A tiny (~1KB) JavaScript utility to sync state across browser tabs and windows using BroadcastChannel with a localStorage fallback. Perfect for SPAs, PWAs, and collaborative apps needing seamless, real-time updates.

Installation

npm install @abuhasanrumi/state-sync

Live Demo

Check it out:

Live Demo on CodeSandbox

Open the demo in multiple tabs to see a todo list sync instantly!

Usage

Create a reactive state that syncs across tabs:

import { syncState } from '@abuhasanrumi/state-sync'

const state = syncState({
  key: 'counter',
  initialState: { count: 0 },
  persist: true,
  onChange: (state) => console.log('State:', state)
})

state.count += 1 // Updates all open tabs

Options

• key (string): Unique identifier for the state.
• initialState (object/array): Initial state to sync.
• persist (boolean, optional): Save state to localStorage (default: false).
• onChange (function, optional): Callback triggered on state changes.

Example: Todo List

Build a synced todo list:

import { syncState } from '@abuhasanrumi/state-sync'

const todos = syncState({
  key: 'todos',
  initialState: [],
  persist: true,
  onChange: (state) => {
    document.getElementById('list').innerHTML = state
      .map((todo) => `<li>${todo}</li>`)
      .join('')
  }
})

document.getElementById('add').onclick = () => {
  const input = document.getElementById('input')
  if (input.value.trim()) {
    todos.push(input.value.trim())
    input.value = ''
  }
}

// Initial render
document.getElementById('list').innerHTML = todos
  .map((todo) => `<li>${todo}</li>`)
  .join('')

Notes

• initialState must be an object or array. For primitives (numbers, strings), wrap them:

  const count = syncState({ key: 'count', initialState: { value: 0 } })
  count.value += 1

• Nested updates require replacing properties:

  state.nested = { ...state.nested, prop: value }

• Persisted state must match initialState type, or clear localStorage[key].

Error Handling

• Throws Error('initialState must be object/array') if initialState is null, undefined, or a primitive.
• Warns in console if persisted state fails to parse.

Why I Built This

I wanted to tackle a common web dev challenge: keeping state consistent across browser tabs. Whether it’s a shared shopping cart, a live form, or a collaborative tool, users expect tabs to stay in sync without lag or complexity. I created state-sync to deliver that experience in a lightweight ~1KB package, using browser-native APIs to keep it fast and dependency-free.

Why It’s Different Than Others

Unlike heavy state management libraries like Redux or Zustand, or sync solutions like y-js and localforage, state-sync is hyper-focused on cross-tab synchronization. At just ~1KB, it’s a fraction of the size of alternatives, leveraging BroadcastChannel for instant updates and localStorage for broader compatibility. It’s not a full store—just a minimal, reactive utility with TypeScript support, designed to slot into any project without bloat.

Why I Came Up With This Idea

While working on single-page apps, I noticed how often tabs fall out of sync—think adding a todo in one tab but not seeing it in another. Browser APIs like BroadcastChannel offered a solution, but I couldn’t find a simple, tiny library to harness them. After prototyping a todo list that synced across tabs, I was hooked on the idea of making tab-syncing effortless. That spark led to state-sync, a micro-tool to bring that magic to every developer.

Features

• Lightweight: ~1KB, smaller than most scripts.
• Real-Time: BroadcastChannel for instant sync, localStorage fallback for compatibility.
• Reactive: Proxy-based state updates.
• Persistent: Optional localStorage to retain state.
• TypeScript-Ready: Full type definitions.
• Zero Dependencies: Pure JavaScript, built for browsers.

Development

Get started:

git clone https://github.com/abuhasanrumi/state-sync.git
cd state-sync
npm install
npm test
npm run build

Contributing

Have ideas to make state-sync even better? Please:

1. Fork the repo.
2. Create a branch: git checkout -b feature/cool-idea.
3. Commit changes: git commit -m 'Add cool idea'.
4. Push: git push origin feature/cool-idea.
5. Open a Pull Request.

License

MIT © Abu Hasan Rumi

Related

Check out my other micro-utility:

• @abuhasanrumi/micro-flow: Tiny debounce, throttle, and rate-limiting (~364B).