Question 1

What counts as a provider in Nest?

Accepted Answer

Injectable classes (services/repos), custom provider objects (useClass/useValue/useFactory/useExisting), and more. Anything that can be injected. Register in module providers (or export from imported modules).

Question 2

useFactory vs useClass vs useValue — when?

Accepted Answer

useValue for constants/mocks; useClass for swappable implementations; useFactory for async/config-dependent construction. Factories enable ConfigService-driven setup. Prefer explicit tokens for interfaces.

Question 3

What is a custom injection token and why use string/symbol tokens?

Accepted Answer

Tokens identify providers when you inject interfaces or multiple alike dependencies. TypeScript interfaces disappear at runtime — tokens preserve DI identity. Use symbols/consts to avoid collisions.

Question 4

Transient vs request vs singleton scope — trade-offs?

Accepted Answer

Default singleton is shared; request scope per HTTP request; transient new per injection — scopes affect memory and graph. Request-scoped providers force dependents request-scoped (bubble). Use sparingly for tenancy needs; prefer ALS or explicit params when possible.

Question 5

Debugging: Nest says provider not found at runtime.

Accepted Answer

Module didn't register/export/import correctly, or wrong token, or circular dependency needing forwardRef. Read the exact token in the error. Check testing module overrides. Circular DI often needs redesign, not only forwardRef.

Question 6

Lead: provider design for ports-and-adapters in Nest?

Accepted Answer

Define port interfaces + tokens; adapters as providers; domain services depend on ports only. Swap infra in tests with useValue mocks. Keep ORM details in adapters. Enforce in code review.

Providers NestJS Interview Questions

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