Union

Summary

A type formed by combining multiple types with `|`, meaning a value can be one of several possible types (e.g. `string | number`).

• 1Model a function parameter that accepts either a `string` or `number`.
• 2Practice narrowing a union type using `typeof` or `in` checks.
• 3Model a set of related states as a union of object shapes — a 'discriminated union'.