Auto saving to localStorage with MobX

autoStore.js

import mobx from "mobx"
import store from "store"

export default function(_this) {
  let firstRun = true

  // will run on change
  mobx.autorun(() => {
    // on load check if there's an existing store on localStorage and extend the store
    if (firstRun) {
      const existingStore = store.get("store")
      
      if (existingStore) {
        mobx.extendObservable(_this, existingStore)
      }
    }

    // from then on serialize and save to localStorage
    store.set("store", mobx.toJS(_this))
  })

  firstRun = false
}

todoStore.js

import mobx, { computed, observable } from "mobx"
import autoSave from "./autoSave"

const initialTodoStore = {
  todos: ["buy milk", "buy eggs"]
}

class TodoStore {
  constructor() {
    // set initial mock up examples
    this.todos = initialTodoStore  // or []
    
    // in the future it will over run it and save to store
    autoSave(this)
  }

  @observable todos
  @observable filter = ""

  @computed get filteredTodos() {
    const filter = new RegExp(this.filter, "i")

    return this.todos.filter(todo => !this.filter || filter.test(todo))
  }
}

const todoStore = window.todoStore = new TodoStore

export default todoStore

I have found an even more simpler solution using toJSON():

import { autorun } from 'mobx'

const name = 'Store'

export class Store {
	constructor() {
		const storedJson = localStorage.getItem(name)
		if (storedJson) Object.assign(this, JSON.parse(storedJson))
		autorun(() => {
			localStorage.setItem(name, JSON.stringify(this))
		})
	}

	toJSON() {
		const { id, background } = this
		return {
			id,
			background
		}
	}

	reset() {
		localStorage.removeItem(name)
	}
}

Works like a charm with MobX 6 🎉

My helper function as a universal solution:

import {autorun, toJS} from 'mobx';

export function makeLocalStorage<T extends object, K extends keyof T>(
  obj: T,
  prefix: string,
  keys: K[],
): void {
  for (const key of keys) {
    const localKey = `${prefix}_${key}`;

    const valueStr = localStorage.getItem(localKey);

    if (!valueStr) {
      continue;
    }

    const value = JSON.parse(valueStr);
    obj[key] = value;
  }

  autorun(() => {
    for (const key of keys) {
      const localKey = `${prefix}_${key}`;

      localStorage.setItem(localKey, JSON.stringify(toJS(obj[key])));
    }
  });
}

Usage:

import {makeObservable, observable} from 'mobx';

class CounterStore {
  count = 0;
  countThatDoesNotNeedToBeLocallyStored = 0;
  
  constructor() {
    makeObservable(this, {
      count: observable,
      countThatDoesNotNeedToBeLocallyStored: observable
    });

    makeLocalStorage(this, 'counterStore', ['count']); // Only store property `count`
  }
}

@dizys nice one. Currently, I use mobx-persist-store as it's a library plus battle-tested with edge-cases. Works with TypeScript as well 🎉

Hi there
I'm trying to read read/write in store but from differents browser tabs:
URL for tab1: /mydomain
URL for tab2: /mydomain/test

I'm trying to figure out how to set reaction to listen changes in localstorage. The flow will be like this:
tab1 set var visible in store to true, then tab2 listen the change (from localstorage I suppose) and show something into tab2.

Is it possible to do that? Any idea how to do it?

thanks in advance.

@hartum websockets maybe? idk why you wanna persist such data in localstorage but i guess hmr in react/next works like that only. they use websockets to listen if ui in one browser changed & update it everywhere i guess :)

Using websockets seems to me a bit drastic solution. The reason is two browser windows/tabs of the same SPA, because the client wants to open several instances of components and be able to use it on several monitors.

In case anyone is interested what I did to solve my problem was to use the 'mobx-persist-store' library to write my store data to the localStorage.
Then (inside my store) I added a store event listener and update it every time I detect a change.

in mobxStore.ts

import { makePersistable, hydrateStore } from 'mobx-persist-store';

class Store {
  // --- STORE STATE VARS ---
  dashboard = {
    isVisible: true,
  };

  constructor() {
    makePersistable(this, { name: 'mobxStore',
      properties: ['dashboard'],
      storage: window.localStorage }).finally(() => {});
   }

  // --- SHOW/HIDE DASHBOARD ---
  setDashboard(visible: boolean) {
    this.dashboard.isVisible = visible;
  }
}

const myStore = new Store();

window.addEventListener('storage', (e) => {
  if (e.key === 'mobxStore') {
    myStore.hydrateStore().catch(() => {});
  }
});

export default myStore;

in a different window (but same domain http://localhost:3000/test )the component:

import React from 'react';

import { observer } from 'mobx-react';
import store from 'store/mobxStore';

const FakeComponent = () => {
  const visible = store.dashboard.isVisible;
  return (
    <div>
      {store.dashboard.isVisible ? <span>Dashboard visible</span> : <span>Dashboard Invisible</span>}
      <br />
      <div onClick={() => store.setDashboard(!visible)} >"Switch dashboard visibility"</div>
    </div>
  );
};

export default observer(FakeComponent);