wait-on is a cross-platform CLI tool and Node.js library that polls resources (files, TCP ports, HTTP(S) endpoints, Unix sockets, and shell commands) until they become available, then exits or resolves. Built on RxJS for reactive polling with configurable delay, interval, timeout, and stability window. Supports both direct and reverse mode (wait for resources to go away). The architecture centers on `lib/wait-on.js`, a single module that validates options via Joi schemas, dispatches per-resource polling observables via a prefix-switch factory, and merges a timeout error observable into the combineLatest stream. CLI argument parsing uses Node's `util.parseArgs` with careful backward-compatibility handling for unknown flags. The codebase demonstrates strong engineering: comprehensive test suite with real servers/temp files, exported pure parser functions, and a clean extensibility pattern for new resource types. However, the core logic is tightly coupled in one file, limiting component reusability. The project is under active development with multiple feature lanes (planned major 10.0.0 replacing axios with undici, dropping lodash, etc.) and a mature CI/release pipeline using semantic-release, commitlint, and GitHub Actions.