Rxjs Observable Vs Subject

 Observables are unicast by design and Subjects are multicast by design.

if you look at the below example - each subscription receives the different values as observables developed as unicast by design.

import {Observable} from 'rxjs';

let obs = Observable.create(observer=>{
   observer.next(Math.random());
})

obs.subscribe(res=>{
  console.log('subscription a :', res); //subscription a :0.2859800202682865
});

obs.subscribe(res=>{
  console.log('subscription b :', res); //subscription b :0.694302021731573
});

this could be weird if you are expecting the same values on both the subscription.

we can overcome this issue using Subjects. Subjects is similar to event-emitter and it does not invoke for each subscription. consider the below example.

import {Subject} from 'rxjs';

let obs = new Subject();

obs.subscribe(res=>{
  console.log('subscription a :', res); // subscription a : 0.91767565496093
});

obs.subscribe(res=>{
  console.log('subscription b :', res);// subscription b : 0.91767565496093
});

obs.next(Math.random());

both the subscription are got the same output value!.