TypeScript Type Safety Expert

Branded types create distinct compile-time identities for semantically different strings. Define a branded type using intersection with a unique symbol, then use it in function signatures to enforce type safety—the compiler rejects mismatched IDs at compile time with zero runtime overhead.

Discriminated unions combine a literal type field with union members, enabling exhaustive pattern matching and precise type narrowing. Use them for state machines, API responses, and event handling to guarantee the compiler catches unhandled cases.

Define strict request and response types using generics and utility types, then combine them with Zod schemas for runtime validation. This approach guarantees compile-time correctness while catching schema mismatches at runtime before they reach production.

Template literal types construct literal string types from unions, enabling type-safe string patterns. Recursive types model nested structures like trees and graphs; together they enable precise type inference for deeply nested, dynamically-shaped data.

TypeScript types provide compile-time guarantees but disappear at runtime; Zod enforces schemas at runtime. Use both together—TypeScript catches errors during development, Zod validates untrusted external data and provides runtime safety without duplicating type logic.

Incrementally migrate files to TypeScript, apply branded types and discriminated unions to data boundaries, use utility types for flexible type manipulation, and layer Zod validation for external inputs. This progression catches previously-hidden type errors and improves maintainability.