Pipe Observable to Subject without making it hot unneccessarily - rxjs

Question

This question builds upon this one, where it is shown how to feed an Observable into a Subject. My question is similar, but I want to avoid making the Observable hot unless it is necessary, so that it's .pipe() doesn't run needlessly. For example:
const subject$ = new Subject();
const mouseMove$ = Observable.fromEvent(document, 'mousemove')
.pipe(map(it => superExpensiveComputation(it)));
mouseMove$.subscribe(n => subject$.next(n));
Because of the subscription, this will make mouseMove$ hot, and superExpensiveComputation will be run for every mouse move, whether someone is listening for it on subject$ or not.
How can I feed the result of mouseMove$ into subject$ without running superExpensiveComputation unneccessarily?

Answer 1

You can simply use tap instead of subscribe to pass emissions to your subject:
const mouseMove$ = fromEvent(document, 'mousemove').pipe(
map(it => superExpensiveComputation(it)),
tap(subject$)
);
Of course you still need to subscribe to mouseMove$ to make the data flow, but you don't need to have a subscription dedicated to passing the data to your subject.
However, you'll probably want to add share as not to repeat the expensive logic for multiple subscribers.
const mouseMove$ = fromEvent(document, 'mousemove').pipe(
map(it => superExpensiveComputation(it)),
share(),
tap(subject$)
);
But... then in that case, do you really need a subject at all? Unless you are going to be calling .next() from somewhere else, you probably don't.

Answer 2

Because of the subscription, this will make mouseMove$ hot
Wrong. The subscription doesn't change behavior of observable.
Observable.fromEvent(document, 'mousemove') is already hot.
Here's naive cold version of it.
The key takeaway, I have to resubscribe it everytime to get the latest data.
const { of } = rxjs;
const mouseMove$ = of((() => {
let payload = {clientX: 0};
document.addEventListener("mousemove", (ev) => payload.clientX = ev.clientX)
return payload;
})())
let subscriber = mouseMove$.subscribe(pr => console.log(pr));
const loop = () => {
if(subscriber) {
subscriber.unsubscribe();
}
subscriber = mouseMove$.subscribe(pr => console.log(pr));
setTimeout(loop, 1000);
};
setTimeout(loop, 1000);
<script src="https://cdnjs.cloudflare.com/ajax/libs/rxjs/6.5.5/rxjs.umd.js"></script>
and superExpensiveComputation will be run for every mouse move,
whether someone is listening for it on subject$ or not
Then you can get rid of map(it => superExpensiveComputation(it)) and move that lamda to subscription so it still executes on every mouse move but after it has been subscribed..