In addition to the main answer: if you have more than one service on the same server that uses websockets, you might want to do this to separate them, by using a custom path (*):
Node server:
var io = require('socket.io')({ path: '/ws_website1'}).listen(server);
Client HTML:
<script src="/ws_website1/socket.io.js"></script>
...
<script>
var socket = io('', { path: '/ws_website1' });
...
Apache config:
RewriteEngine On
RewriteRule ^/website1(.*)$ http://localhost:3001$1 [P,L]
RewriteCond %{REQUEST_URI} ^/ws_website1 [NC]
RewriteCond %{QUERY_STRING} transport=websocket [NC]
RewriteRule ^(.*)$ ws://localhost:3001$1 [P,L]
RewriteCond %{REQUEST_URI} ^/ws_website1 [NC]
RewriteRule ^(.*)$ http://localhost:3001$1 [P,L]
(*) Note: using the default RewriteCond %{REQUEST_URI} ^/socket.io would not be specific to a website, and websockets requests would be mixed up between different websites!