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1.7.2 • Public • Published


a lightweight module to sync JS objects in realtime across tabs / windows of a browser.

❗️ v1.1.x: Now a modular lerna repo! Synchronization strategies have been split out into separate packages.

See my battle with browser tabs for detailed information regarding the issues localsync solves.


  • Uses local storage event emitters to sync objects in realtime across tabs.
  • Never calls the tab that the event occurred on.
  • Falls back to cookie polling internally if using an unsupported browser (IE 9+ / Edge).
  • Isomorphic.
  • Tested with mocha.

Build Status codecov



npm install -S localsync

How to use

import localsync from 'localsync'
/** Create an action that will trigger a sync to other tabs. */
const action = (userID, first, last) => ({ userID, first, last })
/** Create a handler that will run on all tabs that did not trigger the sync. */
const handler = (value, old, url) => {`Another tab at url ${url} switched user from "${old.first} ${old.last}" to "${value.first} ${value.last}".`)
  // do something with value.userID
/** Create a synchronizer. localsync supports N number of synchronizers for different things across your app. The key 'user' defines a localsync synchronization channel. */
const usersync = localsync('user', action, handler)
 * Start synchronizing.
 * Passing true tells localsync to poll the current storage mechanism once on
 * start for any pre-existing state that may be there (cross session).
 * Defaults to false - may change to true in a future major version.
/** IE / Edge do not support local storage across multiple tabs. localsync will automatically fallback to a cookie polling mechanism here. You don't need to do anything else. */
  console.warn('browser doesnt support local storage synchronization, falling back to cookie synchronization.')
/** Trigger an action that will get handled on other tabs. */
usersync.trigger(1, 'jimmy', 'john') /** => 'storagesync' on chrome, 'cookiesync' on IE */
setTimeout(() => {
  /** Trigger another action in 5 seconds. */
  usersync.trigger(2, 'jane', 'wonka')
}, 5000)
setTimeout(() => {
  /** If its still running, stop syncing in 10 seconds. */
}, 10000)

Structure and Roadmap

localsync has a singular purpose: to synchronize events from one client to many using a common interface and the least invasive mechanism for the current browsing medium.

Internally, localsync is comprised of several small sync packages that all adhere to the common localsync interface. The main localsync package does no actual synchronization on its own but rather determines the most appropriate synchronization strategy and calls upon the necessary packages to invoke it. All the packages with brief descriptions are listed here:


Guaranteed synchronization between clients of the same browser (Chrome ↔️ Chrome, IE ↔️ IE, etc.)

  • localsync - Determines synchronization mechanism and invokes it.

Mechanism packages

  • 🚅 storagesync - Synchronizes data in a push fashion using local storage storage event for a given browser.
  • 🍪 cookiesync - Synchronizes data via cookie polling mechanism for a given browser.
  • 💻 serversync - Mocks the localsync interface on server environments but does no actual synchronization (for now).

2.x.x (In Progress)

The primary goal of 2.0 is to enable cross-browser localsync (Chrome ↔️ IE, Firefox ↔️ Safari, etc.). The following additional mechanisms are being implemented to make this happen:

  • 🚀 webrtcsync - Synchronizes data across any supporting browser using WebRTC technology.
  • ✈️ socketsync - Synchronizes data across any supporting browser using web sockets technology (Fallback for WebRTC).


const sync = localsync(key: string, action: (...args) => payload, handler: payload => {}, [opts: Object])
const { start, stop, trigger, isRunning, isFallback } = sync


key: a string that is used for this synchronization instance (you may have multiple instances of localsync each with different keys to sync different types of data).

action: a function that will be called when this client's trigger function is invoked. The action will be passed any arguments provided to the trigger function and should return the payload to be delivered to other clients for the given localsync key.

handler: a function that will be invoked on this client when any other client's trigger function is invoked. NOTE: This handler will NEVER be called due to this clients trigger function being called, only other clients.

opts: An optional object argument that may be specified to control how localsync operates. Supported values are shown below.

name type default description
tracing boolean false toggles tracing for debugging purposes
logger Object console the logger object to trace to
loglevel string 'info' the log level to use when tracing (error, warn, info, trace)
pollFrequency number 3000 fallback: cookiesync the number in milliseconds that should be used for cookie polling
idLength number 8 fallback: cookiesync the number of characters to use for tracking the current instance (tab)
path string '/' fallback: cookiesync The path to use for cookies
secure boolean false fallback: cookiesync Whether to set the secure flag on cookies or not (not recommended)
httpOnly boolean false fallback: cookiesync Whether to set the http only flag on cookies or not


Interface of returned localsync object

name type defaults description
start function N/A Call to start syncing. Accepts one boolean parameter (default false). If passed true, will run the synchronization on start.
stop function N/A Call to stop syncing
trigger function N/A Call to trigger a sync to occur to all other clients
mechanism string (storage|cookie|server)sync The underlying mechanism that was selected for synchronization
isRunning boolean false Is synchronization currently enabled
isFallback boolean false Is the selected mechanism a fallback strategy
isServer boolean false Is the current client running in a server environment


To setup localsync for use in development run the following steps at CLI:

npm i -g lerna@latest
git clone
cd localsync
lerna bootstrap
lerna run start

Then from your project:

npm link ../localsync/packages/localsync
# start your project, localsync should hot reload as you update its source code. 

Feature requests and pull requests encouraged!


npm i localsync

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