How to mask websockets?

I am trying to write a client in javascript that sends websockets to a node.js/ws server. After a lot of reading I understand that it usefull or even required by RFC to mask all the data that it is sent to the server from client, but I don't understand how to do it.

I have found a few articles like this one:

http://www.htmlgoodies.com/html5/tutorials/making-html5-websockets-work.html

but i couldn't find any starting point or even a function to mask the data. And i really don't understand the concept of masking pretty well. A library / function would be helpful. I'm struggling with this problem for a while and nothing... IF i mask the data that it is sent, i need to 'unmask' on the server side? Do you have any ideas?

Thanks a lot

via John