How to Get the ISO-8601 Week Number in JavaScript?

How to Obtain the ISO-8601 Week Number in JavaScript

To determine the ISO-8601 week number of the year, analogous to PHP's date('W'), consider the following approach:

Refer to the resources at Merlyn's website:

• [Working with Weeks](https://www.merlyn.org/js-date6.htm#YWD)

This JavaScript code demonstrates the concept:

/*
 * Calculates the ISO week number for a given date.
 *
 * Algorithm adopted from:
 * https://www.merlyn.org/weekcalc.htm#WNR
 *
 * Input:
 *   d: Date object representing the date to calculate the week number for.
 *
 * Output:
 *   Array containing the year and week number.
 */
function getWeekNumber(d) {
    // Clone the date to avoid modifying the original.
    d = new Date(d.getTime());

    // Set the date to the nearest Thursday by adding 4 days and subtracting the day of the week.
    d.setDate(d.getDate() + 4 - (d.getDay() || 7));

    // Determine the first day of the year.
    const yearStart = new Date(d.getFullYear(), 0, 1);

    // Calculate the number of full weeks between the current date and the first day of the year.
    const weekNo = Math.ceil(((d - yearStart) / 86400000 + 1) / 7);

    // Return the year and week number in an array.
    return [d.getFullYear(), weekNo];
}

// Example:
const result = getWeekNumber(new Date());
console.log(`Current week: ${result[1]} of year ${result[0]}`);

Using this code, you can obtain the current ISO-8601 week number of the year, which accounts for weeks starting on Mondays, similar to the functionality provided by PHP's date('W').

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