Decorator Metadata and reflect-metadata for DI Frameworks

TypeScript decorators with 'emitDecoratorMetadata' require 'reflect-metadata' polyfill at runtime. Missing this causes dependency injection frameworks (NestJS, TypeORM) to fail silently or with cryptic errors.

Install and import reflect-metadata once at the application entry point before any decorated class is imported.

// main.ts — MUST be first import
import 'reflect-metadata';
import { NestFactory } from '@nestjs/core';
import { AppModule } from './app.module';

async function bootstrap() {
  const app = await NestFactory.create(AppModule);
  await app.listen(3000);
}
bootstrap();

// tsconfig.json
{
  "compilerOptions": {
    "experimentalDecorators": true,
    "emitDecoratorMetadata": true
  }
}

emitDecoratorMetadata emits type information at runtime using Reflect.metadata(). The Reflect API is not built into any JS engine — reflect-metadata provides the polyfill. Without it, Reflect.metadata is undefined.