Node.js socket.io - client emit not seen on server

I'm trying to put together a simple Socket.IO test case in Node.js, but I'm not seeing the events emitted from the client getting to the server.

var http = require("http"),
    io = require("socket.io"),
    io_client = require("socket.io-client"),
    listener, port, server, url,
    socketServer, socketClient,
    i = 0;

port = process.env.LISTEN_PORT || 80;
url = "http://localhost:" + port;
server = http.createServer();
socketServer = io(server);
socketClient = io_client(url);

socketServer.on("connection", function(socket) {
    console.log("server:connection");
    socket.emit("ping", ++i);
    socket.on("pong", function(data) {
        console.log("server:pong", data);
    });
});

socketClient.on("ping", function(data) {
    console.log("client:ping", data);
    socketClient.emit("pong", data);
});

listener = server.listen(port, function() {
    console.log("Listening on port " + listener.address().port);
});

expected output

Listening on port 80
server:connection
client:ping 1
server:pong 1

actual output

Listening on port 80
server:connection
client:ping 1

The connection is made, data is moving from server to client, but I don't see the corresponding events emitted on the server when the client triggers a socket event.

Copyright Notice：Content Author:「rich remer」，Reproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/36229998/node-js-socket-io-client-emit-not-seen-on-server