What is an observer?

Observer is an interface for a consumer of push-based notifications delivered by an Observable. It has below structure,

interface Observer<T> {
closed?: boolean;
next: (value: T) => void;
error: (err: any) => void;
complete: () => void;
}

A handler that implements the Observer interface for receiving observable notifications will be passed as a parameter for observable as below,

myObservable.subscribe(myObserver);

Note: If you don't supply a handler for a notification type, the observer ignores notifications of that type.