Advanced Providers & Factories

Take your provider patterns to the next level with advanced techniques for factories, async providers, and conditional registration.

See also: Providers & Services

Async Factory Providers

You can use useFactory with async functions to provide values that require asynchronous setup (e.g., fetching config from a remote source):

container.set('CONFIG', {
  useFactory: async () => {
    const config = await fetchConfigFromRemote();
    return config;
  },
});

Async Factory Provider vs DynamicModule

When to use each:

• Use an async factory provider for simple, one-off async values or services.
• Use a DynamicModule when you need to configure a group of related providers/services, especially if you want module encapsulation, validation, or imports/exports.

Use Case	Async Factory Provider	DynamicModule
Single async value/service	✅	🚫
Multiple related providers	🚫	✅
Module-level config/validation	🚫	✅
Imports/exports between modules	🚫	✅
Simple, one-off async setup	✅	🚫

Rule of Thumb:

• Use an async factory provider for simple, one-off async values or services.
• Use a DynamicModule for configuring groups of related providers/services.

Conditional Providers

Register providers only if certain conditions are met (e.g., environment, feature flag):

if (process.env.NODE_ENV === 'production') {
  container.set('LOGGER', { useClass: ProdLogger });
} else {
  container.set('LOGGER', { useClass: DevLogger });
}

Environment-based Configuration

Tip: For most projects, just use environment variables and useValue for configuration. Use a factory only if you need to fetch or compute config at runtime.

Simple Pattern (Recommended)

container.set('DB_CONFIG', {
  useValue: {
    host: process.env.DB_HOST,
    port: Number(process.env.DB_PORT),
    database: process.env.DB_NAME,
  },
});

Advanced: Remote or Computed Config

Use this only if you need to fetch config from a remote service or merge multiple sources.

container.set('DB_CONFIG', {
  useFactory: async () => {
    if (process.env.NODE_ENV === 'production') {
      return await fetchProdDbConfig();
    } else {
      return { host: 'localhost', port: 5432 };
    }
  },
});

Real-World Example: Feature Toggle

const FEATURE_FLAG = process.env.FEATURE_X_ENABLED === 'true';
container.set('FeatureService', {
  useClass: FEATURE_FLAG ? FeatureXService : NoopService,
});

Best Practices

• Use async factories for config/services that require I/O.
• Use conditional registration for environment-specific or feature-flagged services.
• Prefer explicit tokens for clarity.

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Next: Multi-injection & Collections