Calculate current week number in JavaScript

Calculating the current week number involves determining which week of the year the current date falls into. This is often based on a system where weeks start on a specific day, typically Sunday or Monday.

Example:

The week number of january 1  = either week 52 or week 1

Explanation: From the below calendar we can see that Jan 1 is in last week52 of the previous year. If it was Monday, then Jan1 would be week 1

Approach:

• Date Initialization: Ensure currentDate is set to either the provided date object or the current date using new Date().
• Calculate January 1st: Initialize januaryFirst to the beginning of the current year using new Date(currentDate.getFullYear(), 0, 1).
• Days to Next Monday: Determine the number of days to the next Monday from January 1st using conditional logic.
• Calculate Next Monday: Set nextMonday to the date of the next Monday following January 1st.
• Return Week Number: Compare currentDate with nextMonday and calculate the week number accordingly, returning the result.

Example: In this example, we are calculating the total number of weeks till now.

Javascript

function getDateWeek(date) {
    const currentDate =
        (typeof date === 'object') ? date : new Date();
    const januaryFirst =
        new Date(currentDate.getFullYear(), 0, 1);
    const daysToNextMonday =
        (januaryFirst.getDay() === 1) ? 0 :
        (7 - januaryFirst.getDay()) % 7;
    const nextMonday =
        new Date(currentDate.getFullYear(), 0,
        januaryFirst.getDate() + daysToNextMonday);

    return (currentDate < nextMonday) ? 52 :
    (currentDate > nextMonday ? Math.ceil(
    (currentDate - nextMonday) / (24 * 3600 * 1000) / 7) : 1);
}

const currentDate = new Date();
const weekNumber = getDateWeek();

console.log("Week number of " + currentDate + " is : " + weekNumber);

Output

Week number of Mon Mar 11 2024 04:57:07 GMT+0000 (Coordinated Universal Time) is : 11

the