In JavaScript, there is no built-in function to get the week number of a given date. This functionality, however, is crucial for many applications that depend on week-based reporting or scheduling.
This article introduces a custom JavaScript function that calculates the ISO week number of a date. We’ll explore the use of this function and understand the principles behind the ISO week date system, which is widely recognized and used across various platforms and standards.
What is the ISO Week Date System?
The ISO week date system is a part of the ISO 8601 date and time standard. It designates the week starting on Monday as the first day of the week. Each week belongs to the year in which the week starts; hence, a week can fall in January but belong to the previous year, or in December and belong to the next year.
For instance, December 29, 2014, is a Monday but falls in the first week of 2015 according to ISO standards. Conversely, January 1, 2012, a Sunday, belongs to the 52nd week of 2011. These cases illustrate the importance of correctly calculating the week number to avoid mismatches in date-related data processing.
JavaScript getWeekNumber() Function
The getWeekNumber(d) function provided below calculates the week number by aligning dates to the nearest Thursday. This is because, by ISO standards, the week that contains the first Thursday of the year is considered the first week.
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];
}
How to Use
To use the getWeekNumber(d) function, simply incorporate it into your JavaScript project. You can call this function with any date object, for example, getWeekNumber(new Date()), to find out the current week number.
The function returns an array containing the year and the ISO week number. For instance, on running the function today, it outputs the current week and year, which helps in easy reference and usage in reports or interfaces.
var result = getWeekNumber(new Date());
document.write('It\'s currently week ' + result[1] + ' of ' + result[0]);
Testing and Validation
To ensure the function’s accuracy, test it with dates known for edge cases, like year transitions. Compare these results with an ISO week number calculator found online or in another reliable source.
Watch out for timezone issues, especially if the system settings might influence the date objects in non-UTC formats. The getWeekNumber(d) function ensures consistency by using UTC dates throughout the calculations.
Modifying the Function for Different Start Days
If your application requires a week starting on a different day, you can adjust the offset used in calculating the nearest Thursday. This change affects the logic slightly but can be adapted to fit various requirements.
Performance Considerations
For applications needing high performance, especially when processing large datasets, consider optimizing the function by caching the results for the start of each year or minimizing the use of heavy date manipulations.
Alternative Libraries
While the custom getWeekNumber(d) function serves its purpose well, JavaScript developers often rely on libraries that simplify date and time manipulations. These libraries come with robust functionalities, including methods to easily retrieve the week number of a date.
Moment.js
One of the most popular libraries in this realm has been Moment.js. It offers comprehensive support for dates and times, including utility functions for getting the week number. To get the week number using Moment.js, you would typically use the week() or isoWeek() methods.
Here’s how it works:
// First, include Moment.js in your project
const moment = require('moment');
// Then, create a moment object for a specific date
const date = moment('2022-12-31');
// Retrieve the ISO week number
const weekNumber = date.isoWeek();
console.log('Week Number:', weekNumber);
This method returns the ISO week number, similar to our custom function but with far less code. Note that as of September 2020, Moment.js was considered a legacy project in maintenance mode. It is no longer recommended for new projects due to size and performance issues, but it remains widely used in existing projects.
Luxon
Developed by one of the original authors of Moment.js, Luxon is a modern alternative designed with a smaller footprint and better handling of the internationalization aspects. Luxon utilizes the native Intl API and provides similar functionalities.
Here’s how to get the week number in Luxon:
const { DateTime } = require('luxon');
// Create a Luxon DateTime object
const dt = DateTime.now();
// Get the ISO week number
console.log('Week Number:', dt.weekNumber);
Date-fns
Another library, date-fns, offers a modular approach, allowing you to include only the parts of the library you need, thereby reducing the size of your application. It provides a function to get the ISO week number as well:
const { getISOWeek } = require('date-fns');
// Use the current date or any date object
const now = new Date();
const weekNumber = getISOWeek(now);
console.log('ISO Week Number:', weekNumber);
These libraries are efficient alternatives to writing custom functions and provide additional functionalities that can handle various date-related operations, making them a valuable toolset for any JavaScript developer.
Conclusion
We discussed a custom JavaScript function to determine the ISO week number of a date. This function is essential due to the absence of a built-in JavaScript get week number function, and it adheres to the ISO week date system, ensuring broad compatibility and accuracy.
Additionally, we examined alternative libraries like Moment.js, Luxon, and date-fns, which offer robust and efficient solutions for handling date and time data, including obtaining week numbers.
by the Bito team. 
