为什么我在Javascript中收到NaN错误？

Question

在以下代码中，NaN获得了fulldata。其他所有内容的值均为0。我不知道为什么会这样。问题是什么？我需要改变什么？

var fulldata = 0;
var pulses = 0;
var flowrate; // liters per minute
var value = 0;
var d = new Date();
var liters = 0;
var gallons = 0;
var cost = 0;
var file = ('/var/lib/cloud9/data.txt');
var oldTime;
var oldAmount = b.readTextFile(file);
var totalCost = oldAmount * 0.00313;

app.listen(8090);
console.log('Server running on: http://' + getIPAddress() + ':8090');

var Sensor = "P8_19";
b.pinMode('P8_19', b.INPUT);
console.log(b.digitalRead('P8_19'));
b.digitalWrite('P8_19', b.HIGH);
oldTime = Date.now();
b.attachInterrupt('P8_19', pulseCounter, b.FALLING);

io.on('connection', function(socket) {

  var tm = setInterval(function() {
    var newTime = Date.now();
    var timeDifference = (newTime - oldTime);

    if (timeDifference > 1000) {
      b.detachInterrupt('P8_19');
      b.pinMode('P8_19', b.INPUT);
      value = b.digitalRead('P8_19');
      //console.log(timeDifference);

      flowrate = (1000.0 / (timeDifference)) * ((pulses) / 4.5);
      //console.log(flowrate);
      oldTime = newTime;

      liters = flowrate / 60;
      console.log(liters + "L");
      gallons = (liters * 0.264);
      var newData = gallons;
      console.log(gallons + "G");
      cost = gallons * 0.00313;
      fulldata += newData;
      totalCost += cost;
      console.log(fulldata);
      console.log("$" + cost);

      socket.emit('water', {'water': fulldata});
      socket.emit('money', {'money': cost});
      socket.emit('month', {'month': totalCost});

      pulses = 0;
      b.attachInterrupt('P8_19', pulseCounter, b.FALLING);
    }
  }, 500);

  socket.on('disconnect', function() {
    oldAmount = (oldAmount + fulldata);
    b.writeTextFile(file, oldAmount);
    clearInterval(tm);
  });

});