When do you need @Inject(TOKEN) vs plain constructor injection in NestJS?

Q: When do you need @Inject(TOKEN) vs plain constructor injection in NestJS?

// Plain injection — works because UsersService is a class constructor(private readonly usersService: UsersService) {} // @Inject() required — token is a string constructor(@Inject('CONFIG') private readonly config: AppConfig) {} // @Inject() required — token is a symbol export const CACHE_MANAGER = Symbol('CACHE_MANAGER'); constructor(@Inject(CACHE_MANAGER) private cache: Cache) {} // Best practice — use InjectionToken for type safety export const DB_CONFIG = new InjectionToken ('DB_CONFIG'); constructor(@Inject(DB_CONFIG) private dbConfig: DbConfig) {} Class tokens — use plain constructor injection. String tokens — always use @Inject('TOKEN_NAME'). Symbol tokens — always use @Inject(SYMBOL). InjectionToken — use @Inject(TOKEN) for type-safe non-class tokens. Prefer InjectionToken over raw strings to avoid typos and get TypeScript type checking.

Difficulty: 5/10

dependency injection, custom providers, NestJS tokens

Plain constructor injection works when the token is a class — TypeScript emits the type automatically. @Inject(TOKEN) is required when the token is a string, symbol, or InjectionToken because those have no TypeScript type that reflect-metadata can read.

Plain injection vs @Inject()

Key rules:

1
Class tokens — use plain constructor injection.
2
String tokens — always use @Inject('TOKEN_NAME').
3
Symbol tokens — always use @Inject(SYMBOL).
4
InjectionToken — use @Inject(TOKEN) for type-safe non-class tokens.
5
Prefer InjectionToken over raw strings to avoid typos and get TypeScript type checking.

Scenario Questions

0-2 years experience

Follow-up Questions