It is under the site's learn tab, but never mentioned in the API/library documentation.
It's also weird because the only difference between a WebSocket client and server is that a server is a connection listener that responds to the handshake. The client is responsible for absolutely everything else: send messages, receive messages, initiate the connection, kill the connection, ping/pong. After the client accepts the server's handshake response the server becomes the other client performing exactly the same functionality. Node could have been doing that all along since adding client support.
