Here are my detailed notes from the fantastic course created by Jurek Wozniak, called RxJS 7 and Observables: Introduction.

In order to understand the concept please consider buying the course yourself, these notes are simply subsidiaries

Observables - How Do They Work?

Array vs Stream

Observables are based around the idea of streams, that means the data can come in different parts of time and number of data can be theoretically infinite

The biggest different between arrays is, in arrays all the values inside are already known.

With the stream approach, we react to the things when they show up

For example we first see a lemon and we react to it or not,

then coconut etc.

We can for example have a stream for mouse positions, text input data change, http requests

Observable, Subscription, Observer, Key Elements

Observable is quite simple once an observable is executed it emits some notifications.

There are 3 types of notifications

1. Next
2. Error
3. Complete

For now we fill focus to the next notification which allows us to emit notifications.

Warm-Up Observable - Observable, Observer, Subscription

Observable doesn’t do anything by itself it just has some logic stored inside. We need to subscribe to it to make it work

Subscription should be closed when we are done with them in order to avoid memory leakage and unexpected behaviorus.

Subscriptions can be stopped in 2 ways.

1. It can stop automatically by the observable logic itself with error or complete notifications
2. We can cancel it by unsubscribing

import { Observable } from "rxjs";

const observable$ = new Observable<string>(subscriber => {
  console.log('Observable executed');
  subscriber.next('Alice');
  setTimeout(() => subscriber.next('Ben'), 2000);
  setTimeout(() => subscriber.next('Charlie'), 4000);
});

const subscription = observable$.subscribe(value => console.log(value));

setTimeout(() => {
  console.log('Unsubscribe');
  subscription.unsubscribe();
}, 3000);

One of the most important thing here is how we unsubscribe to observable by calling the subscription object.

We received subscription object as return value of subscribe function.

and we could also use observer like this

const observer = {
  next: (value) => console.log(value),
};

However since we just need to have a next function we used simple function version.

Usually this small - just function version is used in projects.

Multiple Subscriptions

What happens if 2 observers subscribe to the same observable.

Well, just as you would expect they work independently and execute through all observable stream

import { Observable } from 'rxjs';

const observable$ = new Observable<string>((subscriber) => {
  console.log('Observable executed');
  subscriber.next('Alice');
  setTimeout(() => subscriber.next('Ben'), 2000);
  setTimeout(() => subscriber.next('Charlie'), 4000);
});

console.log('Subscription 1 starts');
observable$.subscribe((value) => console.log('Subscription 1:', value));

setTimeout(() => {
  console.log('Subscription 2 starts');
  observable$.subscribe((value) => console.log('Subscription 2:', value));
}, 1000);

Output

Subscription 1 starts
Observable executed
Subscription 1:
Alice
Subscription 2 starts
Observable executed
Subscription 2:
Alice
Subscription 1:
Ben
Subscription 2:
Ben
Subscription 1:
Charlie
Subscription 2:
Charlie

Complete notification is sent only if all the messages are sent and it is telling us that it has nothing more to share.

What Next?

This is the second article of a series of articles I write about RxJS. Do not stop here! Continue reading.
See the first article

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