https://goodtogostore.com/src/js/utils/geo.js

1/**
2 * geo.js — shared geo helpers.
3 *
4 * Extracted from three identical copies that had drifted into
5 * reservationList.js, reservationDetail.js and storeLocator.js. Each lived
6 * inside its own IIFE, so they could not share code and any fix had to be made
7 * three times.
8 */
9
10/**
11 * Great-circle distance between two points, rounded to whole kilometres.
12 *
13 * The extracted copies all applied Math.abs() to the outlet latitude before
14 * differencing it against the user's, which is wrong in principle — it mirrors
15 * southern-hemisphere outlets onto the northern one. It happened to be harmless
16 * for this platform (every outlet is in India, north of the equator) and is
17 * corrected here rather than preserved.
18 *
19 * @param {number} uLat user latitude
20 * @param {number} uLng user longitude
21 * @param {number} oLat outlet latitude
22 * @param {number} oLng outlet longitude
23 * @returns {number} distance in km, rounded
24 */
25export function distanceKm(uLat, uLng, oLat, oLng) {
26    var rad = Math.PI / 180;
27    var a = Math.pow(Math.sin((uLat - oLat) * rad / 2), 2) +
28            Math.cos(uLat * rad) * Math.cos(oLat * rad) *
29            Math.pow(Math.sin((uLng - oLng) * rad / 2), 2);
30    // Constant kept exactly as the extracted copies had it (3956 mi → km), so
31    // displayed distances don't shift for existing users. Only the Math.abs()
32    // quirk above is corrected here.
33    return Math.round(1.609344 * 3956 * 2 * Math.asin(Math.sqrt(a)));
34}