Decorator metadata và reflect-metadata là gì?

Reflect Metadata API cho phép đọc/ghi metadata trên class/method tại runtime.

• TypeScript emit metadata khi emitDecoratorMetadata: true (design:type, design:paramtypes, design:returntype).
• Dùng bởi dependency injection frameworks (Angular, NestJS) để resolve constructor params tự động.

import 'reflect-metadata';

@Injectable()
class UserService {
  constructor(private repo: UserRepository) {}
  // NestJS đọc design:paramtypes → tự inject UserRepository
}

The Reflect Metadata API allows reading/writing metadata on classes/methods at runtime.

• TypeScript emits metadata when emitDecoratorMetadata: true (design:type, design:paramtypes, design:returntype).
• Used by dependency injection frameworks (Angular, NestJS) to automatically resolve constructor parameters.

import 'reflect-metadata';

@Injectable()
class UserService {
  constructor(private repo: UserRepository) {}
  // NestJS reads design:paramtypes → auto-injects UserRepository
}