JS Practice — Built-in Objects

Type Guard — narrowUnion

📖 Usage Example

type Result<T> = { ok: true; value: T } | { ok: false; error: string }
function isOk<T>(r: Result<T>): r is { ok: true; value: T } {
  return r.ok === true
}

Discriminating on a boolean literal property is a clean way to narrow Result-style union types.

Type Guards: Narrowing Result Union

Challenge: Implement narrowUnion<T, E extends Error>(result: T | E): [T, null] | [null, E] — splitting a "result or error" union into a typed tuple.

narrowUnion('success')     // → ['success', null]
narrowUnion(new Error())   // → [null, Error]

💡 Hints (1)

• instanceof Error distinguishes error from success value.