Svelte writable store that stores its value in localStorage. Does json parsing and stringifying if initial value is an object/array.

storable.js

import { writable } from "svelte/store";

/**
 * Svelte writable store that stores its value in localStorage.
 * Does json parsing and stringifying if initial value is an object/array.
 *
 * @param {string} key localStorage-key
 * @param {string|number|boolean|object|array} initialValue the initial value of the store (also the value that can be reset to with the reset() function)
 **/
export const storable = (key, initialValue) => {
  const storeAsJSON = typeof initialValue === "object";

  function getStoreValue() {
    const storeValue = localStorage.getItem(key);
    if (storeAsJSON) {
      try {
        return JSON.parse(storeValue);
      } catch (ignored) {
        return initialValue;
      }
    }
    return storeValue !== undefined ? storeValue : initialValue;
  }

  const { set, update, subscribe } = writable(getStoreValue());

  subscribe((value) =>
    localStorage.setItem(key, storeAsJSON ? JSON.stringify(value) : value)
  );

  return {
    set,
    update,
    subscribe,
    reset() {
      set(initialValue);
    },
  };
};