TypeScript (Advanced)

Advanced TypeScript is type-level programming. While basic TypeScript (interfaces, type annotations) protects against runtime errors, advanced TypeScript uses generics, conditional types, mapped types, and utility types to encode business logic at the type level. You can write a generic function that works on any array, conditional types that derive the return type from the input, and utility types that transform interfaces (Omit removes a field). In 2026, teams building large codebases (100k+ LOC) without advanced TypeScript drown in any-casts and type errors. TypeScript at senior level becomes a superpower: enforce API contracts, prevent impossible states, generate type-safe SDKs that protect users from mistakes before they code. A senior TypeScript engineer can prove at the type level that a function is correct (no null dereference possible, no invalid state). This skill separates L2 engineers (functional code) from L3+ (provably correct code). Frameworks like tRPC (end-to-end type safety across client-server) and libraries like Zod (type + runtime validation) are built on advanced TypeScript patterns.