Exercise: refactor JS → TS + tests

Goal

Take a small JavaScript utility and make it safe and maintainable with TypeScript, backed by tests.

Task

Create a utility (or take an existing one you wrote) such as:

• parsing a user input payload (unknown → validated/narrowed type)
• mapping API data into a UI-friendly model

Then:

• add types
• remove any
• add Vitest tests

Requirements

• Use unknown at the boundary (input).
• Narrow/validate to a safe type before using it.
• Tests cover happy path + at least 2 edge cases.

Acceptance criteria (entry)

• No any in the implementation.
• Tests pass and meaningfully assert behavior.

Stretch goals (mid)

• Add a generic helper function and justify why it’s worth it.
• Add a test data factory with typed defaults.

Stretch goals (senior)

• Explain where you’d enforce this boundary in a Next.js + tRPC feature.