# Subjects

A Subject is a special type of Observable which shares a single execution path among observers.

You can think of this as a single speaker talking at a microphone in a room full of people. Their message (the subject) is being delivered to many (multicast) people (the observers) at once. This is the basis of **multicasting**. Typical observables would be comparable to a 1 on 1 conversation.

There are 4 variants of subjects:

* **Subject** - No initial value or replay behavior.
* **AsyncSubject** - Emits latest value to observers upon completion.
* **BehaviorSubject** - Requires an initial value and emits its current value (last emitted item) to new subscribers.
* **ReplaySubject** - Emits specified number of last emitted values (a replay) to new subscribers.

## Contents

* [AsyncSubject](https://www.learnrxjs.io/learn-rxjs/subjects/asyncsubject)
* [BehaviorSubject](https://www.learnrxjs.io/learn-rxjs/subjects/behaviorsubject)
* [ReplaySubject](https://www.learnrxjs.io/learn-rxjs/subjects/replaysubject)
* [Subject](https://www.learnrxjs.io/learn-rxjs/subjects/subject)

## Subjects comparison

( [Stackblitz](https://stackblitz.com/edit/rxjs-subjects-comparison?file=index.ts\&devtoolsheight=100) )

```js
/*
                   s1    n(r)   n(x)    s2     n(j)   c    n(s)
Subject            
        s1         ^-----r------x--------------j------|----------
        s2         ---------------------^------j------|----------
AsyncSubject       
        s1         ^----------------------------------j|---------
        s2         ---------------------^-------------j|---------
BehaviorSubject    
        s1         ^a----r------x--------------j------|----------
        s2         ---------------------^x-----j------|----------
ReplaySubject      
        s1         ^-----r------x--------------j------|----------
        s2         ---------------------^r-x---j------|----------
*/

// RxJS v6+
import { Subject, AsyncSubject, BehaviorSubject, ReplaySubject } from 'rxjs';

const subject = new Subject();
const asyncSubject = new AsyncSubject();
const behaviorSubject = new BehaviorSubject('a');
const replaySubject = new ReplaySubject(2);

const subjects = [subject, asyncSubject, behaviorSubject, replaySubject];
const log = subjectType => e => console.log(`${subjectType}: ${e}`);

console.log('SUBSCRIBE 1');
subject.subscribe(log('s1 subject'));
asyncSubject.subscribe(log('s1 asyncSubject'));
behaviorSubject.subscribe(log('s1 behaviorSubject'));
replaySubject.subscribe(log('s1 replaySubject'));

console.log('\nNEXT(r)');
subjects.forEach(o => o.next('r'));

console.log('\nNEXT(x)');
subjects.forEach(o => o.next('x'));

console.log('\nSUBSCRIBE 2');
subject.subscribe(log('s2 subject'));
asyncSubject.subscribe(log('s2 asyncSubject'));
behaviorSubject.subscribe(log('s2 behaviorSubject'));
replaySubject.subscribe(log('s2 replaySubject'));

console.log('\nNEXT(j)');
subjects.forEach(o => o.next('j'));

console.log('\nCOMPLETE');
subjects.forEach(o => o.complete());

console.log('\nNEXT(s)');
subjects.forEach(o => o.next('s'));

/*
OUTPUT:

SUBSCRIBE 1
s1 behaviorSubject: a

NEXT(r)
s1 subject: r
s1 behaviorSubject: r
s1 replaySubject: r

NEXT(x)
s1 subject: x
s1 behaviorSubject: x
s1 replaySubject: x

SUBSCRIBE 2
s2 behaviorSubject: x
s2 replaySubject: r
s2 replaySubject: x

NEXT(j)
s1 subject: j
s2 subject: j
s1 behaviorSubject: j
s2 behaviorSubject: j
s1 replaySubject: j
s2 replaySubject: j

COMPLETE
s1 asyncSubject: j
s2 asyncSubject: j

NEXT(s)
*/
```

### Additional Resources

* [Official Overview](http://reactivex.io/rxjs/manual/overview.html#subject) 📰
* [Official Documentation](http://reactivex.io/documentation/subject.html) 📰
* [On The Subject Of Subjects (in RxJS)](https://medium.com/@benlesh/on-the-subject-of-subjects-in-rxjs-2b08b7198b93) 📰 - Ben Lesh