NestJS

Dependency Injection

• Provider not available — must be in providers array AND exports if used by other modules
• Circular dependency crashes — use forwardRef(() => Module) in both modules
• Default scope is singleton — same instance across requests, careful with state
• Request-scoped provider — @Injectable({ scope: Scope.REQUEST }), propagates to dependents

Module Organization

• Import module, not provider directly — imports: [UserModule] not providers: [UserService]
• exports makes providers available to importers — without it, provider stays private
• Global modules need @Global() decorator — only for truly shared (config, logger)
• forRoot() vs forRootAsync() — async for when config depends on other providers

Validation

• ValidationPipe needs class-validator decorators — plain classes won't validate
• Enable transform: true for auto-transformation — string "1" to number 1
• whitelist: true strips unknown properties — forbidNonWhitelisted: true to error instead
• Nested objects need @ValidateNested() AND @Type(() => NestedDto) — both required