exhaustiveness-checking

Use never to ensure all cases in a union are handled.

When you add a new variant to a union type, TypeScript can automatically flag every switch statement that needs updating. This catches errors of omission at compile time.

Without exhaustiveness checking, new union variants are silently ignored:

在處理標記聯合時使用。將新案例新增至受歧視聯合時使用。使用when switch 語句必須涵蓋所有情況。 來源：marius-townhouse/effective-typescript-skills。