browser.js

var xhr = new XMLHttpRequest();
xhr.open('POST', './mac-address', true);
var formData = new FormData();
formData.append('mac-address', mac);
xhr.send(formData);

server.js

var express = require('express');
var bodyParser = require('body-parser');

app.use(bodyParser.urlencoded({ extended: true }));
app.post('/mac-address', function(req, res) {
  // TODO: check if valid mac
  console.log(req);
  console.log(req.body);
  june_mac = req.body;
  if (june_peripheral !== null) {
    june_peripheral.disconnect(function(error) {
      if (error !== null) {
        res.status(500);
        res.send(error);
        console.log('Failed to diconnect');
      } else {
        res.send('ok');
      }
    });
  }
});