store.js

export class Writable {
  constructor(value) {
    this.value = value;
    this.callbacks = new Set();
  }

  subscribe(callback) {
    this.callbacks.add(callback);
    callback(this.value);

    return () => this.callbacks.delete(callback);
  }

  set(value) {
    this.value = value;

    for (let callback of this.callbacks) {
      callback(this.value);
    }
  }

  get() {
    return this.value;
  }
}

export class Persistable extends Writable {
  constructor(table, value) {
    super(value);
    this.table = table;

    const v = localStorage.getItem(table);
    if (v === null) return;

    try {
      if (this.value instanceof Set) {
        this.value = new Set(JSON.parse(v));
      } else {
        Object.assign(this.value, JSON.parse(v));
      }
    } catch (ex) {}

    this.set(value);
  }

  set(value) {
    super.set(value);

    // update localStorage
    if (this.value instanceof Set) {
      localStorage.setItem(this.table, JSON.stringify([...value]));
    } else {
      localStorage.setItem(this.table, JSON.stringify(value));
    }
  }
}

MIT License