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.

Features

• 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.

Install

yarn add localsync

OR

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) => ({
  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) => {
  console.info(
    `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.
 */
userController.start(true);

/**
 * 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) {
  console.warn(
    "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");

console.info(
  userController.mechanism
); /** => '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) {
    userController.stop();
  }
}, 10000);

API

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;

Input

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.

FallbackOptions

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

Output

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
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

Input

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

---

Contributing

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

git clone https://github.com/noderaider/localsync
cd localsync
yarn
yarn build

Then from your project:

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

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^{Feature / bug fixes via pull requests encouraged!}