RxJS (Reactive Extensions for JavaScript) is a library for composing asynchronous and event-based programs using observable sequences. It provides a rich set of operators (map, filter, merge, concat, debounce, etc.) that can be chained to transform and combine streams of data. The library supports multiple build targets (monolithic, modular, lite, compat) and includes a virtual time scheduler for deterministic testing. It is a direct port of Rx.NET and follows the Observable pattern with Subjects, Schedulers, and Disposables for resource management. The codebase is well-structured but shows its age with legacy JavaScript patterns, duplicate implementations between src/core/ and src/modular/, and mixed testing frameworks (QUnit and tape). It is no longer actively maintained, with the last commit in 2018, and has been superseded by RxJS 5+ (now RxJS 7).