Type Guards

Summary

Runtime checks that let TypeScript narrow a broad type to a specific one — essential for safely handling untyped input like request bodies or third-party API responses.

• 1Write a type guard that validates an incoming request body matches your expected shape.
• 2Use the `in` operator to narrow a union type based on which property exists.
• 3Pair type guards with a validation library (Zod, Joi) for runtime + compile-time safety.