What are the different forms of custom provider syntax in NestJS?

Q: What are the different forms of custom provider syntax in NestJS?

// useClass { provide: LoggerService, useClass: ProductionLoggerService } // useValue { provide: 'CONFIG', useValue: { apiUrl: 'https://api.example.com' } } // useFactory (with dependency injection) { provide: 'DB_CONNECTION', useFactory: async (config: ConfigService) => { return await createConnection(config.get('DB_URL')); }, inject: [ConfigService], } // useExisting (alias) { provide: 'AliasLogger', useExisting: LoggerService } useClass — swap implementations (e.g., mock vs real service). useValue — inject constants, config objects, or mock instances. useFactory — when provider creation requires async work or other dependencies. useExisting — create an alias so two tokens point to the same instance.

Difficulty: 6/10

Provider registration, Injection scopes, Dynamic providers

There are four forms: useClass (provide a class), useValue (provide a static value or mock), useFactory (provide the return value of a factory function — supports async and DI via the inject array), and useExisting (alias one token to another already-registered provider).

All four custom provider forms

When to use each:

1
useClass — swap implementations (e.g., mock vs real service).
2
useValue — inject constants, config objects, or mock instances.
3
useFactory — when provider creation requires async work or other dependencies.
4
useExisting — create an alias so two tokens point to the same instance.

Scenario Questions

0-2 years experience

Follow-up Questions