Key Index is a tiny TypeScript utility that creates on‑demand, memoized key‑value stores backed by either `Map` objects or plain JavaScript objects. It lets you define an initializer (or “initer”) that automatically generates a value the first time a key is accessed, supports nested indexes, and includes a `memoize` helper for safely caching the result of expensive or cyclic functions with optional post‑initialization hooks. The library is aimed at developers who need lightweight, lazy‑loaded lookup tables or want to guard against circular dependencies in configuration‑or service‑creation code.