Narrowing and type guards

Narrowing refines a broad type through control flow. `typeof`, `instanceof`, `in`, equality checks, custom type predicates, and assertion functions help the compiler understand what is safe. Runtime validation is still required at trust boundaries.

					function isUser(value: unknown): value is User {
  return typeof value === 'object' && value !== null &&
    'id' in value && typeof (value as User).id === 'string';
}
				

Start external data as `unknown`, not `any`. A dishonest type predicate can lie to the compiler. Checking only `typeof value === 'object'` is not enough for nested API payloads.

On interviews: explain with a concrete example and name what interviewers probe in production code.

Common pitfalls: mixing similar APIs and forgetting edge cases during live coding.

The trade-off is often clarity versus safety or expressiveness versus maintainability. Checklist:

• Start external data as unknown.
• Keep predicates honest.
• Validate nested shapes.