I'm expecting a different output....

I'm getting undefined for resTempVal and undefined outputs before console.log(resTempVal); So, i'm assuming there's a small delay while taking the sensor reading and the variable resTempVal isn't set. Is there a way to wait until resTempVal gets set before I continue?

code:

Sensor.prototype.takeReading = function() {
  var a = this.sensorAddress;
  var c = this.cmdTable.Reading.R.cmd;
  var w = this.cmdTable.Reading.R.wait;

  var d = "";
  var sensorStatus = "";
  var sensorValue = "";

  var captureSensorData = false;

  I2C1.writeTo(a, c);

  setTimeout(function () {
    d = I2C1.readFrom(a, 9);

    if (d.length > 0) {
      var dContent = d[0];

      switch (dContent) {
        case 1:
          sensorStatus = 1;
          captureSensorData = true;
          break;
        case 255:
          sensorStatus = 255;
          break;
        case 254:
          sensorStatus = 254;
          break;
        case 2:
          sensorStatus = 2;
          break;
      }

      if (captureSensorData === true) {
        for (i = 1; i < d.length; i++) {
          sensorValue += String.fromCharCode(d[i]);
        }
      }

      captureSensorData = false;
      console.log(sensorValue); //<-- I get output
      return sensorValue;
    }
  }, w);
};

var restemp = new Sensor("RES TEMP", "temp", 77);
var ph = new Sensor("PH", "ph", 78);
var ec = new Sensor("EC", "ec", 99);

function getAllSensorData() {
  resTempVal = restemp.takeReading();
  console.log(resTempVal); //<-- undefined
}

setInterval(function () {
  console.log("Taking Reading: ");
  getAllSensorData();
}, 5000);