JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" id=lon value=160.3456789012>
<input type="text" id=lat value=30.3456789012>
<button id="process">submit</button>

<br>

<div id=result></div>
<br>

<div id=original></div>

<input type="text" id=num2>
<button id="process2">submit</button>

JavaScript

enc = function(lon, lat) {
  var shiftedLon = Number.parseFloat(lon) + 180;
  var shiftedLat = Number.parseFloat(lat) + 90;
  
  //var shiftedLon = lon + 180;
  //var shiftedLat = lat + 90;
  
  var expandedLon = shiftedLon * 1000000;
  var expandedLat = shiftedLat * 1000000;

  var truncatedLon = Math.trunc(expandedLon);
  var truncatedLat = Math.trunc(expandedLat);


  //console.log((shiftedLat * Math.pow(10, 16)));
  console.log(truncatedLon);
  console.log(truncatedLat);
  var num = (truncatedLon * Math.pow(10, 9)) + truncatedLat;
  //console.log((shiftedLat * Math.pow(10, 16)) + shiftedLon)

  return num;
};

dec = function(num) {
debugger;

  var lat = num % 1000000000; // extract latitude int
  lat = Number.parseFloat(lat) / 1000000; // get decimals
  lat = lat - 90; // shift back  
  lat = Math.trunc(lat * 1000000) / 1000000;
  console.log(lat)
  var lon = (num - (num % 1000000000)) / Math.pow(10,9); // extract longitude int
  lon = Number.parseFloat(lon / 1000000); // get decimals
  lon = lon - 180; // shift back
  lon = lon.toFixed(6);
  //lon = Math.trunc(lon * 1000000) / 1000000;
console.log(lon)
  
  return [lon, lat];
}

$("#process").click(function() {
  var e = enc($("#lon").val(), $("#lat").val());
  $("#num2").val(e)
var res = dec(e);
  $("#original").text(res[0] + ", " + res[1]);
});

$("#process2").click(function() {
var res = dec($('#num2').val());
  $("#original").text(res[0] + ", " + res[1]);
});