index.js

var serialport = require('serialport'),
   SerialPort = serialport.SerialPort, // make a local instance
   portName = "/dev/cu.usbmodem1451",  // replace the string with your own
   latestData = 0;

var myPort = new SerialPort(portName, {
  baudRate: 9600,
  // look for return and newline at the end of each data packet
  parser: serialport.parsers.readline("\r\n")
});

myPort.on('open', showPortOpen);
myPort.on('data', saveLatestData);
myPort.on('close', showPortClose);
myPort.on('error', showError);

function showPortOpen() {
   console.log('port open. Data rate: ' + myPort.options.baudRate);
}

function saveLatestData(data) {
   console.log(data);
   latestData = data;
}

function showPortClose() {
   console.log('port closed.');
}

function showError(error) {
   console.log('Serial port error: ' + error);
}