A Svelte store that saves to localStorage

App.svelte

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

	import { saveStore } from "localstorage.js";
  
  let features = writable([])
  
  onMount(() => {
		saveStore(features, "$features", { storage: localStorage });
  }
</script>

localstorage.js

export function saveStore(
	store,
	key,
	{ storage = sessionStorage, serialize = JSON.stringify, deserialize = JSON.parse }
) {
	if (!store || !key) return;

	const saved = storage.getItem(key);
	if (saved) {
		store.set(deserialize(saved));
	}

	return store.subscribe($value => {
		storage.setItem(key, serialize($value));
	});
}