Validation & Shutdown

Preview — 3 of 10 questions

Using a custom validate function with class-transformer and class-validator, why is plainToInstance(EnvVars, config, { enableImplicitConversion: true }) important?

javascript

import { plainToInstance } from 'class-transformer';
import { IsInt, IsBoolean, validateSync } from 'class-validator';

class EnvVars {
  @IsInt() PORT!: number;
  @IsBoolean() FEATURE_X!: boolean;
}

export function validate(config: Record<string, unknown>): EnvVars {
  const v = plainToInstance(EnvVars, config, { enableImplicitConversion: true });
  const errors = validateSync(v, { skipMissingProperties: false });
  if (errors.length) throw new Error(errors.toString());
  return v;
}

AIt encrypts the env values before validation

BEnv vars are always strings, so implicit conversion lets decorators like @IsInt()/@IsBoolean() coerce "3000"/"true" to their real types before validation

CIt disables validation for missing properties

DIt makes validation asynchronous

What is the main benefit of enabling webpack Hot Module Replacement (HMR) in NestJS during development?

javascript

// main.ts (HMR setup)
declare const module: any;

async function bootstrap(): Promise<void> {
  const app = await NestFactory.create(AppModule);
  await app.listen(3000);

  if (module.hot) {
    module.hot.accept();
    module.hot.dispose(() => app.close());
  }
}
void bootstrap();

AIt enables zero-downtime deploys in production

BIt reloads only changed modules without a full process restart, dramatically reducing rebuild/restart time for large apps

CIt automatically writes migrations when entities change

DIt replaces the DI container with a faster runtime

With expandVariables: true, what does .env containing BASE_URL=http://localhost\nAPI_URL=${BASE_URL}/api produce for API_URL?

javascript

ConfigModule.forRoot({ expandVariables: true });
// .env:
// BASE_URL=http://localhost
// API_URL=${BASE_URL}/api  -> resolves to http://localhost/api

A${BASE_URL}/api (literal, unexpanded)

Bundefined, because expansion only works for process.env

Chttp://localhost/api (the reference is expanded)

DAn error, because variable references aren't allowed in .env

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