WebMercatorToLatLon.js

/**
 * webMercatorToLatLong
 *
 * Convert supplied Web Mercator coordinates to WGS84-compatible latitude and longitude coordinates.
 *
 * Sources: http://www.neercartography.com/latitudelongitude-tofrom-web-mercator/
 *          https://developers.arcgis.com/javascript/3/jsapi/esri.geometry.webmercatorutils-amd.html#xytolnglat
 *
 * @param double x  The X coordinate in Web Mercator format
 * @param double y  The Y coordinate in Web Mercator format
 *
 * @return object   {x, y, lat, lon} The given X, Y, and their equivalent
 *                  Latitude and Longitude (WGS84)
 */
function webMercatorToLatLong(x, y) {
  if ((Math.abs(x) > 20037508.3427892) || (Math.abs(y) > 20037508.3427892)) {
    return false;
  }

  const SEMIMAJOR_AXIS    = 6378137.0;

  let lat = (((x / SEMIMAJOR_AXIS) * (180.0 / Math.PI)) - (Math.floor((((x / SEMIMAJOR_AXIS) * (180.0 / Math.PI)) + 180.0) / 360.0) * 360.0)),
      lon = (((Math.PI / 2) - (2.0 * Math.atan(Math.exp((-1.0 * y) / SEMIMAJOR_AXIS)))) * (180 / Math.PI));

  return  { x: x, y: y, lat: lat, lon: lon };
}