Publish the last value of an observable

Question

I have a hot observable (a subject in this case):

var subject = new Rx.Subject(); 

I want to create another observable that every time a new subscriptions is being made immediately fires out the last value that was produced.

So in pseudo code:

var myObservableWithLastValue = subject.publishLast();  subject.onNext(3);  myObservableWithLastValue.subscribe(function(x){     console.log(x); //should write 3 });  myObservableWithLastValue.subscribe(function(x){     console.log(x); //should write 3, too });  subject.onNext(4);  myObservableWithLastValue.subscribe(function(x){     console.log(x); //should write 4 }); 

This is roughly what I want and it seems to work. However, I guess there must be some built in mechanism to achieve the same

Rx.Observable.prototype.keepLatest = function () {     var latestValue;      var disposable = this.subscribe(function (value) {         latestValue = value;     });      return Rx.Observable.create(function (observer) {         observer.onNext(latestValue);         return disposable.dispose;     }); }; 

Answer 1

RxJs now has the ReplaySubject. Initialize it with 1 buffer and you have the BehaviorSubject.

// as an example, use buffer size of 2  var subject = new Rx.ReplaySubject(2 /* buffer size */);    subject.onNext('a');  subject.onNext('b');  subject.onNext('c');    subject.subscribe(function (x) { document.write('x1:' + x + '<br>'); });    subject.onNext('d');    subject.subscribe(function (x) { document.write('x2:' + x + '<br>'); });

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Answer 2

BehaviorSubject:

Initializes a new instance of the Rx.BehaviorSubject class which creates a subject that caches its last value and starts with the specified value.

var subject = new Rx.BehaviorSubject('a' /* initial value */);    subject.subscribe(function (x) {      console.log('x1:' + x);  });    subject.onNext('d');    // Will produce the last value.    subject.subscribe(function (x) {      console.log('x2:' + x);  });

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