rxjs

RxJS (Reactive Extensions for JavaScript) is a library for composing asynchronous and event-based programs using observable sequences. At its core, RxJS provides the Observable primitive—a lazy, push-based data structure that can emit multiple values over time. Unlike Promises which resolve once, Observables can represent ongoing streams like user clicks, WebSocket messages, or interval timers.

The library exists to solve the complexity of managing asynchronous data flows in modern applications. Traditional callback pyramids and Promise chains become unwieldy when dealing with multiple concurrent events, cancellation, retry logic, or time-based coordination. RxJS offers a declarative alternative through operators—pure functions like map, filter, debounceTime, and switchMap that transform streams without imperative state management.

With 71+ million weekly downloads, RxJS is ubiquitous in the JavaScript ecosystem. Angular ships with RxJS as its reactive foundation for HTTP, forms, and router events. React developers use it for complex state management and side effects. It's equally valuable in Node.js for streaming data processing, event coordination across microservices, or rate-limiting API calls. The learning curve is real—thinking in streams requires a mental model shift from imperative to reactive programming—but the payoff is maintainable code for scenarios where events, time, and asynchrony intersect.

Real-time search with debouncing: Prevent excessive API calls by debouncing user input in a search box. RxJS operators like debounceTime and switchMap cancel previous requests automatically when new keystrokes arrive, ensuring only the latest query executes.

Coordinating multiple HTTP requests: When displaying a dashboard requiring data from several endpoints, use forkJoin or combineLatest to wait for all responses or react to each as it arrives. Operators handle errors, retries, and cancellation without nested callbacks.

Drag-and-drop interactions: Model mousedown, mousemove, and mouseup as observable streams. Use takeUntil to stop listening to move events after mouseup fires, and map operators to calculate element positions—all without manual event listener cleanup.

WebSocket message processing: Transform incoming WebSocket events into typed message streams, filter by message type, apply throttling to prevent UI overload, and automatically reconnect on disconnection using retry operators.

Polling with exponential backoff: Create an interval observable that fetches server status every N seconds. Use retryWhen with delay operators to implement exponential backoff when requests fail, stopping after a threshold without complex timer management.