How to Calculate ISO Week Numbers in JavaScript, Just Like PHP's `date('W')`?

Determining Week Number in JavaScript Like PHP's date('W')

PHP's date('W') function calculates the week number of the year based on ISO-8601 standards. The ISO-8601 week number begins on Monday and runs through Sunday. This article demonstrates how to achieve similar functionality in JavaScript.

Solution

Merlyn's website provides a comprehensive guide to working with weeks in JavaScript:

• [Working with weeks](https://www.merlyn.org/2005/11/04/working-with-weeks/)

Based on this guide, the following code snippet returns the ISO-8601 week number of a given date:

/* For a given date, get the ISO week number
 *
 * Based on information at:
 *
 *    https://www.merlyn.org/weekcalc.htm#WNR
 *
 * Algorithm is to find nearest thursday, it's year
 * is the year of the week number. Then get weeks
 * between that date and the first day of that year.
 *
 * Note that dates in one year can be weeks of previous
 * or next year, overlap is up to 3 days.
 *
 * e.g. 2014/12/29 is Monday in week  1 of 2015
 *      2012/1/1   is Sunday in week 52 of 2011
 */
function getWeekNumber(d) {
    // Copy date so don't modify original
    d = new Date(Date.UTC(d.getFullYear(), d.getMonth(), d.getDate()));
    // Set to nearest Thursday: current date + 4 - current day number
    // Make Sunday's day number 7
    d.setUTCDate(d.getUTCDate() + 4 - (d.getUTCDay()||7));
    // Get first day of year
    var yearStart = new Date(Date.UTC(d.getUTCFullYear(),0,1));
    // Calculate full weeks to nearest Thursday
    var weekNo = Math.ceil(( ( (d - yearStart) / 86400000) + 1)/7);
    // Return array of year and week number
    return [d.getUTCFullYear(), weekNo];
}

var result = getWeekNumber(new Date());
console.log('It\'s currently week ' + result[1] + ' of ' + result[0]);

This code relies on UTC methods to avoid issues during daylight saving and specific year-start scenarios. The output displays both the current year and week number, similar to PHP's date('W') function.

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