WebSocket emulation - Javascript client
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SockJS family:


SockJS is a browser JavaScript library that provides a WebSocket-like object. SockJS gives you a coherent, cross-browser, Javascript API which creates a low latency, full duplex, cross-domain communication channel between the browser and the web server.

Under the hood SockJS tries to use native WebSockets first. If that fails it can use a variety of browser-specific transport protocols and presents them through WebSocket-like abstractions.

SockJS is intended to work for all modern browsers and in environments which don't support WebSocket protcol, for example behind restrictive corporate proxies.

SockJS-client does require a server counterpart:


  • The API should follow HTML5 Websockets API as closely as possible.
  • All the transports must support cross domain connections out of the box. It's possible and recommended to host SockJS server on different server than your main web site.
  • There is a support for at least one streaming protocol for every major browser.
  • Streaming transports should work cross-domain and should support cookies (for cookie-based sticky sessions).
  • Polling transports are be used as a fallback for old browsers and hosts behind restrictive proxies.
  • Connection establishment should be fast and lightweight.
  • No Flash inside (no need to open port 843 - which doesn't work through proxies, no need to host 'crossdomain.xml', no need to wait for 3 seconds in order to detect problems)

Subscribe to SockJS mailing list for discussions and support.

Live QUnit tests and smoke tests

SockJS comes with some QUnit tests and a few smoke tests (using SockJS-node on the server side). At the moment they are deployed in few places:


SockJS mimics WebSockets API but instead of WebSocket there is a SockJS Javascript object.

First, you need to load SockJS JavaScript library, for example you can put that in your http head:

<script src="http://cdn.sockjs.org/sockjs-0.1.min.js">

After the script is loaded you can establish a connection with the SockJS server. Here's a simple example:

   var sock = new SockJS('http://mydomain.com/my_prefix');
   sock.onopen = function() {
   sock.onmessage = function(e) {
       console.log('message', e.data);
   sock.onclose = function() {

SockJS-client API

SockJS class

Similar to 'WebSocket' class 'SockJS' constructor takes one, or more arguments:

var sockjs = new SockJS(url, protocols, options);

Where options is a hash which can contain:

debug (boolean)
Print more debugging messages using 'console.log'.
devel (boolean)
Development mode. Currently settint it affects only caching of 'iframe.html'.
cookie (boolean)
Disables transports which doesn't support cookies (ie: XDR on IE). Usefull for load balancing based on sticky sessions provided by JSESSIONID cookie.

Although the 'SockJS' object tries to emulate the 'WebSocket' behaviour, it's impossible to support all features. One of the important SockJS limitations is the fact that you're not allowed to open more than one SockJS connection to a single domain at a time. This limitation is caused by a in-browser limit of outgoing connections - usually browsers don't allow opening more than two outgoing connections to a single domain. Single SockJS session requires those two connections - one for downloading data, other for sending messages. Opening second SockJS session at the same time would most probably block and can result in both sessions timing out.

Opening more than one SockJS connection at a time is generally a bad practice. If you absolutely must do it, you can use mutliple subdomains, using different subdomain for every SockJS connection.

Currently SockJS-client is only able to send valid Unicode messages. Never try to send unicode surrogates, Less common characters can be badly encoded into UTF-8 due to various browser quirks, we're working a fix.

Supported transports (#1)

Protocol Browser
WebSocket hixie-76 Chrome 6-12, Safari 5, Firefox 4 (disabled), Opera 11 (disabled)
WebSocket hybi-10 Chrome 14+, Firefox 6+
IFrame via postMessage + EventSource Opera 10.70+, Firefox 3.5+
XDR (CORS) streaming IE 8 (no cookies), Firefox 3.5+, Safari 4+, Chrome 3+
IFrame via postMessage + HtmlFile IE 8 (with cookies)
XDR (CORS) polling IE 8, Firefox 3.5+, Safari 4+, Chrome 3+ (through misbehaving proxy)
IFrame via postMessage + XHR polling Opera 9+
JsonP polling (rough and slow fallback)

Supported transports (#2)

Transport Target browsers Good loadbalancer required Behaving proxy required
WebSocket Chrome, Safari, Firefox 6+ yes yes
XHR streaming (CORS) IE 8 (cookie=no), Firefox <6 no yes
IFrame + EventSource Opera 10.70+ no yes
IFrame + HtmlFile IE 8 (cookie=yes) no yes
XHR polling (CORS) Chrome, Safari, Firefox, IE 8 no no
IFrame + XHR polling Opera no no
JsonP polling any no no


In order to utilize best performance you should use the SockJS-client releases hosted on SockJS CDN. You should use a version of sockjs-client that supports the protocol used by your server. For example:

<script src="http://cdn.sockjs.org/sockjs-0.1.min.js">

A list of files hosted on a CDN is available here: http://sockjs.github.com/sockjs-client/ .

You can also use or CDN via https (using Cloud Front domain name):

<script src="https://d1fxtkz8shb9d2.cloudfront.net/sockjs-0.1.js">

For server-side deployment tricks, especially about load balancing and session stickiness, take a look at the SockJS-node readme.


SockJS-client uses Node.js for testing and Javascript minification. If you want to play with SockJS code, check out the git repo and follow this steps:

cd sockjs-client
npm install

(SockJS-client uses SockJS-node for testing, you may want to link 'node_modules/sockjs' to directory with cloned SockJS-node.)

To generate javascript run:

make sockjs.js

To generate minified javascript run:

make sockjs.min.js

(To generate both run make build.)


To run qunit tests, type:

make test

This command runs script 'tests/server.js' which starts a web server that listens on . It serves static QUnit files and serves a simple SockJS.

To run QUnit tests simply point your browser at

If you want the javascript to be recompiled when the source files are modified and automatically restart the http server run make serve. You will need inotifywait command from package inotify-tools.

Browser Quirks

There are various browser quirks which we don't intend to address:

  • Pressing ESC in Firefox closes SockJS connection (described in socket.io thread).
  • Some older transports may result in a browser showing "spinning wheel" or "busy indicator".
  • In most of the browsers you can't open more than one SockJS connection to one domain at the same time.
  • You shouldn't start SockJS connection before body loads - don't create SockJS objects within head (see #15).