How to access a JSON object?

Properties - value (state) and function (behavior) properties - of a JavaScript object ("object" === typeof objVariable) can be accessed in two ways: static and dynamic.

Example to access a value property - read and 'write' (assign):

1. static get and set access:

   
   2. dynamic get and set access:
   
   

   var phPropName = "phValue"; ph = sensorObject[phPropName]; sensorObject[phPropName] = 7.0;```

Same static (hard coded) and dynamic (by variable for property name) access goes for functions (behavior). The example below uses the SensorReading prototype based object-oriented approach to have multiple (maxNumOfLastReadings) sensorReading s (objects / instances of SensorReading prototype / class) stored in sensorReadings singleton object (singleton - only one instance exists). Two sensorObjects read values, create a reading with the read values and store it in the readings in different intervals.

var sensorModule = require("sensorModuleName");

// constructor for a reading accepting a sensor id and read restTemp, ph and ec
var SensorReading = function(id, restTemp, ph, ec) {
  this.ts = getTime(); // set current system time stamp in seconds
  this.id = sensorId; // set sensor id
  this.restTemp = restTemp; // set restTemp
  this.ph = ph; // set ph
  this.ec = ec; // set ec
}

// constructor for a sensor (prototype - or class - definition)
var SensorObject = function(sensorId, sensorModule, connectInfo1,... ) {
  this.sensorId = sensorId;
  this.sensor = sensorModule.connect(connectInfo1, conectInfo2,...);
}
// method (function / behavior) to take a reading and store it in readingsObject
SensorObject.prototype.readAndStoreIn = function(readingsObject) {
  var restTemp = this.sensor.read("temp"); // ...code detail depends on the module
  var ph = this.sensor.read("ph"); // ...code detail depends on the module
  var ec = this.sensor.read("ec"); // ...code detail depends on the module
  var reading = new SensorReading(this.sensorId, restTemp, ph, ec); // create a reading
  readingsObject.store(reading);
};
  
// literal 'constructor' for sensorReadings singleton object
var sensorReadings =
{ maxNumOfLastReadings: 10 // keep max 10 readings
, readings: [] // most recent reading is in readings[0]
, store: function(reading) {
    if (this.readings.length >= this.maxNumOfLastReadings) { // chop oldest off
      this.readings = this.slice(0, this.maxNumOfLastReadings - 1); }
    this.readings.splice(o, 0, reading);
  }
, clearReadings: function() { this.readings = []; }
, getReadings: function() { return [].concat(this.readings); }
, getReadingByIndex: function(x) { // return null if x 'outside' of readings
    return (((x>=0)&&(x<this.readings.length)) ? this.readings[x] : null); }
, getLastReading: function() { // return most recent reading
    return this.getReadingByIndex(0); }
, getReadingsAsJSON: function() { return JSON.stringify(this.readings); }
, getReadingsAsHtmlTableString() {
    h  = '<table><tr><th>#</th>';
    h += '<th>ts</th><th>id</th>';
    h += '<th>restTemp</th><th>ph</th><th>ec</th>­</tr>';
    this.readings.forEach(function(r, x) { 
      h += '<tr><td>'+x+'</td>';
      h += '<td>'+r.ts+'</td><td>'+r.id+'</td>';
      h += '<td>'+r.restTemp+'</td><td>'+r.ph+'</td­><td>'+r.ec+'</td></tr>';
    }); h += '</table>';
    return h;
  }
, getReadingsAsHtmlTableString2() { // dynamic access in loop
    var ps = ["ts","id","restTemp","ph","ec"];
    h  = '<table><tr><th>#</th>';
    ps.forEach(function(p){ h += '<th>'+p+'</th>'; });
    h += '</tr>';
    this.readings.forEach(function(r, x) { 
      h += '<tr><td>'+x+'</td>';
      ps.forEach(function(p){ h += '<th>'+r[p]+'</th>'; });
      h += '</tr>';
    }); h += '</table>';
    return h;
  }
}

var sensorAtLocation1 = new SensorObject("LocInt",<connect parmsInt>,...); 
var sensorAtLocation2 = new SensorObject("LocOut",<connect parmsOut>,...); 
setTimeout(function(){ 
    sensorAtLocation1.readAndStoreIn(sensorR­eadings);
  }, 15000); // every 15 seconds
setTimeout(function()setTimeout(function­(){
    sensorAtLocation2.readAndStoreIn(sensorR­eadings);
  }, 45000); }, 5000); first time after 50 seconds, after that every 45 seconds

Interval settings produce readings at 15, 30, 45, 50, 60, 75, 90, 95, 105, 120, 135, 140,... seconds (italics are LOC1 readings, bold are LOC2 readings).

Complete above code with sensor specifics, upload / run it, and then ask readingsObjects for different things - (first) one line after the other:

readingsObject.getLastReading();
readingsObject.getReadings();
readingsObject.getReadingByIndex(4);
readingsObject.getReadingsAsJSON();
readingsObject.getReadingsAsHtmlTableStr­ing2();
...
readingsObject["getLastReading"]();
var getReadingsMethodName = "getReadings";
readingsObject[getReadingsMethodName]();­
var rx = readingsObject["getReadingByIndex"](4);
var ps = ["ts","sensorId","restTemp","ph"];
if (rx) ps.forEach(function(p){ console.log(p+"="+rx[p]); });
readingsObject["getReadingsAsJSON"]();
...

PS:

• code not fully tested (yet)
  
• in 1v88, '<td>${r[p]}</td>' can replace '<th>'+r[p]+'</th>' in line 59