useReducer

import React, { useReducer } from 'react'

const initialState = { count: 0 }

function reducer(state, action) {
    switch (action.type) {
        case 'increment':
            return { count: state.count + 1 }
        case 'decrement':
            return { count: state.count - 1 }
        default:
            throw new Error()
    }
}

export default function Counter() {
    const [state, dispatch] = useReducer(reducer, initialState)
    return (
        <>
            Count: {state.count}
            <button onClick={() => dispatch({ type: 'decrement' })}>-</button>
            <button onClick={() => dispatch({ type: 'increment' })}>+</button>
        </>
    )
}

<script>
	import { writable } from "svelte/store"

	const initialValue = 0
	const count = writable(initialValue)

	count.increment = () => {
		count.update((state) => state + 1)
	}
	count.decrement = () => {
		count.update((state) => state - 1)
	}
</script>

Count: {$count}
<button on:click={count.decrement}>-</button>
<button on:click={count.increment}>+</button>