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:

在处理标记联合时使用。将新案例添加到受歧视联合时使用。使用when switch 语句必须涵盖所有情况。 来源：marius-townhouse/effective-typescript-skills。