TypeScript's strict null checks are not enough to prevent runtime null errors

A developer working with a Next.js Server Action encountered a runtime crash despite clean TypeScript compilation and no lint errors, highlighting a critical gap in static type checking. The article argues that enabling strict null checks in TypeScript is a necessary but insufficient safeguard, as the compiler only validates static types and cannot verify data arriving from external sources like APIs, databases, or HTTP headers. Four specific patterns are identified where TypeScript reports no errors yet production code fails, using a Next.js 16, Prisma ORM 5, and strict TypeScript stack as reference. One key example involves assertion functions where a missing throw causes TypeScript to trust a false type contract, leading to silent runtime failures. The author concludes that real null safety requires runtime validation at system boundaries, not just compiler-level checks.