convert Date object to ISO string with zone info

zonedIso.js

const n = $ => Number.isNaN($);

const p = $ => {
    if ($ < 1) {
        return '00';
    }
    if ($ < 10) {
        return '0' + $;
    }
    return '' + $;
};

const q = $ => {
    if ($ < 1) {
        return '000';
    }
    if ($ < 10) {
        return '00' + $;
    }
    if ($ < 100) {
        return '0' + $;
    }
    return '' + $;
};


// @see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/toISOString

const zonedIso = (date) => {
    //
    if (!(date instanceof Date)) {
        return '';
    }

    // first part
    const Y = date.getFullYear();
    const M = date.getMonth() + 1;
    const D = date.getDate();

    if (n(Y) || n(M) || n(D)) {
        return '';
    }

    // T part
    const h = date.getHours();
    const m = date.getMinutes();
    const s = date.getSeconds();

    // . part
    const k = date.getMilliseconds();

    if (n(h) || n(m) || n(s) || n(k)) {
        return '';
    }

    // Z part
    const tzo = -date.getTimezoneOffset();
    const abs = Math.abs(tzo);

    if (n(tzo)) {
        return '';
    }

    const tzs = tzo >= 0 ? '+' : '-';
    const tzh = Math.floor(abs / 60);
    const tzm = abs % 60;

    // fragments
    const dateFragment = `${p(Y)}-${p(M)}-${p(D)}`;
    const timeFragment = `${p(h)}:${p(m)}:${p(s)}.${q(k)}`;
    const zoneFragment = `${tzs}${p(tzh)}:${p(tzm)}`;
    //
    return `${dateFragment}T${timeFragment}${zoneFragment}`;
};


const now = zonedIso(new Date());
console.log(now, '.length:', now.length);
console.log(zonedIso());
console.log(zonedIso(new Date(NaN)));
console.log(zonedIso(new Date('2222-03-31')));