How does NestJS resolve constructor injection at runtime?

Difficulty: 5/10

Dependency Injection, Provider Resolution, Reflection

TypeScript emits constructor parameter types as metadata via emitDecoratorMetadata: true in tsconfig. NestJS reads this using the reflect-metadata package to determine what tokens to inject. It then looks up those tokens in the module's provider registry and resolves the dependency graph recursively before instantiating the class.

NestJS leverages TypeScript's metadata reflection system to automatically detect constructor dependencies. When a class is decorated with @Injectable(), TypeScript emits type metadata that NestJS reads at runtime.

Required tsconfig settings

How the resolution works:

1
TypeScript emits design:paramtypes metadata for decorated classes.
2
NestJS reads this metadata using Reflect.getMetadata().
3
Each type is used as a DI token to look up a registered provider.
4
The container resolves the full dependency graph before instantiating any class.

Scenario Questions

0-2 years experience

1You need to inject a service into a controller in NestJS. Walk me through what happens when the app starts and how NestJS knows which instance to provide.
2If you forget to add a provider to the module's providers array, what runtime error will you see when trying to inject it via the constructor?
3How would you inject a plain config object using a value provider and constructor injection?

Follow-up Questions

What role does the Reflect metadata play in this process?
How does Nest handle circular dependencies during injection?
What changes when a provider is marked as request‑scoped?