Core Concepts in RxJS - Part 2 - Observer.md

Core Concepts in RxJS - Part 2 - Observer

What is Observable?

• 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.

  var 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 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.

• When subscribing to an Observable, you may also just provide the callbacks as arguments, without being attached to an Observer object: (Note: If you provide the Observer with the form below, not an object format with key-value pair, the order of the callbacks matter, which means the callbacks would be considered as next - error - complete in order.)

  observable.subscribe(
    x => console.log('Observer got a next value: ' + x),
    err => console.error('Observer got an error: ' + err),
    () => console.log('Observer got a complete notification')
  );

References

• Observer in RxJS