type-safety

Prevent TypeScript runtime type errors by validating external JSON at I/O boundaries using runtime schemas before trusting internal types. Ensure compile-time soundness by replacing unchecked casts with narrowing and discriminated unions to avoid unsafe assumptions.

Soundness in gradual type systems means compile-time types guarantee correctness, but leaks occur through escape hatches like `any`, assertions, and unchecked assumptions. Enforce soundness by preferring `unknown` over `any` and applying runtime boundary validation.

The best way to handle missing cases in a TypeScript union is exhaustiveness verification, which enforces complete handling of unions so missing cases fail at compile time. Combine this with narrowing and discriminated unions rather than unchecked casts.

TypeScript assumes network responses match interfaces because it provides compile-time checks without runtime guarantees. Apply runtime boundary discipline by validating external data with runtime validators before trusting internal types, ensuring actual runtime correctness.

Use `unknown` instead of `any` in TypeScript when receiving untrusted external data to enforce safer narrowing. `unknown` requires explicit type checking before use, preserving compile-time soundness and preventing unsafe assumptions that lead to runtime defects.