如何根据指定日期计算对应的日历周？

I struggled for a while finding a simple yet accurate solution when searching for "Calculating the Week for a given Date"—most either didn't handle edge cases properly or were overly complex. Here's the implementation I came up with that follows ISO week rules (Monday as the first day of the week, Week 1 containing January 4th of the year):

The Implementation #

getWeekOfDay: function(d){ 
    var date1 = new Date(d.getFullYear(), d.getMonth(), d.getDate()); 
    var yearWeek; 
    var firstMondayOfWeek1 = this.getMondayOfWeekOneOfAGivenYear(date1.getFullYear()); 
    var weekNumber = Math.ceil((((date1 - firstMondayOfWeek1) / 86400000) + 1) / 7); 
    yearWeek = date1.getFullYear() + "." + weekNumber; 

    // Handle dates that fall in the last week of the previous year (e.g., Jan 1, 2012)
    if(weekNumber <= 0 ){ 
        firstMondayOfWeek1 = this.getMondayOfWeekOneOfAGivenYear(date1.getFullYear() - 1); 
        weekNumber = Math.ceil((((date1 - firstMondayOfWeek1) / 86400000) + 1) / 7); 
        yearWeek = (date1.getFullYear() - 1) + "." + weekNumber; 
    } 

    // Handle years where Week 53 rolls over to next year (e.g., Dec 29, 2014; Dec 28, 2020)
    if(weekNumber === 53 ){ 
        var Dec31 = new Date(date1.getFullYear(), 11, 31); 
        if(Dec31.getDay() < 4){ 
            yearWeek = (date1.getFullYear() + 1) + "." + 1; 
        } 
    } 
    return yearWeek; 
}, 

getMondayOfWeekOneOfAGivenYear: function(year){ 
    // ISO rule: January 4th always falls in Week 1 of the year
    var fourthJan = new Date(year, 0, 4);
    var day = fourthJan.getDay(); 
    // Treat Sunday as day 7 (since ISO weeks start on Monday)
    if (day === 0) { 
        day = 7; 
    } 
    var firstMondayOfW1 = new Date(fourthJan); 
    firstMondayOfW1.setDate(firstMondayOfW1.getDate() + (1 - day)); 
    return firstMondayOfW1; 
}, 

Key Logic Breakdown #

• getMondayOfWeekOneOfAGivenYear: The foundation of this implementation relies on the ISO standard rule that January 4th is always in Week 1 of the year. We calculate the first Monday of Week 1 by adjusting from January 4th—if January 4th is a Sunday, we treat it as day 7 to correctly shift back to the preceding Monday.

• getWeekOfDay:

  1. Create a copy of the input date to avoid modifying the original object.
  2. Calculate the week number by finding the day difference between the input date and the first Monday of Week 1, then converting that to weeks.
  3. Edge case 1: If the calculated week number is ≤0, the date belongs to the last week of the previous year—we recalculate using the prior year's Week 1 Monday.
  4. Edge case 2: If the week number is 53, we check if December 31st of the current year is a Wednesday or earlier (since such years don't have a full Week 53, so those days belong to Week 1 of the next year).

内容的提问来源于stack exchange，提问作者Arvind