Rx.ReplaySubject subscription callback not being called

Question

I have an rxjs ReplaySubject that will either emit a value or null depending on what environment I am on. Meaning if in a certain environment I make a service call and get the data. If I am not in that environment I just call next(null) on the replay subject. In the component where I subscribe to this all works fine if the service call was made. If I had done next(null) then my callback is never fired.

My assumption is that the null value will not be returned because it was set before I subscribed but I thought that was the entire point of ReplaySubject. I have googled then until I ran out of different ways to explain the issue and all I get is the read me file for ReplaySubject. Hoping somebody can tell me what I am doing wrong. I thought maybe it was because I passed in null so I passed in "Hello" instead and still nothing. Here is a code sample:

public Data = new ReplaySubject<any>(1);
init(){
 if(!window.Something){
   //subscribe callback is never fired in this case.
   this.Data.next(null);
 }else{
    this.http.get(...).map(...{
     //this works 
     this.Data.next(result);
    });
 }}

The mistake I made actually had nothing to do with Rx. I can't remember exactly but it was making so the condition was never true. This kept the code from executing.

Answer 1

Using .next(null) on any Observable is correct. Everything is a value in RxJS including null and undefined. So the problem is probably somewhere else than this.

This is what you're doing right now and it both cases (when you uncomment the second if) it works as should. The value is stored in the ReplaySubject and then replayed or pushed directly to the current observer:

let subject = new ReplaySubject(1);

// if (!window.document) {
if (!window.whatever) {
  subject.next(null);
} else {
  setTimeout(() => {
    subject.next('Hello');
  }, 500);
}

subject.subscribe(val => console.log(val));

See live demo: https://jsbin.com/cigocak/edit?js,console