Runtime type guards, narrowing, and exhaustiveness checking for TypeScript — typeof/instanceof/in guards, custom and generic type predicates (value is T), discriminated union narrowing, assertion functions (asserts value is T), branded/nominal types validated at runtime, and exhaustive switch statements with a never guard. Use when replacing an unsafe `as` assertion with a real runtime check, fixing "Object is possibly undefined", modeling impossible states out of existence, writing an `is*` predicate, or making a switch over a union fail to compile when a case is missing.