import { Observable } from 'rxjs'; const myObservable$ = Observable.create((observer) => { observer.next('Hello, world!'); observer.complete(); return { unsubscribe: () => { console.log('Observable unsubscribed.'); }, }; }); myObservable$.subscribe((value) => console.log(value)); // Output: "Hello, world!"In this example, the Observable creates an Observer using the provided function. The Observer emits a single value using the next method, and then completes using the complete method. When the Observable is subscribed to, the Observer's next and complete methods are called, and the value is logged to the console. The RxJS package library provides a collection of functions and classes for working with reactive programming and data streams in JavaScript.