutable cache state for Lambda warm starts
 * @returns Async function that returns the cached or newly created server
 */
const createServerGetter = (): (() => Promise<ServerlessHandler>) => {
  // eslint-disable-next-line functional/no-let -- Required for Lambda warm start caching
  let cachedServer: ServerlessHandler | null = null;

  return async (): Promise<ServerlessHandler> => {
    if (cachedServer) {
      return cachedServer;
    }

    const nestApp = await NestFactory.create(AppModule, {
      cors: {
        origin: "*",
        methods: "GET,HEAD,PUT,PATCH,POST,DELETE",
        preflightContinue: false,
        optionsSuccessStatus: 204,
      },
    });

    await nestApp.init();
    const app = nestApp.getHttpAdapter().getInstance();
    cachedServer = serverlessExpress({ app });
    return cachedServer;
  };
};

const getServer = createServerGetter();

/**
 * Lambda handler function
 * @param event - AWS Lambda event object
 * @param context - AWS Lambda context object
 * @returns Promise resolving to Lambda response
 */
export const handler = async (
  event: unknown,
  context: unknown
): Promise<unknown> => {
  const server = await getServer();
  return server(event, context);
};

Key Lambda Concepts

Warm Start Caching: Use closure pattern to cache the NestJS application between invocations
Serverless Express: Use @vendia/serverless-express for Express adapter compatibility
CORS Configuration: Configure CORS in NestFactory.create(), not in serverless.yml

Configuration Files

NestJS CLI Configuration

// nest-cli.json
{
  "$schema": "https://json.schemastore.org/nest-cli",
  "collection": "@nestjs/schematics",
  "sourceRoot": "src",
  "compilerOptions": {
    "deleteOutDir": true
  }
}

TypeScript Configuration

Required settings for NestJS decorators:

// tsconfig.json (additions)
{
  "compilerOptions": {
    "emitDecoratorMetadata": true,
    "experimentalDecorators": true,
    "strictPropertyInitialization": false
  }
}

Serverless Framework Configuration

# serverless.yml
service: project-name
frameworkVersion: "^4.0.0"

custom:
  esbuild:
    bundle: true
    minify: false
    sourcemap: true
    keepNames: true
    platform: node
    target: node20
    external:
      - "fsevents"
      - "@nestjs/websockets"
      - "@nestjs/microservices"
      - "@apollo/gateway"
      - "@apollo/subgraph"
      - "@as-integrations/fastify"
      - "class-transformer/storage"

plugins:
  - serverless-esbuild
  - serverless-offline

provider:
  name: aws
  runtime: nodejs22.x
  region: us-east-1
  httpApi:
    cors: true

functions:
  main:
    handler: src/main.handler
    timeout: 29
    memorySize: 1024
    events:
      - httpApi:
          method: any
          path: /{proxy+}

Documentation Standards

JSDoc for All Components

Every exported function, class, and type must have JSDoc documentation:

/**
 * Service for managing user accounts
 * @description Provides CRUD operations for user entities
 * @remarks
 * - All methods are idempotent
 * - Throws NotFoundException for missing resources
 */
@Injectable()
export class UserService {
  /**
   * Retrieves a user by their unique identifier
   * @param id - The unique identifier of the user
   * @returns The user if found, null otherwise
   */
  async findById(id: string): Promise<User | null> {
    return this.repository.findOne({ where: { id } });
  }
}

Module File Preambles

Every file should have a preamble comment:

/**
 * @file user.service.ts
 * @description Service providing user account management
 * @module users
 */

DataLoader Integration

Batch Method Pattern

Services that support DataLoader must implement batch methods:

/**
 * Batch loads entities by IDs (for DataLoader)
 * @param ids - Array of entity IDs to load
 * @returns Promise resolving to array of entities in same order as input
 * @remarks Used by DataLoader for batching - maintains input order
 */
async findByIds(ids: readonly string[]): Promise<Entity[]> {
  const entities = await this.repository.findBy({ id: In([...ids]) });
  const entityMap = new Map(entities.map(e => [e.id, e]));
  return ids.map(id => entityMap.get(id) ?? null);
}

Order Preservation

Batch functions must return results in the same order as input keys. Always map input IDs to results to maintain order.

Common Patterns

Immutable Code

Use const instead of let or var:

// Good
const users = await this.userService.findAll();
const filtered = users.filter(u => u.active);

// Bad
let users = await this.userService.findAll();
users = users.filter(u => u.active);

Functional Transformations

Use reduce instead of push or pop:

// Good
const userMap = users.reduce(
  (acc, user) => ({ ...acc, [user.id]: user }),
  {} as Record<string, User>
);

// Bad
const userMap: Record<string, User> = {};
users.forEach(user => {
  userMap[user.id] = user;
});

Error Handling

Use GraphQL errors with codes:

import { GraphQLError } from "graphql";

throw new GraphQLError("User not found", {
  extensions: { code: "NOT_FOUND", id },
});

Configuration Management

Never Use process.env Directly

Always use NestJS ConfigService instead of accessing process.env directly. This provides:

Type safety with full autocomplete
Centralized configuration management
Easier testing (mock ConfigService instead of environment)
Validation at startup

ConfigService in NestJS Context

For services, resolvers, and controllers running within NestJS:

import { Injectable } from "@nestjs/common";
import { ConfigService } from "@nestjs/config";
import { Configuration } from "../config/configuration";

@Injectable()
export class MyService {
  constructor(
    private readonly configService: ConfigService<Configuration, true>
  ) {}

  someMethod(): void {
    // Type-safe configuration access with autocomplete
    const host = this.configService.get("database.host", { infer: true });
    const isOffline = this.configService.get("app.isOffline", { infer: true });
  }
}

ConfigService in Module Factories

For dynamic module configuration (e.g., TypeOrmModule.forRootAsync):

import { Module } from "@nestjs/common";
import { ConfigService } from "@nestjs/config";
import { TypeOrmModule } from "@nestjs/typeorm";
import { Configuration } from "../config/configuration";

@Module({
  imports: [
    TypeOrmModule.forRootAsync({
      inject: [ConfigService],
      useFactory: (configService: ConfigService<Configuration, true>) =>
        createTypeOrmOptionsFromConfigService(configService),
    }),
  ],
})
export class DatabaseModule {}

Standalone Configuration (Lambda Handlers)

For code running outside NestJS context (Lambda authorizers, WebSocket handlers):

import { getStandaloneConfig } from "../../config/configuration";

// Use getStandaloneConfig() for type-safe access outside NestJS
const config = getStandaloneConfig();
const host = config.valkey.host;
const port = config.valkey.port;

Configuration Schema

All configuration is defined in src/config/configuration.ts:

export interface Configuration {
  readonly app: {
    readonly nodeEnv: string;
    readonly isOffline: boolean;
  };
  readonly database: {
    readonly host: string;
    readonly port: number;
    readonly username: string;
    readonly password: string;
    readonly name: string;
    // ... other database config
  };
  readonly valkey: {
    readonly host: string;
    readonly port: number;
    readonly maxRetriesPerRequest: number;
  };
  // ... other configuration namespaces
}

Adding New Configuration

Add the new property to the Configuration interface
Add default values in the configuration() factory function
Access via configService.get("namespace.property", { infer: true })

Testing with ConfigService

Create mock ConfigService in tests:

const createMockConfigService = (): ConfigService<Configuration, true> => {
  const config = {
    valkey: { host: "localhost", port: 6379, maxRetriesPerRequest: 3 },
  };

  return {
    get: jest.fn((key: string) => {
      const keys = key.split(".");
      return keys.reduce((obj, k) => obj?.[k], config);
    }),
  } as unknown as ConfigService<Configuration, true>;
};

// In test setup
const module = await Test.createTestingModule({
  providers: [
    MyService,
    { provide: ConfigService, useValue: createMockConfigService() },
  ],
}).compile();

Verification Checklist

After creating or modifying NestJS components:

Unit tests pass: bun run test:unit
Integration tests pass: bun run test:integration
Linting passes: bun run lint
Type check passes: bun run build
Local server starts: bun run start:local

Agent Skills: NestJS Development Rules

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