Using zipcode lat and lon to calculate distance between two points

distance.js

// using geometry to calculate distance between to points by latitude and longitude,
//units are of type enum, 'M' for distance in miles, 'K' for distance in kilometers or 'N' in Nautic Miles 

module.exports.calculateDistance = (a, b, unit = 'M') => {
    //if points received are not valid
    if (!a || !b || !a.lat || !a.lon || !b.lat || !b.lon) {
        return 0
    }
    let lat1 = a.lat;
    let lon1 = a.lon;
    let lat2 = b.lat;
    let lon2 = b.lon

    if ((lat1 == lat2) && (lon1 == lon2)) {
        return 0;
    }
    else {
        let radlat1 = Math.PI * lat1 / 180;
        let radlat2 = Math.PI * lat2 / 180;
        let theta = lon1 - lon2;
        let radtheta = Math.PI * theta / 180;
        let dist = Math.sin(radlat1) * Math.sin(radlat2) + Math.cos(radlat1) * Math.cos(radlat2) * Math.cos(radtheta);
        if (dist > 1) {
            dist = 1;
        }
        dist = Math.acos(dist);
        dist = dist * 180 / Math.PI;
        dist = dist * 60 * 1.1515;
        if (unit == "K") { dist = dist * 1.609344 }
        if (unit == "N") { dist = dist * 0.8684 }
        return dist;
    }
}