Calculating week numbers is essential for building calendar applications, scheduling systems, and time-based analytics in JavaScript applications. As the creator of CoreUI, a widely used open-source UI library, I’ve implemented week number calculations in numerous date-related components over 25 years of development. From my expertise, the most reliable approach is calculating the days between the target date and the first day of the year, then dividing by 7. This method handles edge cases correctly and works consistently across different years.