useEffect

import React, { useState, useEffect } from 'react'

export default function Timer() {
    const [count, setCount] = useState(0)

    useEffect(() => {
        const interval = setInterval(() => {
            setCount(count => count + 1)
        }, 1000)

        return () => clearInterval(interval)
    }, [])

    return (
        <>
            Timer: {count}
        </>
    )
}

<script>
	import { onMount } from "svelte"

	let count = 0

	onMount(() => {
		const interval = setInterval(() => {
			count += 1
		}, 1000)

		return () => clearInterval(interval)
	})
</script>

Timer: {count}