wait-on is a cross-platform CLI tool and Node.js library that polls various resource types (files, TCP ports, Unix domain sockets, HTTP/HTTPS endpoints) until they become available (or unavailable in reverse mode). It uses RxJS observables for reactive polling with configurable delay, interval, stability window, timeout, and simultaneous connection limits. The library supports both callback and promise APIs. The codebase demonstrates solid engineering: input validation via Joi, functional utilities from lodash, HTTP requests via axios with TLS/proxy/auth support, and comprehensive test coverage (API and CLI). It is a well-maintained, production-ready utility commonly used in CI/CD pipelines and development workflows to synchronize service startup.