@milajs/mobx-helpers

@milajs/mobx-helpers is a companion package that provides lightweight utilities for observing MobX observable property changes and composing disposer functions.

Installation

npm i @milajs/mobx-helpers mobx

onChange

Reacts to changes on one or more observable properties. Returns a disposer function.

import { makeAutoObservable } from 'mobx';
import { onChange } from '@milajs/mobx-helpers';

class UserStore {
  name = '';
  age = 0;
  constructor() { makeAutoObservable(this); }
}

const store = new UserStore();

// Track a single property
const off = onChange(store, 'name', ({ name }) => {
  console.log('Name changed to', name);
});

// Track multiple properties
const off2 = onChange(store, ['name', 'age'], ({ name, age }) => {
  console.log('Updated:', name, age);
});

// Dispose when no longer needed
off();
off2();

trackChanges

Like onChange, but fires the callback immediately with the current values of the tracked properties before listening for subsequent changes:

import { trackChanges } from '@milajs/mobx-helpers';

const off = trackChanges(store, ['name', 'age'], (values) => {
  // Both first call and subsequent calls: values === { name, age }
  console.log(values);
});

This is useful when you need to run initialization logic with the current state and then continue reacting to future changes.

combineFns

Combines multiple disposer functions into one. This is the key utility for cleanly managing multiple MobX reactions inside a React useEffect.

import { combineFns } from '@milajs/mobx-helpers';

const off = combineFns(disposer1, disposer2, disposer3);

// Calling off() disposes all three
off();

Using with React useEffect

The most common and powerful pattern: set up several onChange reactions inside a useEffect and compose all their disposers with combineFns so a single cleanup function tears everything down.

import { useEffect } from 'react';
import { observer } from 'mobx-react-lite';
import { onChange, combineFns } from '@milajs/mobx-helpers';

const Dashboard = observer(({ authStore, projectStore, notifStore }) => {
  useEffect(() => {
    const off = combineFns(
      onChange(authStore, 'user', ({ user }) => {
        analytics.identify(user);
      }),
      onChange(projectStore, ['selectedId', 'filter'], ({ selectedId, filter }) => {
        analytics.track('project_view', { selectedId, filter });
      }),
      onChange(notifStore, 'unreadCount', ({ unreadCount }) => {
        document.title = unreadCount > 0
          ? `(${unreadCount}) Dashboard`
          : 'Dashboard';
      }),
    );

    // Single disposer cleans up all three reactions on unmount
    return off;
  }, []);

  return <div>{/* ... */}</div>;
});

:::tip Why combineFns? Without combineFns, you'd need to track each disposer individually and call them all in your cleanup function. combineFns reduces boilerplate and ensures nothing is accidentally left undisposed. :::