index.js

'use strict';

module.exports = function(dmsString) {

    dmsString = dmsString.trim();

    // Inspired by https://gist.github.com/JeffJacobson/2955437
    // See https://regex101.com/r/kS2zR1/3
    var dmsRe = /([NSEW])?\s?(-)?(\d+(?:\.\d+)?)[°º:d\s]?\s?(?:(\d+(?:\.\d+)?)['’‘′:]?\s?(?:(\d{1,2}(?:\.\d+)?)(?:"|″|’’|'')?)?)?\s?([NSEW])?/i;

    var result = {};

    var m1, m2, decDeg1, decDeg2, dmsString2;

    m1 = dmsString.match(dmsRe);

    if (!m1) throw 'Could not parse string';

    // If dmsString starts with a hemisphere letter, then the regex can also capture the 
    // hemisphere letter for the second coordinate pair if also in the string
    if (m1[1]) {
        m1[6] = undefined;
        dmsString2 = dmsString.substr(m1[0].length - 1).trim();
    } else {
        dmsString2 = dmsString.substr(m1[0].length).trim();
    }

    decDeg1 = decDegFromMatch(m1);

    m2 = dmsString2.match(dmsRe);

    decDeg2 = m2 ? decDegFromMatch(m2) : {};

    if (typeof decDeg1.latLon === 'undefined') {
        if (!isNaN(decDeg1.decDeg) && isNaN(decDeg2.decDeg)) {
            // If we only have one coordinate but we have no hemisphere value,
            // just return the decDeg number
            return decDeg1.decDeg;
        } else if (!isNaN(decDeg1.decDeg) && !isNaN(decDeg2.decDeg)) {
            // If no hemisphere letter but we have two coordinates,
            // infer that the first is lat, the second lon
            decDeg1.latLon = 'lat';
            decDeg2.latLon = 'lon';
        } else {
            throw 'Could not parse string';
        }
    }

    // If we parsed the first coordinate as lat or lon, then assume the second is the other
    if (typeof decDeg2.latLon === 'undefined') {
        decDeg2.latLon = decDeg1.latLon === 'lat' ? 'lon' : 'lat';
    }

    result[decDeg1.latLon] = decDeg1.decDeg;
    result[decDeg2.latLon] = decDeg2.decDeg;

    return result;

};

function decDegFromMatch(m) {
    var signIndex = {
        "-": -1,
        "N": 1,
        "S": -1,
        "E": 1,
        "W": -1
    };

    var latLonIndex = {
        "N": "lat",
        "S": "lat",
        "E": "lon",
        "W": "lon"
    };
    var degrees, minutes, seconds, sign, latLon;

    sign = signIndex[m[2]] || signIndex[m[1]] || signIndex[m[6]] || 1;
    degrees = Number(m[3]);
    minutes = m[4] ? Number(m[4]) : 0;
    seconds = m[5] ? Number(m[5]) : 0;
    latLon = latLonIndex[m[1]] || latLonIndex[m[6]];

    if (!inRange(degrees, 0, 180)) throw 'Degrees out of range';
    if (!inRange(minutes, 0, 60)) throw 'Minutes out of range';
    if (!inRange(seconds, 0, 60)) throw 'Seconds out of range';

    return {
        decDeg: sign * (degrees + minutes / 60 + seconds / 3600),
        latLon: latLon
    };
}

function inRange(value, a, b) {
    return value >= a && value <= b;
}