What is an Observable?

See also the pattern description on Wikipedia​
If you're already familiar with the concept as how it is realised in RxJS, go read next section about it.
RxJS familiarities and differences
Observables allow you to push messages from your publishers to your subscribers in a way where publishers do not have to care for what subscriber they post messages to and subscribers don't have to know where and what the publisher is. That's because publishers, subscribers and connection between them are three different things. Let's see the example of it:
import { Observable } from '@hullo/core';
​
// first - create a subject with some publisher
const answersToAll = new Observable(
    observer => {
        // observer represents future subscriber
        // let's send it a simple message
        observer.next(42);
    }
);
​
// second - there is a subscriber
const subscriber = {
  next: message => {
    // do sth with the received message
  },
  complete: () => {
    // do sth when publisher decided
    // to not push any more messages
  }
};
​
// third - connect subscriber to the subject
const subscription = answersToAll.subscribe(subscriber);
// (bonus) you can stop receiving messages
// externally to both publisher and subscriber
// through subscription object
subscription.cancel();
You'll find yourself writing second step and the third next to themselves, usually as one step. So there will be two steps, but those are still three separate concepts. First step might be even more than often in a completely separate file and for a reason!
The separation is a key to understand usefulness of this pattern. Due to a standard interface of Observer there is no need for the publisher to know where and why the subscription starts as the only responsibility of that is to emit specific to it messages. You can find similar concepts made with EventEmitter both on web and in Node.
In Node you can have ReadableStream that is a specific emitter which by itself doesn't know about what is reading the stream other  than that it has to provide functions that'll be run on specific events. 
In the browsers JS, any DOM node can be an emitter of events and then it just awaits for the developer to submit a function that should express a reaction to a click, a keystroke etc. Event emission can be done with Observables as well and in respective Hullo packages, there are helper function to get around in these environments.

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Intro to Core

RxJS familiarities and differences

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