RxJS - Observer and Subscription

A beginner-friendly guide to two core RxJS concepts: Observers and Subscriptions. Observers are sets of callbacks (next, error, complete) that consume values emitted by Observables. Subscriptions control when an Observable is listened to and when to stop, using the subscribe and unsubscribe methods. The guide covers partial observers (NextObserver, ErrorObserver, CompletionObserver), grouping multiple subscriptions with the add method, and running cleanup code on unsubscription by returning a teardown function from the Observable constructor.