The storage Event and Limits

The `storage` event

When a page changes localStorage, the browser fires a storage event in every other tab and window open on the same origin — but not in the tab that made the change. This asymmetry surprises most developers the first time.

// Register this listener in Tab B
window.addEventListener('storage', (event) => {
  console.log('key changed:', event.key);
  console.log('old value:', event.oldValue);
  console.log('new value:', event.newValue);
  console.log('storage area:', event.storageArea); // localStorage object
});

// In Tab A, write to localStorage — Tab B's listener fires; Tab A's does NOT

The StorageEvent properties are:

Property	Type	Description
`key`	`string \| null`	The key that changed. `null` when `clear()` is called.
`oldValue`	`string \| null`	The previous value, or `null` if the key was newly created.
`newValue`	`string \| null`	The new value, or `null` if the key was removed.
`url`	`string`	The URL of the document that made the change.
`storageArea`	`Storage \| null`	The `localStorage` object (never `sessionStorage` — that event only fires within the same tab).

The storage event fires only for localStorage. sessionStorage changes do not propagate across tabs because each tab has its own isolated session store.

Cross-tab signalling pattern

sequenceDiagram
  participant A as Tab A (writer)
  participant LS as localStorage
  participant B as Tab B (listener)
  A->>LS: setItem("demo:signal", "ping")
  LS-->>B: storage event fires
  Note over B: key="demo:signal"<br/>newValue="ping"
  Note over A: No event fires in Tab A

The storage event fires in other tabs, not the writer

A common use case is broadcasting a logout across tabs:

// In any tab: signal logout
localStorage.setItem('demo:auth-event', 'logout:' + Date.now());

// In every other tab: react to it
window.addEventListener('storage', (e) => {
  if (e.key === 'demo:auth-event' && e.newValue?.startsWith('logout:')) {
    // redirect to login page
    window.location.href = '/login';
  }
});

The ~5 MB quota

The Web Storage specification does not mandate a specific quota size, but browsers universally implement approximately 5 MB per origin for localStorage (some give 10 MB; mobile browsers may give less).

When you exceed the quota, setItem throws a QuotaExceededError (a DOMException):

try {
  const big = 'x'.repeat(6 * 1024 * 1024); // 6 MB string
  localStorage.setItem('demo:big', big);
} catch (e) {
  if (e instanceof DOMException && e.name === 'QuotaExceededError') {
    console.error('Storage quota exceeded — cannot save data');
  }
}

Always wrap writes that could be large (e.g., serialised arrays or objects that grow over time) in a try/catch to handle quota errors gracefully.

The synchronous-blocking caveat

All Web Storage operations — setItem, getItem, removeItem, clear — run synchronously on the main thread. This is normally fine for small values, but it becomes a problem when:

You are storing large strings (hundreds of KB or more) — each call blocks the main thread while serialising and writing.
You are calling setItem in a tight loop — the cumulative blocking adds up.
You are reading a large value inside a requestAnimationFrame or event handler — frame drops become visible.

If you find yourself storing large blobs, consider IndexedDB (asynchronous, supports binary data) or the Cache API instead.

Runnable: storage event listener + write

The snippet below registers a storage listener and then writes to localStorage. Because the listener only fires in other tabs, you will see the write confirmation but not the event in this window. Open the same page in a second tab to observe the cross-tab event.

Browser Storage

// Register a storage listener
// Note: this fires in OTHER tabs, not in this one
window.addEventListener('storage', function(e) {
  console.log('[storage event] key:', e.key, '| new:', e.newValue);
});

// Write to localStorage — the listener above will NOT fire here
localStorage.setItem('demo:signal', 'hello-from-tab');
console.log('Written demo:signal =', localStorage.getItem('demo:signal'));
console.log('(Open this page in another tab to see the storage event fire there)');

// Clean up
localStorage.removeItem('demo:signal');
console.log('Cleaned up. demo:signal =', localStorage.getItem('demo:signal'));