How to extend types?

0-2 years experience

1. 1We have a function that takes a parameter of type `User` with fields `id` and `name`. We now need to add an optional `email` field without modifying the original `User` definition. How would you extend the type?
2. 2Given an interface `Config` with properties `url: string` and `timeout: number`, you need to create a new type that includes all `Config` properties plus a `retries?: number`. Show the TypeScript code you would write.

2-5 years experience

1. 1Our codebase uses a third‑party library that declares an interface `RequestOptions`. We need to add a custom `authToken` property for our internal usage without forking the library. How would you extend the type, and what pitfalls should you watch for?
2. 2You refactored a component to accept a generic `Props` type, but later you need to add a new optional prop `theme` that should be available to all components. Explain how you would extend the existing props type and why a simple intersection might cause issues.

5-8 years experience

1. 1We're building a shared UI component library used across several services. Each service wants to augment the base `ButtonProps` with its own styling fields. How would you design the type extensions to allow safe augmentation while preventing breaking changes for existing consumers?
2. 2During a migration from a legacy JavaScript module to TypeScript, you need to add additional fields to a type exported by the legacy module without editing its source. Discuss how you would use declaration merging or module augmentation, and the trade‑offs regarding build time and type safety.

8+ years experience

1. 1Our organization is standardizing on a core domain model defined in a central package. Multiple product teams need to extend these domain types with product‑specific attributes. How would you structure the type extensions and package boundaries to support independent evolution while keeping a single source of truth?
2. 2We plan to deprecate a set of internal types and replace them with a new version across dozens of microservices. Describe a strategy using TypeScript's module augmentation and versioned type namespaces to roll out the change gradually without breaking existing services.