71. implement Observable Subject

This is a follow-up on 57. create an Observable.

Plain Observables are unicast, meaning every subscription is independent. To create multicast, you need to use Subject.

Following code is easier to understand.

// default behavior with plain Observable
const observable = from([1,2,3])
observable.subscribe(console.log)
observable.subscribe(console.log)
// 1
// 2
// 3
// 1
// 2
// 3

You can see that two subscriptions are independent so the logs are grouped by subscription.

with Subject, it works like Event Listeners in DOM world.

const subject = new Subject()
subject.subscribe(console.log)
subject.subscribe(console.log)
 
const observable = from([1, 2, 3])
observable.subscribe(subject)

// 1
// 1
// 2
// 2
// 3
// 3

Now the logs are different! That is because Subject first works as a observer, get the values, then works as an Observable and dispatch the value to different observers.

Cool right? Ok, you are asked to implement a simple Subject Class.

1. Observable is given for you, you can just use it.
2. you can use new Observer({next,error,complete}) or new Observer(function) to create an observer.