eventListener on button to exchange data between HTML client and node.js server

I'm trying to pass a json object between a server and client. The server-side this far looks like:

var express = require('express'); // Express web server framework
var request = require('request'); // "Request" library
var querystring = require('querystring');
var cookieParser = require('cookie-parser');

var app = express();
app.use(express.static("public"))

app.get("/", function (req, res) {
})

app.get("/callback", function(req, res){

    console.log('Request received');

    res.writeHead(200, { 
        'Content-Type': 'text/plain',
        'Access-Control-Allow-Origin': '*' 
    });
    req.on('data', function (chunk) {
        console.log('GOT DATA!');
    });

    res.end('{"msg": "OK"}'); 

})

var server = app.listen("8080")
console.log('Server running port 

With the following function attached to a button.

<script type="text/javascript">
document.getElementById("myBtn").addEventListener("click", passArg);

function passArg() {
    console.log("I'm here")
    var xmlhttp = new XMLHttpRequest();

    xmlhttp.onreadystatechange = function() {
        if (xmlhttp.readyState == XMLHttpRequest.DONE ) {
           if (xmlhttp.status == 200) {
               xmlhttp.open("POST", "localhost:8080")

           }
           else if (xmlhttp.status == 400) {
              alert('There was an error 400');
           }
           else {
               alert('something else other than 200 was returned');
           }
        }
    };
    xmlhttp.open("POST", "/callback", true);
    xmlhttp.setRequestHeader("Content-Type", "application/json;charset=UTF-8")
    xmlhttp.send(JSON.stringify({myname: "somename"}))
}
</script>

Now upon clicking the console log "I'm here" triggers fine, but the function returns a 404 error on the POST part of the code. I also don't entirely understand how to pass data back to the client from the server. Would the method res.end() be the correct way?

via Copperwire