[RxJS] Observables can complete

The Observer object has the functions next() and error(). In this lesson we will see the other (and last) function available on observers, complete(), and its purpose.

Completion is an important concept, as we will see later on. Imagine if you want to concatenate two observables. You can only do that if the first one ends. Then you know that the second observable takes over after that.

Completion is also important in other ways. For instance, let‘s say that observer is only interested in the last value that observable produced. This last can only be determined if there is a way to know that the observable has finished and won‘t deliver any values anymore.

var bar = Rx.Observable.create(function (observer) {
  try {
    console.log(‘Hello‘);
    observer.next(42);
    observer.next(100);
    observer.next(200);
    setTimeout(function () {
      observer.next(300);
      observer.complete();
    }, 1000);
  } catch (err) {
    observer.error(err);
  }
});

bar.subscribe(
  function nextValueHandler(x) {
    console.log(x);
  },
  function errorHandler(err) {
    console.log(‘Something went wrong: ‘ + err);
  },
  function completeHandler() {
    console.log(‘done‘);
  }
);