storage-watcher

storage-watcher

A zero-dependency, lightweight npm package that lets you subscribe to localStorage and sessionStorage changes in real-time within the same browser tab.

Normally, there's no way to know when a value in storage changes within the same tab — the browser's native storage event only fires in other tabs. This package solves that by intercepting storage write operations and notifying your callbacks instantly.

Installation

npm install storage-watcher

Quick Start

import { StorageWatcher } from 'storage-watcher';

const watcher = new StorageWatcher();

// Watch a specific key
const unsub = watcher.on('user-token', (newValue, oldValue) => {
  console.log(`Token changed: ${oldValue} → ${newValue}`);
});

// Later, when done watching
unsub();

// Clean up everything
watcher.destroy();

API

new StorageWatcher(options?)

Creates a new watcher instance.

Option	Type	Default	Description
storageType	'localStorage' | 'sessionStorage'	'localStorage'	Which storage backend to watch

// Watch localStorage (default)
const watcher = new StorageWatcher();

// Watch sessionStorage
const sessionWatcher = new StorageWatcher({ storageType: 'sessionStorage' });

watcher.on(key, callback) → Unsubscribe

Watch a specific key for changes. Works even if the key doesn't exist yet — the callback fires the moment the key is created.

// Watch an existing key
watcher.on('theme', (newValue, oldValue) => {
  console.log(`Theme changed from ${oldValue} to ${newValue}`);
});

// Watch a key that doesn't exist yet (future key)
watcher.on('future-config', (newValue, oldValue) => {
  // oldValue will be null (key didn't exist before)
  console.log(`Config was created with value: ${newValue}`);
});

Callback signature: (newValue: string | null, oldValue: string | null) => void

• newValue is null when the key is removed or cleared
• oldValue is null when the key is newly created

watcher.onAny(callback) → Unsubscribe

Watch all key changes across the storage.

watcher.onAny((key, newValue, oldValue) => {
  console.log(`${key} changed: ${oldValue} → ${newValue}`);
});

Callback signature: (key: string, newValue: string | null, oldValue: string | null) => void

watcher.onNew(callback) → Unsubscribe

Watch for brand-new keys being added to storage. Fires only when a key that didn't previously exist is created via setItem().

watcher.onNew((key, value) => {
  console.log(`New key "${key}" created with value: ${value}`);
});

localStorage.setItem('brand-new', 'hello');  // ✅ fires (new key)
localStorage.setItem('brand-new', 'world');  // ❌ doesn't fire (key already exists)
localStorage.removeItem('brand-new');
localStorage.setItem('brand-new', 'again');  // ✅ fires (key was removed, now re-created)

Callback signature: (key: string, value: string) => void

watcher.destroy()

Destroy the watcher instance. Removes all listeners and cleans up internal patches. After calling destroy(), any calls to on(), onAny(), or onNew() will throw an AlreadyDestroyedError.

watcher.destroy();

This method is idempotent — calling it multiple times is safe.

Unsubscribe

All watch methods return an unsubscribe function:

const unsub = watcher.on('key', callback);

// Stop watching this specific callback
unsub();

// Calling again is safe (idempotent)
unsub();

How It Works

The package monkey-patches Storage.prototype.setItem, .removeItem, and .clear to intercept mutations. Key details:

• Reference counted — patches are installed when the first StorageWatcher instance is created and restored when the last instance is destroyed
• Multiple instances — you can create multiple watchers (even for different storage types) and they coexist safely
• clear() handling — when clear() is called, individual events fire for each key that existed, so per-key watchers work correctly
• Error isolation — if one callback throws, other callbacks and the storage operation itself are not affected

Events Fired

Storage Operation	on(key)	onAny()	onNew()
setItem(key, value) — new key	✅	✅	✅
setItem(key, value) — existing key	✅	✅	❌
removeItem(key)	✅	✅	❌
clear()	✅ (per key)	✅ (per key)	❌

Error Handling

Error	When
StorageUnavailableError	Storage backend is not available (e.g., private browsing)
AlreadyDestroyedError	Method called on a destroyed instance

Both extend StorageWatcherError, which extends Error.

import { StorageWatcherError, AlreadyDestroyedError } from 'storage-watcher';

try {
  watcher.on('key', cb);
} catch (err) {
  if (err instanceof AlreadyDestroyedError) {
    // handle destroyed instance
  }
}

TypeScript

Full TypeScript support with exported types:

import type {
  StorageWatcherOptions,
  StorageType,
  WatchCallback,
  WatchAnyCallback,
  WatchNewCallback,
  Unsubscribe,
} from 'storage-watcher';

License

MIT