How to access a JSON object?

sensorReadings is already an object... and JSON.parse(aJSONString) expects a string as argument... ;-), therefore, expression var sensorObj = JSON.parse(sensorReadings); throws error (in Browser: ...Uncaught SyntaxError: Unexpected token o in JSON at position 1(…); what error is thrown in Espruino I cannot figure out right now, because have no Espruino (or emulator) at hand).

This code (see second line)

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

fixes your issue, but there is not point to first build a JSON string from sensorReadings object and second - right away - parse that JSON string back into an object... ;-)

Plain assignment gets you there - because it is the same:

var sensorObj = sensorReadings;

Therefore, you can drop sensorReadings completely and just use:

var sensorObject = { "restempvalue" : 0, "phvalue" : 0, "ecvalue" : 0 };

It is a different story though when you want to write you readings onto a SD memory card, or transmit to wherever. For that you would

A) for initialization

1. create a sensorReadings object
   
2. stringify the object using 'JSON.stringify() to a sensorReadingsAsString string
   
3. write or send that sensorReadingsAsString string to the storage;
   
   

B) for update that readings on the storage, you

1. retrieve them from storage as sensorReadingsAsString
   
2. parse it into the `sensorReadings' object
   
3. update the desired property with value
   
4. stringify the object using 'JSON.stringify() to a sensorReadingsAsString string
   
5. write or send that sensorReadingsAsString string 'back' to the storage