`annojs/annotate` is a JavaScript library that allows developers to annotate function definitions with metadata such as descriptions, type checkers, and invariants. It's designed for developers who want to document and enforce function contracts, and can be used with tools like `annofuzz` to generate tests or accessed via REPL for runtime inspection. The library supports features like type checking, invariant assertions, and chaining guards, making it a powerful tool for robust JavaScript development.