Questions
5 of 11
1What is a provider in NestJS?
2What are the different forms of custom provider syntax in NestJS?
3How does NestJS handle async providers and what problem do they solve?
4What are provider scopes in NestJS and when would you change the default?
5How do you resolve a circular dependency between two providers in NestJS?
6Walk through all four custom provider types: useClass, useValue, useFactory, useExisting.
7How do you create a multi-provider pattern in NestJS to inject multiple implementations for the same token?
8How does an async provider work in NestJS and what guarantees does it give?
9What is the shorthand provider syntax in NestJS and what does it expand to?
10What is the difference between useExisting and useClass when aliasing a provider in NestJS?
11How do you make a provider accessible across the entire module graph using ModuleRef.get() with strict: false?
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05 / 11

How do you resolve a circular dependency between two providers in NestJS?

Use forwardRef() on both sides of the circular dependency. However, the real fix is to refactor and extract the shared logic into a third provider to eliminate the cycle entirely.

Circular dependencies occur when Provider A depends on Provider B and Provider B depends on Provider A. NestJS cannot resolve these without a hint because neither can be instantiated first.

Using forwardRef() to break the cycle
Best practices:
  1. 1

    forwardRef() is a workaround, not a solution.

  2. 2

    Prefer refactoring — extract the shared logic into a third, independent service.

  3. 3

    Apply forwardRef() on both sides of the dependency.

  4. 4

    Also applies at the module level when two modules import each other.

Difficulty: 6/10
Topics: forwardRef, module design, dependency injection

Scenario Questions

0-2 years experience
  1. 1

    You have ServiceA that injects ServiceB and ServiceB that injects ServiceA. How would you change the code so Nest can start the application?

  2. 2

    If you forget to wrap one of the providers with forwardRef, what error does Nest throw and why?

2-5 years experience
  1. 1

    During a feature rollout you see an error like "Cannot resolve dependencies of the XProvider" caused by a circular import. Walk me through how you would debug and fix it.

  2. 2

    Explain the trade‑offs between using forwardRef versus extracting a common interface to break the circular dependency.

5-8 years experience
  1. 1

    In a large codebase you discover several feature modules that depend on each other, creating a web of circular dependencies. How would you redesign the module layout to eliminate the cycles while keeping the public API stable?

  2. 2

    Discuss any performance or startup‑time implications of using forwardRef extensively in a production NestJS service.

8+ years experience
  1. 1

    Your team is migrating legacy NestJS services into a micro‑service architecture, and many shared libraries introduce circular dependencies across modules. How would you plan a refactor at scale and put safeguards in place to prevent new cycles?

  2. 2

    What governance or tooling would you introduce to detect and manage circular dependencies across multiple teams over time?

Follow-up Questions

  • ❓What are the downsides of relying on forwardRef for many providers?
  • ❓How would you test that the circular dependency is truly resolved?
  • ❓Can you think of a scenario where breaking the cycle is preferable to using forwardRef?