You’ve tested your app across browsers, devices, and screen sizes. It handles slow networks, aggressive typing, and even partial form input. But have you tested what happens when a user opens a second tab?

Here’s a real story.

A colleague once complained that our internal admin panel was acting strangely. He’d opened two tabs: one to view logs, another to manage users. He made a config change in the first tab, but it didn’t reflect in the second. Then he logged out from the second tab, thinking he was done - but the first tab stayed logged in and allowed actions for another ten minutes.

This wasn’t a bug in the feature. It was a bug in the assumption that one tab is all that matters.

The truth is: in modern web apps, multi-tab usage is the norm. People open new tabs to multitask, compare, or keep context while exploring other parts of the UI. And most of the time, we as developers ignore that.

Until something breaks.

Table of Contents

1. The Forgotten Problem
2. The Hidden UX Cost
3. Why React Doesn’t Solve This
4. Limitations of storage and postMessage
5. BroadcastChannel API
6. Why It’s Rarely Used
7. Coming Up in Part 2

The Hidden UX Cost of Multi-Tab Desync

When each tab is out of sync with the others, subtle and serious issues start to appear:

• A user logs out in one tab, but another tab still thinks they're authenticated and keeps making API calls
• A notification shows up twice, once in each tab
• A local draft is saved in one tab, and then overwritten by an older state from another
• A filter applied in one view is missing in another, leading to confusion

These aren’t just technical issues. They erode trust. They create support tickets. They make users feel like your app is unreliable, even if your backend and UI code are solid.

The root of the problem is surprisingly simple: browser tabs don’t share state. And most frontend frameworks, React included, don’t offer any solution out of the box.

Why React Doesn’t Solve This

React provides excellent tools for managing state within a tab: useState, useReducer, the Context API, or external stores like Redux or Zustand. But all of these live entirely inside the JavaScript runtime of a single browser tab.

Each tab gets its own React app instance, with isolated memory and state. That means even if you’re using the same global store or persist your data in localStorage, each tab still runs independently, unaware of changes happening elsewhere.

Let’s walk through a common example: logging out.

You may think you’ve got it covered:

// AuthContext.js
const AuthContext = createContext()

export const AuthProvider = ({ children }) => {
  const [isLoggedIn, setIsLoggedIn] = useState(() => {
    return localStorage.getItem('token') !== null
  })

  const logout = () => {
    localStorage.removeItem('token')
    setIsLoggedIn(false)
  }

  return (
    <AuthContext.Provider value={{ isLoggedIn, logout }}>
      {children}
    </AuthContext.Provider>
  )
}

Now, imagine this:

1. Tab A logs in → stores token in localStorage
2. Tab B opens later → reads token from localStorage → thinks user is logged in
3. Tab A logs out → removes token → updates its state
4. Tab B? Still thinks user is logged in

To fix this, one might try using the storage event:

useEffect(() => {
  const handleStorage = (event) => {
    if (event.key === 'token' && event.newValue === null) {
      setIsLoggedIn(false)
    }
  }

  window.addEventListener('storage', handleStorage)
  return () => window.removeEventListener('storage', handleStorage)
}, [])

That helps - if the logout happens via localStorage. But there are caveats:

• storage events don’t fire in the same tab that made the change
• It’s fragile - what if multiple values need to be synced? Manual sync logic must be implemented for each feature

This becomes even harder when dealing with more complex state - filters, drafts, modals. storage events can’t support real-time sync for everything. And even when used creatively, they amount to a custom messaging system layered on top of a key-value store.

Another messaging option is window.postMessage, which enables communication between Window objects, including iframes or tabs opened via window.open. For example:

// Tab A opens Tab B
const newTab = window.open('/another-tab')
newTab.postMessage({ type: 'init' }, '*')

// In Tab B
window.addEventListener('message', (event) => {
  console.log(event.data)
})

This works well when one tab opens the other and maintains a reference. However, it breaks down in common multi-tab use cases where tabs are opened independently (e.g., via bookmarks, navigation, or external links). Without a reference to the target Window, there is no way to initiate communication. Moreover, the opened tab can’t reliably send messages back unless it uses window.opener, which may be null due to browser restrictions or security settings. For this reason, window.postMessage is not suitable for general-purpose multi-tab communication.

React, for all its strengths, wasn’t designed with multi-tab coordination in mind.

The BroadcastChannel API - A Native Tool Worth Knowing

To address the problem of isolated browser tabs, modern browsers provide a built-in solution: the BroadcastChannel API. It enables communication between different browsing contexts, including tabs, windows, and iframes, under the same origin. This makes it an ideal candidate for implementing real-time synchronization without requiring a backend or external service.

Using BroadcastChannel is straightforward. A named channel is opened, and any tab that posts a message to that channel immediately triggers onmessage listeners in other tabs.

const channel = new BroadcastChannel('auth-channel')

channel.postMessage({ type: 'logout' })

channel.onmessage = (event) => {
  if (event.data.type === 'logout') {
    // log out in this tab as well
  }
}

Messages are transmitted almost instantly. Unlike localStorage-based communication, there is no need to serialize strings or rely on passive change listeners. Any serializable object can be sent through the channel, and tabs listening on the same name will receive the broadcast.

The simplicity of this interface is appealing. It is native, fast, and requires no third-party tools. It also avoids many of the limitations of alternatives like storage events, which are less consistent across browsers and harder to manage reactively.

However, once this API is introduced into a React application, new challenges emerge - not because of the API itself, but because of how it fits (or fails to fit) into component-based architectures.

So Why Isn’t BroadcastChannel Used More Often?

While the BroadcastChannel API offers a clean, native solution for inter-tab messaging, integrating it into a React application reveals several architectural tensions.

React’s component model encourages scoped, lifecycle-driven logic. Components mount, update, and unmount independently, often driven by state or route changes. In contrast, BroadcastChannel is a long-lived, process-level resource. Using it directly within component effects can lead to unintended side effects.

A typical integration might look like this:

useEffect(() => {
  const channel = new BroadcastChannel('auth-channel')

  channel.onmessage = (event) => {
    if (event.data.type === 'logout') {
      logout()
    }
  }

  return () => {
    channel.close()
  }
}, [])

At first glance, this seems sufficient: a single useEffect creates the channel, listens for messages, and cleans up when the component unmounts. However, this approach assumes that:

• The component is always mounted when relevant messages are sent
• No other parts of the app are listening to the same channel
• All messages should be handled the same way, regardless of context or origin

In larger applications, these assumptions often break.

Consider a case where two tabs send a logout message at the same time:

channel.postMessage({ type: 'logout' })

channel.onmessage = (event) => {
  logout() // Gets called twice in each tab
}

If the component is conditionally rendered, the channel might be closed before a message arrives. If multiple components create the same channel, each will subscribe independently, leading to overlapping listeners and unpredictable behavior.

There is also no built-in support for tracking the source of a message, which makes it impossible to distinguish between messages originating from the current tab and those from others:

channel.postMessage({ type: 'update-settings' })

channel.onmessage = (event) => {
  applyUpdate(event.data) // Executes in the sender tab too
}

To build something reliable, developers must implement:

• Message deduplication
• Cleanup logic tied to React lifecycles
• Source tracking
• Scoped message handling
• Expiration and filtering logic

Because this infrastructure is non-trivial and error-prone, most teams avoid using BroadcastChannel directly or use it only in isolated scenarios. For reliable multi-tab sync in production, a React-friendly abstraction is needed - one that wraps this API safely and integrates with the component model.

TL;DR

Most modern apps ignore what happens when users open multiple tabs. This leads to subtle bugs and broken UX, especially in React apps where state is isolated per tab. While the BroadcastChannel API provides a native solution, integrating it into a React architecture safely requires careful design. This post breaks down the problem and paves the way for a React-friendly abstraction coming in Part 2.

Coming Up in Part 2

🔔 Stay tuned for Part 2: A React hook built for multi-tab communication. Subscribe or follow to get notified!