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RxJS (Reactive Extensions for JavaScript) is a library for reactive programming using Observables, which makes it easier to compose asynchronous or event-based programs by using functional programming concepts. RxJS provides a way to represent asynchronous data streams as Observables, which can be transformed, combined, and consumed in a declarative way using operators. RxJS is based on the ReactiveX API, which was originally developed for .NET, but has since been implemented in several programming languages, including Java, Swift, and JavaScript. In essence, RxJS provides a powerful set of tools for managing asynchronous data streams in JavaScript, enabling developers to create more efficient and flexible applications. It is commonly used in web applications to manage events and network requests, but can also be used in other contexts where reactive programming is beneficial. Here is an example of using RxJS in JavaScript to create an Observable that emits a sequence of numbers and filters out even numbers: 

  const { Observable } = require('rxjs');

  const source$ = new Observable(subscriber => {
  let count = 0;
  const intervalId = setInterval(() => {
    subscriber.next(count);
    count++;
  }, 1000);
  
  return () => clearInterval(intervalId);
  });

 const filtered$ = source$.pipe(
  filter(num => num % 2 !== 0)
  );

 const subscription = filtered$.subscribe(
  num => console.log(num),
  err => console.error(err),
  () => console.log('Complete')
  );

  setTimeout(() => {
  subscription.unsubscribe();
  }, 5000);
In this example, we create an Observable called source$ that emits a sequence of numbers using the setInterval function. We then create a new Observable called filtered$ by applying the filter operator to the source$ Observable, which removes even numbers from the sequence. Finally, we subscribe to the filtered$ Observable and log the emitted values to the console, and after 5 seconds we unsubscribe from the subscription to stop the Observable from emitting any more values.