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    2.0.6 • Public • Published


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

    See this blog post 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 Jest.
    • Written with TypeScript.

    Build Status



    yarn add localsync


    npm install localsync

    Version 2

    How to use

    import { localsync } from "localsync";
    /** Create a publisher that optionally transforms some data to a payload to be transmitted to other tabs. */
    const publisher = (userID, firstName, lastName) => ({
    /** Create a subscriber that will receive published payloads on all tabs that did not trigger the sync. */
    const subscriber = (current, previous, url) => {
        `Another tab at url ${url} switched user from "${previous.firstName} ${previous.lastName}" to "${current.firstName} ${current.lastName}".`
      // do something with `current.userID`
     * Create a synchronizer channel.
     * @remarks
     * `localsync` supports N number of synchronizers for different things across your app.
     * The key 'user' defines a localsync synchronization channel.
    const createUserSync = localsync({ channel: "user" });
     * Create a controller for the synchronizer.
     * @remarks
     * To create the controller, you must pass the publisher and the subscriber.
    const userController = createUserSync(publisher, subscriber);
     * 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.
    if (userController.isFallback) {
        "browser doesnt support local storage synchronization, falling back to cookie synchronization."
     * `isServer` can be used to detect if the current code is running in a server environment (for SSR).
    if (userController.isServer) {
      console.debug("Detected a SSR environment.");
    /** Trigger an action that will get handled on other tabs. */
    userController.trigger(1, "jimmy", "john");
    /** Trigger an action that will get handled on other tabs. */
    userController.trigger(1, "jimmy", "john");

    ); /** => 'storagesync' on chrome, 'cookiesync' on IE */
    setTimeout(() => {
      /** Trigger another action in 5 seconds. */
      userController.trigger(2, "jane", "wonka");
    }, 5000);
    setTimeout(() => {
      /** If its still running, stop syncing in 10 seconds. */
      if (userController.isRunning) {
    }, 10000);


    const createSync = localsync({ channel: string, level?: LogLevel, fallback?: FallbackOptions })
    const sync = createSync((publisher: (...args) => TMessage, subscriber: (message: TMessage) => void);
    const { start, stop, trigger, isRunning, isFallback } = sync;


    name type default description
    channel string required a string that is used for this synchronization instance.
    level string 'INFO' the log level to use when tracing (ERROR, WARN, INFO, DEBUG)
    fallback object see below optional fallback options if you need more control over cookie fallback.


    name type default description
    pollFrequency number 3000 The number in milliseconds that should be used for cookie polling
    path string '/' The path to use for cookies
    secure boolean false Whether to set the secure flag on cookies or not (not recommended)
    httpOnly boolean false Whether to set the http only flag on cookies or not


    Interface of localsync returned controller

    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

    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).

    Version 1 API

    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


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

    git clone
    cd localsync
    yarn build

    Then from your project:

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

    Feature / bug fixes via pull requests encouraged!


    npm i localsync

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