How do you write a dynamic module that supports both sync (register) and async (registerAsync) configuration sharing the same internal providers?

Q: How do you write a dynamic module that supports both sync (register) and async (registerAsync) configuration sharing the same internal providers?

@Module({}) export class ThrottlerModule { static register(options: ThrottlerOptions): DynamicModule { return { module: ThrottlerModule, providers: [ { provide: THROTTLER_OPTIONS, useValue: options }, ...ThrottlerModule.createCoreProviders(), ], exports: [ThrottlerService], }; } static registerAsync(options: ThrottlerAsyncOptions): DynamicModule { return { module: ThrottlerModule, imports: options.imports || [], providers: [ { provide: THROTTLER_OPTIONS, useFactory: options.useFactory, inject: options.inject || [], }, ...ThrottlerModule.createCoreProviders(), ], exports: [ThrottlerService], }; } private static createCoreProviders(): Provider[] { return [ThrottlerService, ThrottlerGuard]; } } Single source of truth — adding or removing a core provider only requires changing the helper. Both sync and async paths stay automatically synchronized. Reduces code duplication and the risk of the two paths drifting apart over time. This is the pattern used inside NestJS framework modules like @nestjs/throttler.

Difficulty: 6/10

Dynamic modules, Async providers, Configuration sharing

Extract the core provider creation into a private static helper method so both sync and async configuration paths produce the same set of providers. This prevents duplication and keeps both paths in sync when core providers change.

Shared private helper for sync and async config paths

Benefits of the private helper pattern:

1
Single source of truth — adding or removing a core provider only requires changing the helper.
2
Both sync and async paths stay automatically synchronized.
3
Reduces code duplication and the risk of the two paths drifting apart over time.
4
This is the pattern used inside NestJS framework modules like @nestjs/throttler.

Scenario Questions

0-2 years experience

1Suppose you need to expose a configuration value to other modules, and you have both a register and a registerAsync method. How would you structure the module so that the same provider token is used in both cases?
2If you call MyModule.register({ apiKey: 'abc' }) and later inject the config service in another module, what will Nest resolve and why?
3What happens if you forget to export the internal provider from the dynamic module when using registerAsync?

Follow-up Questions

Can you sketch a minimal code snippet of the module’s registerAsync implementation?
What would you adjust if the configuration needed to be refreshed at runtime?
How does Nest treat provider scope when the same token is used in both sync and async registrations?