Observer

• An Observer is a consumer of values delivered by an Observable.
• An Observer’s job is to perform a certain action(s) whenever a value comes from the data stream.
• New data may not always be a value it may be an error or a signal that observable is complete(done emitting values).
• Hence, we need to connect the observer and the observable. This can be done using Subscription.
• With subscribe() method we tell the observable that someone is listening out for these values.
• Observers can implement three methods:
  • next(): called whenever a new value is emitted
  • error(): called whenever an observable throws an error
  • complete(): called whenever an observable is done
• Some observables may never finish (click listener).

RxJS doesn’t deliver notification after error() or complete() is called. This means emitting next value after error() or complete() is not allowed.