hmmmm. after a little search it appears that the socket.io website is down and has been for quite a while so no api reference. So I went to socket.io github:

https://github.com/socketio/socket.io/issues/2883

and there others have reported the same problem so in place of cdn.sock.io-1.3.4.js

there suggested to use https://cdnjs.com/libraries/socket.io/

for the libraries and specifically for 1.3.4: https://cdnjs.cloudflare.com/ajax/libs/socket.io/1.3.4/socket.io.js