Why does RxJS interval begin from last value but not from "0"? - reactjs

Question

I need to reset my custom timer to "0 seconds" when I click the Reset button. But when I press Start my timer continues from last value, not from "0 seconds".
const [time, setTime] = useState (0);
const [timerOn, setTimerOn ] = useState (false);
let observable$ = interval(1000);
let subscription = observable$.subscribe(result =>{
if (timerOn) {
setTime(result);
}
});
return () => subscription.unsubscribe();
}, [timerOn]);
return (
<div>
{!timerOn && (
<button onClick={() => setTimerOn(true)}>Start</button>
)}
{ time > 0 && (
<button onClick={() => setTime(0)}>Reset</button>
)}

Answer 1

Problem
Let's take a look at the Reset button:
<button onClick={() => setTime(0)}>Reset</button>
When you click this button it runs the following function:
() => setTime(0)
This just sets the time state back to 0. That's all. It doesn't touch the subscription to the interval observable at all. As a result, the interval subscription will continue emitting numbers in sequence which is why it appears to continue from the last value.
Solution
Your reset function will have to do more than just setting time back to 0. What it does exactly will be up to your specific use case. For example, it could end the subscription, end the subscription and create a new one, or reset the existing subscription. I've included a code example for a basic way of ending the subscription:
const {
StrictMode,
useCallback,
useRef,
useState,
} = React;
const { interval } = rxjs;
const rootElement = document.getElementById("root");
function useCounter() {
const [time, setTime] = useState(0);
const [timerOn, setTimerOn] = useState(false);
const observable$ = interval(1000);
const subscription = useRef();
const start = useCallback(() => {
setTimerOn(true);
subscription.current = observable$.subscribe((result) => {
console.log(result);
setTime(result);
});
});
const stop = useCallback(() => {
setTimerOn(false);
subscription.current.unsubscribe();
});
const reset = useCallback(() => {
setTime(0);
setTimerOn(false);
subscription.current.unsubscribe();
});
return {
time,
timerOn,
start,
stop,
reset
};
}
function Counter() {
const { time, timerOn, start, stop, reset } = useCounter();
return (
<div>
<h1>{time}</h1>
<br />
{!timerOn && <button onClick={() => start()}>Start</button>}
{time > 0 && <button onClick={() => reset()}>Reset</button>}
</div>
);
}
function App() {
return <Counter />;
}
ReactDOM.render(
<StrictMode>
<App />
</StrictMode>,
rootElement
);
<div id="root"></div>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/17.0.1/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/17.0.1/umd/react-dom.production.min.js"></script>
<script src="https://unpkg.com/rxjs#^7/dist/bundles/rxjs.umd.min.js"></script>