nodemcu, esp8266, ds18s20

The callback needs some sort of identifier, as you can't assume the order of the readings in the callbacks. I have two sensors connected:

var pin=D5; // update for your hardware!
var ow = new OneWire(pin);

ds = require("https://raw.githubusercontent.com/espruino/EspruinoDocs/master/devices/DS18B20.js");

function cb(t) {  
  print({cb_temp:t});
}
var s=ow.search();

console.log( { search:s } );

var sensors = s.map(function (device) {
  return ds.connect(ow, device);
});

setInterval(function() {
   sensors.forEach(function (sensor, index) {
      console.log(index + ": " + sensor.getTemp(cb));
  });
},2000);

{
  "search": [
    "28cc2e230500006b",
    "283260dc04000001"
   ]
 }
=undefined
0: undefined
1: undefined
{ "cb_temp": 25.8125 }
{ "cb_temp": 24.9375 }
0: undefined
1: undefined
{ "cb_temp": 25.8125 }
{ "cb_temp": 24.9375 }
0: undefined
1: undefined

There are a couple of options:

if (callback) callback(temp);

if (callback) callback(this);
function cb(t) {
print({cb_temp:t.getTemp()});
}

if (callback) callback(temp,this.sCode);

or probably most flexible:

if (callback) callback(temp,this);
And then you have the object if you need it...