How to access a JSON object?

...with that said - and to come full circle - your initial code is missing only two (2) single quotes in the first line...

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

Note: JSON is always a String (object)... JS Object Notated (as a string). It is the literal representation of an object, like the literal representation of a simple string is always in "DoubleQuotes" or 'SingleQuotes', or like literal number, for example 6, is just the 'naked' number, or like the literal booleans true and false ('naked' symbols / reserved words), or the null and undefined object null and undefined (also 'naked' symbols / reserved words)... versus the variable referencing - or value holder - of an object, string, boolean, number, null and undefined respectively.