TypeScript · Narrowing and unions

Exhaustive switch

A never assignment in the default branch makes the compiler flag any unhandled union member.

Syntax

const _never: never = value

Example

type Status = "draft" | "published";
function label(status: Status) {
  switch (status) {
    case "draft": return "Draft";
    case "published": return "Published";
    default: { const _never: never = status; return _never; }
  }
}

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Exhaustive switch

Example

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