How to calculate business days in JavaScript

Date math in JS is famously sharp-edged; business-day math more so.

1. Plain JS (UTC-safe skeleton)

function addBusinessDays(iso, n, holidays = new Set()) {
  const d = new Date(iso + "T00:00:00Z");
  const step = Math.sign(n); let left = Math.abs(n);
  while (left > 0) {
    d.setUTCDate(d.getUTCDate() + step);
    const dow = d.getUTCDay();
    const key = d.toISOString().slice(0, 10);
    if (dow !== 0 && dow !== 6 && !holidays.has(key)) left--;
  }
  return d.toISOString().slice(0, 10);
}

Works, for Saturday and Sunday countries, with a holiday set you maintain. Other weekends, substitute-day rules, and market calendars are where it quietly breaks.

2. One fetch

const r = await fetch(
  `https://<host>/v1/add?region=SA&start=2027-01-06&days=2`,
  { headers: { "X-Api-Key": KEY } });
(await r.json()).result   // "2027-01-10", Fri-Sat weekend handled