solution.js

/**
 * @param {string} IP
 * @return {string}
 */
const numMap = {
    0: true,
    1: true,
    2: true,
    3: true,
    4: true,
    5: true,
    6: true,
    7: true,
    8: true,
    9: true,
};
function isIPv4 (IP) {
    const arr = IP.split('.');
    if (arr.length !== 4) {
        return false;
    }
    for (let i = 0; i < 4; i++) {
        if (arr[i].length < 1 || arr[i].length > 3 || (arr[i].length > 1 && arr[i][0] === '0')) {
            return false;
        }
        for (let j = 0; j < arr[i].length; j++) {
            if (numMap[arr[i][j]] === undefined) {
                return false;
            }
        }

        if (+arr[i] > 255) {
            return false;
        }
    }
    return true;
}
const map = {
    0: true,
    1: true,
    2: true,
    3: true,
    4: true,
    5: true,
    6: true,
    7: true,
    8: true,
    9: true,
    a: true,
    A: true,
    b: true,
    B: true,
    c: true,
    C: true,
    d: true,
    D: true,
    e: true,
    E: true,
    f: true,
    F: true,
};

function isIPv6 (IP) {
    const arr = IP.split(':');
    if (arr.length !== 8) {
        return false;
    }
    for (let i = 0; i < 8; i++) {
        if (arr[i].length < 1 || arr[i].length > 4) {
            return false;
        }
        for (let j = 0; j < arr[i].length; j++) {
            if (map[arr[i][j]] === undefined) {
                return false;
            }
        }
    }

    return true;
}

var validIPAddress = function (IP) {
    if (IP.includes('.')) {
        return isIPv4(IP) ? 'IPv4' : 'Neither';
    } else if (IP.includes(':')) {
        return isIPv6(IP) ? 'IPv6' : 'Neither';
    }
    return 'Neither';
};