How to access a JSON object?

My goal is to calculate sensor readings and then save them to a JSON object then console.log output a specific value. I'm not sure why I'm not able to access my json value. I keep getting error:

Uncaught Error: Field or method "restempvalue" does not already exist, and can't create it on undefined
 at line 153 col 10
sensorObj.restempvalue = val;
         ^

code:

var sensorReadings = { "restempvalue" : 0, "phvalue" : 0, "ecvalue" : 0 };
var sensorObj = JSON.parse(sensorReadings);

function getAllSensorData() {
  /*restemp.takeReading(function(val) {
    console.log(val);
  });

  ph.takeReading(function(val) {
    console.log(val);
  });*/

  ec.takeReading(function(val) {
    sensorObj.restempvalue = val;
    console.log(val);
  });
}