What does @Optional() decorator do in NestJS?

Q: What does @Optional() decorator do in NestJS?

@Injectable() export class ReportService { constructor( private readonly usersService: UsersService, @Optional() private readonly loggerService?: LoggerService, ) {} generateReport() { // LoggerService may be undefined if not provided this.loggerService?.log('Generating report...'); return this.usersService.findAll(); } } Building reusable library modules where consumers may not configure all providers. Feature flags — optionally inject a service only when a feature module is loaded. Fallback behavior — use a default implementation when the provider is absent. Always use TypeScript optional typing (?) alongside @Optional() for type safety.

Difficulty: 5/10

dependency injection, parameter decorators, module design

The @Optional() decorator marks a constructor dependency as optional. If the provider is not registered in the DI container, NestJS injects undefined instead of throwing an error. Useful when building libraries or optional integrations where a provider may or may not be configured by the consumer.

Using @Optional()

When to use @Optional():

1
Building reusable library modules where consumers may not configure all providers.
2
Feature flags — optionally inject a service only when a feature module is loaded.
3
Fallback behavior — use a default implementation when the provider is absent.
4
Always use TypeScript optional typing (?) alongside @Optional() for type safety.

Scenario Questions

0-2 years experience

Follow-up Questions