Calculate current week number in JavaScript

Calculating the current week number involves determining which week of the year the current date falls into. The method of calculation can vary slightly depending on the rules you follow, such as which day starts the week (Sunday or Monday) and how the first week of the year is defined.

For example:

• January 1st might fall into the last week (week 52 or 53) of the previous year or the first week of the current year, depending on the day of the week it lands on.
• If January 1st is a Monday, then it will be considered the first week of the new year.

Here's an approach to calculate the current week number based on the assumption that weeks start on Monday.

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.

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);

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