WebSocket emulation - Javascript client
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SockJS family:

Work in progress:


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-0.2.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, _reserved, options);

Where options is a hash which can contain:

  • debug (boolean)

    Print some debugging messages using 'console.log'.

  • devel (boolean)

    Development mode. Currently setting it disables caching of the 'iframe.html'.

  • protocols_whitelist (list of strings)

    Sometimes it is useful to disable some fallback protocols. This option allows you to supply a list protocols that may be used by SockJS. By default all available protocols will be used, which is equivalent to supplying: "['websocket', 'xdr-streaming', 'xhr-streaming', 'iframe-eventsource', 'iframe-htmlfile', 'xdr-polling', 'xhr-polling', 'iframe-xhr-polling', 'jsonp-polling']"

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.

Supported transports, by browser

Browser Websockets Streaming Polling
IE 6, 7 no no jsonp-polling
IE 8, 9 (cookies=no) no xdr-streaming † xdr-polling †
IE 8, 9 (cookies=yes) no iframe-htmlfile iframe-xhr-polling
Chrome 6-12 hixie-76 xhr-streaming xhr-polling
Chrome 14+ hybi-10 xhr-streaming xhr-polling
Firefox <10 no ‡ xhr-streaming xhr-polling
Firefox 10+ hybi-10 xhr-streaming xhr-polling
Safari 5 hixie-76 xhr-streaming xhr-polling
Opera 10.70+ no ‡ iframe-eventsource iframe-xhr-polling
Konqueror no no jsonp-polling
  • : IE 8+ supports XDomainRequest, which is esentially a modified AJAX/XHR that can do requests across domains. But unfortunately it doesn't send any cookies, which makes it inaproppriate for deployments when the load balancer uses JSESSIONID cookie to do sticky sessions.

  • : Firefox 4.0 and Opera 11.00 and shipped with disabled Websockets "hixie-76". They can still be enabled by manually changing a browser setting.

Supported transports, by name

Transport References
websocket (hixie-76) draft-hixie-thewebsocketprotocol-76
websocket (hybi-10) draft-ietf-hybi-thewebsocketprotocol-10
xhr-streaming Transport using Cross domain XHR streaming capability (readyState=3).
xdr-streaming Transport using XDomainRequest streaming capability (readyState=3).
iframe-eventsource EventSource used from an iframe via postMessage.
iframe-htmlfile HtmlFile used from an iframe via postMessage.
xhr-polling Long-polling using cross domain XHR.
xdr-polling Long-polling using XDomainRequest.
iframe-xhr-polling Long-polling using normal AJAX from an iframe via postMessage.
jsonp-polling Slow and old fashioned JSONP polling. This transport will show "busy indicator" (aka: "spinning wheel") when sending data.


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-0.2.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-0.2.js">

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

Development and testing

SockJS-client needs Node.js for running a test server and JavaScript minification. If you want to work on SockJS-client source code, check out the git repo and follow this steps:

cd sockjs-client
npm install --dev

To generate JavaScript run:

make sockjs.js

To generate minified JavaScript run:

make sockjs.min.js

(To generate both run make build.)


Once you compiled SockJS-client you may want to check if your changes pass all the tests. To run the tests you need a server that can answer various SockJS requests. A common way is to use SockJS-node test server for that. To run it (by default it will be listening on port 8081):

cd sockjs-node
npm install --dev
make build
make test_server

At this point you're ready to run a SockJS-client server that will server your freshly compiled JavaScript and various static http and javscript files (by default it will run on port 8080).

cd sockjs-client
make test

At that point you should have two web servers running: sockjs-node on 8081 and sockjs-client on 8080. When you open the browser on http://localhost:8080/ you should be able run the QUnit tests against your sockjs-node server.

If you look at your browser console you will see warnings like that:

Incompatibile SockJS! Main site uses: "a", the iframe: "b".

This is due to a fact that SockJS-node test server is using compiled javascript from CDN, rather than your freshly compiled version. To fix that you must amend sockjs_url that is used by SockJS-node test server. Edit the config.js file:

vim sockjs-node/examples/test_server/config.js

And replace sockjs_url setting which by default points to CDN:

sockjs_url: 'http://cdn.sockjs.org/sockjs-0.2.min.js',

to a freshly compiled sockjs, for example:

sockjs_url: 'http://localhost:8080/lib/sockjs.js',

Also, if you want to run tests agains SockJS server not running on localhost:8081 you may want to edit the tests/config.js file.

Additionally, if you're doing more serious development consider using make serve, which will automatically reload the server when you modify the source code.

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).
  • Jsonp-polling transport will show a "spinning wheel" (aka. "busy indicator") when sending data.
  • In most of the browsers you can't open more than one SockJS connection to one domain at the same time (with the exception of native websockets).
  • Although SockJS is trying to escape any strange Unicode characters (even invalid ones - like surrogates \xD800-\xDBFF or \xFFFE and \xFFFF) it's advisable to use only valid characters. Using invalid characters is a bit slower, and may not work with SockJS servers that have a proper Unicode support.
  • Having a global function called onmessage or such is probably a bad idea, as it could be called by the built-in postMessage API.
  • Serving an html page that uses SockJS from file:// url will not work. This is due to a badly thought through CORS specification It is impossible to receive response to an Ajax request with cookies set (withCredentials set to true) sent from a file:// origin.
  • From SockJS point of view there is nothing special about SSL/HTTPS. Connecting between unencrypted and encrypted sites should work just fine.