Question 1

What are decorators in TypeScript?

Accepted Answer

Functions that wrap/annotate classes and members for metaprogramming. Used heavily by Angular/Nest. Syntax evolved (legacy experimental vs Stage 3).

Question 2

legacy experimentalDecorators vs new decorators?

Accepted Answer

Old TS emit differs from TC39 decorators; frameworks may depend on legacy. Know which your toolchain expects. Don't mix casually.

Question 3

Common decorator use cases?

Accepted Answer

Dependency injection tokens, route metadata, validation, logging, ORM columns. Prefer explicit config when magic hurts readability/testability.

Question 4

How do class decorators run conceptually?

Accepted Answer

Receive the class (or replace it) at definition time — not per instance. Order matters with multiple decorators. Understand compose bottom-up/top-down rules.

Question 5

Decorator metadata reflection caveats?

Accepted Answer

reflect-metadata + emitDecoratorMetadata can expose design:type but is limited/imperfect. Don't treat metadata as fully sound typing. Prefer explicit schemas.

Question 6

Testing code that relies on decorators?

Accepted Answer

May need framework testing utilities or to test behavior not decorator wiring. Prefer injecting deps over reading decorator metadata in unit tests.

Question 7

When should a team avoid decorators?

Accepted Answer

If they obscure control flow, hurt onboarding, or lock you to compiler flags. Use for established DI frameworks; avoid novel DIY decorator DSLs without need.

Decorators TypeScript Interview Questions

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