I'm expecting a different output....

As you say, your problem is that takeReading returns immediately - even though it sets a timeout using setTimeout.

This is classic JS though. You need to use a callback function:

Sensor.prototype.takeReading = function(callback) {
  // ...
  setTimeout(function () {
    // ...
    callback(sensorValue);
  }, w);
};


function getAllSensorData() {
  restemp.takeReading(function(resTempVal)­ {
    console.log(resTempVal);
  });
}