@burglekitt/gmt/dist/plain/calculate/getWeekNumber.js

Temporal-based date and time utilities with timezone support and polyfill integration

import { Temporal } from "@js-temporal/polyfill";
/**
 * Calculate the week number based on configurable week start day.
 *
 * - ISO (monday): Week 1 is the week containing the first Thursday (has at least 4 days in the year)
 * - US (sunday): Week 1 is the week containing Jan 1 (first week has at least 1 day)
 *
 * @param dateStr ISO PlainDate string (e.g. "2024-03-15")
 * @param weekStartsOn optional: "monday" (default) or "sunday"
 * @returns Week number (1-53), or null on invalid input
 *
 * @example getWeekNumber("2024-01-01") // 1
 * @example getWeekNumber("2024-01-08") // 2
 * @example getWeekNumber("2024-12-31") // 1
 * @example getWeekNumber("2024-01-01", "sunday") // 1
 * @example getWeekNumber("invalid") // null
 */
export function getWeekNumber(dateStr, weekStartsOn = "monday") {
    try {
        const date = Temporal.PlainDate.from(dateStr);
        if (weekStartsOn === "monday") {
            const isoWeek = date.weekOfYear;
            return isoWeek ?? null;
        }
        const yearStart = Temporal.PlainDate.from({
            year: date.year,
            month: 1,
            day: 1,
        });
        const firstDayOfWeek = yearStart.dayOfWeek;
        const daysSinceSunday = firstDayOfWeek === 7 ? 0 : firstDayOfWeek;
        const daysSinceJan1 = date.dayOfYear - 1;
        const result = Math.floor((daysSinceJan1 + daysSinceSunday) / 7) + 1;
        return result;
    }
    catch {
        return null;
    }
}