useState

import React, { useState } from "react"

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

    return (
        <>
            Count: {count}
            <button onClick={() => setCount(0)}>Reset</button>
            <button onClick={() => setCount((prevCount) => prevCount - 1)}>-</button>
            <button onClick={() => setCount((prevCount) => prevCount + 1)}>+</button>
        </>
    )
}

<script>
	let count = 0
</script>

Count: {count}
<button on:click={() => (count = 0)}>Reset</button>
<button on:click={() => count--}>-</button>
<button on:click={() => count++}>+</button>