Observer

What is an Observer? An Observer is a consumer of values delivered by an Observable. Observers are simply a set of callbacks, one for each type of notification delivered by the Observable: next, error, and complete. The following is an example of a typical Observer object:

const observer = {
  next: x => console.log('Observer got a next value: ' + x),
  error: err => console.error('Observer got an error: ' + err),
  complete: () => console.log('Observer got a complete notification'),
};

To use the Observer, provide it to the subscribe of an Observable:

observable.subscribe(observer);

Observers are just objects with three callbacks, one for each type of notification that an Observable may deliver.

Observers in RxJS may also be partial. If you don't provide one of the callbacks, the execution of the Observable will still happen normally, except some types of notifications will be ignored, because they don't have a corresponding callback in the Observer.

The example below is an Observer without the complete callback:

const observer = {
  next: x => console.log('Observer got a next value: ' + x),
  error: err => console.error('Observer got an error: ' + err),
};

When subscribing to an Observable, you may also just provide the next callback as an argument, without being attached to an Observer object, for instance like this:

observable.subscribe(x => console.log('Observer got a next value: ' + x));

Internally in observable.subscribe, it will create an Observer object using the callback argument as the next handler.