[RxJS] Subject basic

A Subject is a type that implements both Observer and Observable types. As an Observer, it can subscribe to Observables, and as an Observable it can produce values and have Observersw subscribe it.

First time I read 

 implements both Observer and Observable types

I was quite confused. 

As a Observable:

var subject = new Rx.Subject();
var subscription = subject.subscribe(
   function onNext(x) { console.log('onNext: ' + x); },
   function onError(e) { console.log('onError: ' + e.message); },
   function onCompleted() { console.log('onCompleted'); }
);
subject.onNext('Our message #1');
subject.onNext('Our message #2');

/*
"onNext: Our message #1"
"onNext: Our message #2"
*/

Every time we call onNext() message to yield the value.

As a Observer, so we use 'source' to sebscribe 'subject', then subscribe 'subject' again to get the side effect

var subject = new Rx.Subject();
var source = Rx.Observable.interval(300)
   .map(function(v) { return 'Interval message #' + v; })
   .take(5);
source.subscribe(subject);
var subscription = subject.subscribe(
   function onNext(x) { console.log('onNext: ' + x); },
   function onError(e) { console.log('onError: ' + e.message); },
   function onCompleted() { console.log('onCompleted'); }
);
setTimeout(function() {
   subject.onCompleted();
}, 1000);

/*
"onNext: Interval message #0"
"onNext: Interval message #1"
"onNext: Interval message #2"
"onCompleted"
*/