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Sysend.js logo

npm bower downloads jsdelivr

Web application synchronization between different tabs

sysend.js is a small library that allows to send messages between pages that are open in the same browser. It also supports Cross-Domain communication (Cross-Origin). The library doesn't have any dependencies and uses the HTML5 LocalStorage API or BroadcastChannel API. If your browser don't support BroadcastChannel (see Can I Use) then you can send any object that can be serialized to JSON. With BroadcastChannel you can send any object (it will not be serialized to string but the values are limited to the ones that can be copied by the structured cloning algorithm). You can also send empty notifications.

Tested on:

GNU/Linux: in Chromium 34, FireFox 29, Opera 12.16 (64bit)
Windows 10 64bit: in IE11 and Edge 38, Chrome 56, Firefox 51
MacOS X El Captain: Safari 9, Chrome 56, Firefox 51

Note about Safari 7+ and Cross-Domain communication

All cross-domain communication is disabled by default with Safari 7+. Because of a feature that block 3rd party tracking for iframe, and any iframe used for cross-domain communication runs in sandboxed environment. That's why this library like any other solution for cross-domain comunication, don't work on Safari.


Include sysend.js file in your html, you can grab the file from npm:

npm install sysend

or bower

bower install sysend

you can also get it from CDN:

or jsDelivr:

jsDelivr will minify the file. From my testing it's faster than


window.onload = function() {
    sysend.on('foo', function(data) {
    var input = document.getElementsByTagName('input')[0];
    document.getElementsByTagName('button')[0].onclick = function() {
        sysend.broadcast('foo', { message: input.value });

If you want to add support for Cross-Domain communication, you need to call proxy method with url on target domain that have proxy.html file.


on Firefox you need to add CORS for the proxy.html that will be loaded into iframe (see Cross-Domain LocalStorage)

if you want to send custom data you can use serializer (new in 1.4.0). Example serializer can be json-dry.

sysend.serializer(function(data) {
    return Dry.stringify(data);
}, function(string) {
    return Dry.parse(string);


Screen capture of Operating System Windows dragging and moving around animation



sysend object:

function description arguments Version
on(name, callback) add event handler for specified name name - string - The name of the event
callback - function (object, name) => void
off(name [, callback]) remove event handler for given name, if callback is not specified it will remove all callbacks for given name name - string - The name of the event
callback - optional function (object, name) => void
broadcast(name [, object]) send any object and fire all events with specified name (in different pages that register callback using on). You can also just send notification without an object name - string - The name of the event
object - optional any data
proxy(<urls>) create iframe proxy for different domain, the target domain/URL should have proxy.html
file. If url domain is the same as page domain, it's ignored. So you can put both proxy calls on both
url - string 1.3.0
serializer(to_string, from_string) add serializer and deserializer functions both arguments are functions (data: any) => string 1.4.0
emit(name, [, object]) same as broadcast() but also invoke the even on same page name - string - The name of the event
object - optional any data
post(<window_id>, [, object]) send any data to other window window_id - string of the target window
object - any data
list() returns a Promise of objects {id:<UUID>, primary} for other windows, you can use those to send a message with post() NA 1.6.0
track(event, callback) track inter window communication events event - any of the strings: "open", "close", "primary",
"secondary", "message"
callback - different function depend on the event:
* "message" - {data, origin} - where data is anything the post() sends, and origin is id of the sender.
* "open" - {count, primary, id} when new window/tab is opened
* "close" - {count, primary, id, self} when window/tab is closed
* "primary" and "secondary" function has no arguments and is called when window/tab become secondary or primary.
* "ready" - event when tracking is ready.
1.6.0 except ready - 1.10.0
untrack(event [,callback]) remove sigle event listener all all listeners for a given event event - any of the strings 'open', 'close', 'primary', 'secondary', or 'message'. 1.6.0
isPrimary() function returns true if window is primary (first open or last that remain) NA 1.6.0
channel() function restrict cross domain communication to only allowed domains. You need to call this function on proxy iframe to limit number of domains (origins) that can listen and send events. any number of origins (e.g. 'http://localhost:8080' or '') you can also use valid URL. 1.10.0

To see details of using the API, see demo.html source code or TypeScript definition file.


Copyright (C) 2014-2022 Jakub T. Jankiewicz
Released under the MIT license

This is free software; you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.