从星期天开始获取星期几

Question

I currently have this code where I get the week number alongside the start/end day of that week with add and substract buttons:

 Date.prototype.getWeekNumber = function() { var d = new Date(Date.UTC(this.getFullYear(), this.getMonth(), this.getDate())); var dayNum = d.getUTCDay() || 7; d.setUTCDate(d.getUTCDate() + 4 - dayNum); var yearStart = new Date(Date.UTC(d.getUTCFullYear(), 0, 1)); return Math.ceil((((d - yearStart) / 86400000) + 1) / 7); }; var curr; weekReset(); function display() { start.textContent = curr; end.textContent = endOfWeek(curr); week.textContent = ("Week " + curr.getWeekNumber()); } function weekReset() { curr = startOfWeek(new Date()); display(); } function startOfWeek(date) { var start = new Date(date); start.setHours(0, 0, 0, 0); start.setDate(start.getDate() - start.getDay()); return start; } function endOfWeek(date) { date = startOfWeek(date); date.setDate(date.getDate() + 6); return date; } function weekPlus(weeks) { curr.setDate(curr.getDate() + 7 * weeks); display(); } 

 <div id="start">start</div> <div id="end">end</div> <div id="week">week</div> <button onclick="weekReset()">current</button> <button onclick="weekPlus(1)">add</button> <button onclick="weekPlus(-1)">substract</button> 

The problem is that it show's the current week number as 34 ;

starting on Sun Aug 27

ending on Sat Sep 02

... when this should be the week number 35 .

So how can I change this function to count sunday as the first day of the week and saturday as the last day of the week?

Any help is appreciated.

Answer 1

Your code d.setUTCDate(d.getUTCDate() + 4 - dayNum); is setting your date back to three days earlier. I think that's the reason you are getting previous week number. I checked removing the code and it worked fine.

Answer 2

Your issue is here:

var dayNum = d.getUTCDay() || 7;
d.setUTCDate(d.getUTCDate() + 4 - dayNum);

That code is for ISO weeks and adjusts Sunday's day number to 7 and sets the date to Thursday, where the week is Monday to Sunday.

You want weeks Sunday to Saturday and want the week of the Sunday, so use:

var dayNum = d.getUTCDay();
d.setUTCDate(d.getUTCDate() - dayNum);

which can be reduced to:

d.setUTCDate(d.getUTCDate() - d.getUTCDay());

  Date.prototype.getWeekNumber = function () { var d = new Date(Date.UTC(this.getFullYear(), this.getMonth(), this.getDate())); d.setUTCDate(d.getUTCDate() - d.getUTCDay()); var yearStart = new Date(Date.UTC(d.getUTCFullYear(), 0, 1)); return Math.ceil((((d - yearStart) / 86400000) + 1) / 7); }; var c = new Date(2017,7,26); console.log(c.getWeekNumber() + ':' + c.toString()); var d = new Date(2017,7,27); console.log(d.getWeekNumber() + ':' + d.toString());