transform epsg:4326 <=> epsg:3857

transform_4326_3857.js

/**
 * epsg:3857 <=> epsg:4326
 * https://wiki.openstreetmap.org/wiki/Mercator
 */

const EARTH_RADIUS = 6378137; //equatorial radius in m

function degree_to_radian(a) {
  return a / (180 / Math.PI);
}

function radian_to_degree(a) {
  return a * (180 / Math.PI);
}

function transform_3857_to_4326(x, y) {
  const lon = radian_to_degree(x / EARTH_RADIUS);
  const lat = radian_to_degree(2 * Math.atan(Math.exp(y / EARTH_RADIUS)) - Math.PI / 2);
  return { lon, lat };
}

function transform_4326_to_3857(lon, lat) {
  const x = degree_to_radian(lon) * EARTH_RADIUS;
  const y = Math.log(Math.tan(degree_to_radian(lat) / 2 + Math.PI / 4)) * EARTH_RADIUS;
  return { x, y };
}