formatDuration.js

function formatDuration (seconds) {
  if (seconds === 0) {
    return 'now';
  }
  const dif = seconds * 1000;
  const difSeconds =  Math.floor(dif / 1000) % 60;
  const difMinutes =  Math.floor(dif / 60000) % 60;
  const difHours =  Math.floor(dif / 3.6e+6) % 24;
  const difDays =  Math.floor(dif / 8.64e+7) % 365;
  const difYears = Math.floor(dif / 3.154e+10);
  const format = (val, str) => {
    if (Number(val) === 0) return;
    if (Number(val) === 1) return `1 ${str}`;
    return `${val} ${str}s`
  }
  const arr = [format(difYears, 'year'), format(difDays, 'day'), format(difHours, 'hour'), format(difMinutes, 'minute'), format(difSeconds, 'second')].filter(Boolean);

  if (arr.length === 1) {
    return arr.join();
  } else if (arr.length === 2) {
    return arr.join(' and ');
  }

  return arr.map((c, i) => i === arr.length - 1 ? `and ${c}` : c).join(', ').replace(/\, and/, ' and');
}