`codependency` is a tiny utility that lets Node.js middleware declare **optional peer dependencies** in a custom `optionalPeerDependencies` field of their `package.json`. It provides a `requirePeer` function that validates semver, gives clear install‑or‑upgrade messages, and can optionally return `undefined` instead of throwing—making it ideal for middleware authors who want to support many optional back‑ends (databases, caches, etc.) without forcing users to install every driver.