NestJS · Senior · Question 1 of 75

How does Nest know what to inject into `constructor(private users: UsersService)`?

✓Correct answer: TypeScript emits design:paramtypes metadata (with emitDecoratorMetadata), and Nest reads it via reflect-metadata to look up providers by class token
2Nest parses the source file at runtime
3Nest injects by parameter name

Explanation

That's why type-only imports (import type) or interfaces break injection: there is no runtime value to use as a token.

Further reading

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