MobX is a production-grade, transparent reactive state management library for JavaScript and TypeScript, widely regarded for its minimal boilerplate and automatic dependency tracking. The monorepo contains the core reactivity engine (observable, computed, action, reaction, flow), official React bindings (mobx-react-lite for functional components, mobx-react for class components), an ESLint plugin enforcing best practices, and a codemod tool (mobx-undecorate) to migrate legacy decorator syntax to the modern makeObservable API. The codebase is architected around a small set of well-defined primitives: observable values (via Proxy for arrays, objects, and dedicated classes for maps/sets), derivations (ComputedValue, Reaction) with a deterministic update scheduler, and batching via startBatch/endBatch. React integration leverages hooks like useObserver and a UniversalFinalizationRegistry for reaction cleanup. The library has extensive test coverage (unit, integration, performance benchmarks) and is actively maintained with a clear deprecation path. Notable technical strengths include the lazy evaluation of computed values, the efficient binding algorithm for dependency tracking, and the decoupled annotation system that supports both decorator and makeObservable styles.
RxJava 4 is a reactive programming library for the JVM, implementing the Reactive Streams specification via `java.util.concurrent.Flow` (Publisher/Subscriber). It provides six core reactive types: `Flowable` (backpressured), `Observable` (unbounded), `Single` (single value), `Maybe` (optional value), `Completable` (completion signal), and the new `Streamable` (pull-based streaming with `CompletionStage`). The library offers a comprehensive set of operators (map, filter, flatMap, concat, merge, window, buffer, etc.) for composing asynchronous data flows, along with schedulers for threading control (`Schedulers.computation()`, `Schedulers.io()`, `Schedulers.virtual()` for virtual threads). It includes advanced features like operator fusion (`QueueFuseable`), backpressure strategies, and configurable operator parameters via immutable records. The architecture is modular: each operator is a final class extending the base reactive type with an inner observer/subscriber class managing state. External dependencies are minimal (only JDK standard library), and the library is designed for embedding in applications, frameworks, and Android. Key entry points are the static factory methods on `Flowable`, `Observable`, `Single`, `Maybe`, `Completable`, and `Streamable`. Data flows are constructed by chaining operators that transform and combine reactive streams, with lifecycle managed through `Disposable` and `Subscription` contracts.
@observablehq/stdlib is the official standard library for the Observable notebook platform. It provides a comprehensive set of utilities for building interactive data visualizations and applications directly in the browser. The library covers everything from DOM element creation (HTML/SVG/Markdown/LaTeX rendering), file access (FileAttachments with CSV, JSON, SQLite, Excel, Arrow, Parquet support), interactive inputs and reactive programming via generators (observe, queue, range), promise utilities (delay, tick, when), and database clients (SQLite via sql.js, DuckDB via WebAssembly). It is architecturally a monolithic client-side library that exports a main `Library` class, composed from small, single-purpose modules under `src/`. The primary entry point is `src/index.js` (package module) which re-exports all public symbols. The library is consumed primarily by the Observable runtime, but it can also be used standalone via the `Library` constructor. External dependencies are minimal (d3-array, d3-dsv, d3-require); heavy parts like DuckDB and Plot load dynamically from a CDN at runtime. The codebase demonstrates high craftsmanship in its generator implementation (push-to-pull adaptation) and schema inference system for tabular data. However, some modules are tightly coupled to the Observable operation model (e.g., `table.js`). Overall, it is a well-structured, extensively tested library that serves a specific ecosystem but contains several reusable patterns.
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).