JavaScript Week Number

function getIsoWeek(dateString) {
  const date = new Date(dateString + "T00:00:00.000Z");
  const weekday = date.getUTCDay() || 7;
  date.setUTCDate(date.getUTCDate() + 4 - weekday);
  const weekYear = date.getUTCFullYear();
  const weekOne = new Date(Date.UTC(weekYear, 0, 4));
  const weekOneDay = weekOne.getUTCDay() || 7;
  weekOne.setUTCDate(weekOne.getUTCDate() + 1 - weekOneDay);
  const weekNumber = Math.floor((date - weekOne) / 86400000 / 7) + 1;
  return { weekYear, weekNumber };
}

const result = getIsoWeek("2026-05-03");
console.log(result);
// { weekYear: 2026, weekNumber: 18 }

Use explicit YYYY-MM-DD strings and append a UTC time when the calculation should be timezone-neutral.

Avoid passing a bare date string into business logic if the result depends on a user's local timezone. Decide whether the input means a UTC date or a local calendar date before calculating the week.

For current-week features, calculate the user's local calendar date first, then calculate the week number from that date.

Use the WhatWeek API when you need stable JSON responses for applications or internal tools.