week-of-year.ts

const FIRST_MONTH_OF_THE_YEAR = 0;
const FIRST_DAY_OF_THE_MONTH = 1;
const DAYS_IN_A_WEEK = 7;
const THURSDAY_DAY_OF_THE_WEEK = 4;
const SUNDAY_DAY_OF_THE_WEEK = 7;
const TEN_DAYS_MILISECONDS = 86400000;

export type WeekOfYear = {
	week: number;
	year: number;
};

/**
 * @see https://en.wikipedia.org/wiki/ISO_week_date
 * @see https://stackoverflow.com/questions/6117814/get-week-of-year-in-javascript-like-in-php
 */
export function getWeekOfYear(date: Date): WeekOfYear {
	const dateUTC = new Date(Date.UTC(date.getFullYear(), date.getMonth(), date.getDate()));

	const nearestThursday = new Date(Date.UTC(
		dateUTC.getFullYear(),
		dateUTC.getMonth(),
		dateUTC.getDate() + THURSDAY_DAY_OF_THE_WEEK - (dateUTC.getUTCDay() || SUNDAY_DAY_OF_THE_WEEK)
	));

	const firstDayOfYear = new Date(Date.UTC(
		nearestThursday.getFullYear(),
		FIRST_MONTH_OF_THE_YEAR,
		FIRST_DAY_OF_THE_MONTH
	));

	const weekNumber = Math.ceil(
		(((nearestThursday.getTime() - firstDayOfYear.getTime()) / TEN_DAYS_MILISECONDS) + 1) / DAYS_IN_A_WEEK
	);

	return {
		week: weekNumber,
		year: nearestThursday.getUTCFullYear()
	};
}