Questions
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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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What is the difference between useExisting and useClass when aliasing a provider in NestJS?

useClass with a class always creates a new independent instance managed by the container. useExisting resolves and returns the same singleton instance of the referenced token — no new instantiation occurs. Use useExisting when you need backward-compatible aliases without duplicating state.

useClass vs useExisting — instance behavior
When to use each:
  1. 1

    useClass — when you want an independent instance with its own state.

  2. 2

    useExisting — when you want two token names to point to the exact same runtime object.

  3. 3

    useExisting is useful for maintaining backward-compatible tokens in evolving APIs.

  4. 4

    useExisting requires the referenced token to already be registered in the same module or an imported module.