How to get the previous 8 Sundays in JavaScript?

Question

I'm trying to get the previous 8 Sundays in JavaScript. Not only did this not work, but it looks very clunky. I feel something with arrays would work cleaner. What did I do wrong?

var now = new Date();
var today = new Date(now.getFullYear(), now.getMonth(), now.getDate());
var lastSunday = new Date(today.setDate(today.getDate() - today.getDay()));
lastSunday = lastSunday.format("mm/dd/yyyy");

var lastSunday2 = new Date(today.setDate(today.getDate() - today.getDay() - 7));
lastSunday2 = lastSunday2.format("mm/dd/yyyy");
var lastSunday3 = new Date(today.setDate(today.getDate() - today.getDay() - 14));
lastSunday3 = lastSunday3.format("mm/dd/yyyy");
var lastSunday4 = new Date(today.setDate(today.getDate() - today.getDay() - 21));
lastSunday4 = lastSunday4.format("mm/dd/yyyy");
var lastSunday5 = new Date(today.setDate(today.getDate() - today.getDay() - 28));
lastSunday5 = lastSunday5.format("mm/dd/yyyy");
var lastSunday6 = new Date(today.setDate(today.getDate() - today.getDay() - 35));
lastSunday6 = lastSunday6.format("mm/dd/yyyy");
var lastSunday7 = new Date(today.setDate(today.getDate() - today.getDay() - 42));
lastSunday7 = lastSunday7.format("mm/dd/yyyy");
var lastSunday8 = new Date(today.setDate(today.getDate() - today.getDay() - 49));
lastSunday8 = lastSunday8.format("mm/dd/yyyy");

console.log(lastSunday8); //this gives 06/25/2017 Obviously not what I wanted.

@Andy Please don't just suggest he does this with a library, that's a bit unnecessary. — user4020527, Jan 11 '18 at 22:25
Are you using a library? `lastSunday.format()` would normally throw an error. — Rick Hitchcock, Jan 11 '18 at 22:27
Each time you do `today.setDate(today.getDate() - today.getDay() - 7)` you are modifying *today*, so just keep subtracting 7 (use a loop per [Jacob Wood's answer](https://stackoverflow.com/a/48216894/257182)). — RobG, Jan 11 '18 at 22:36

score 4 · Accepted Answer · answered Jan 11 '18 at 22:34

4

No need to create individual variables for each date, you can use a for loop and an array to store them.

var date = new Date();
date.setDate(date.getDate() - date.getDay()); //start at last sunday

var dates = [];
for (var i = 0; i < 8; i++) {
    dates.push(new Date(date));
    date.setDate(date.getDate() - 7); //subtract a week
}

answered Jan 11 '18 at 22:34

user4020527

1
8
20

This works nicely but it gives me the Mondays instead of Sundays. `var now = new Date();` `var date = new Date(now.getFullYear(), now.getMonth(), now.getDate());` Added this and it gives me Sundays, thank you! – Dong Jan 12 '18 at 02:40
@Dong Changing it to `date.setUTCDate(date.getUTCDate() - date.getUTCDay());` also fixed it for me, seems to be an issue with UTC/locale being mixed causing it to function incorrectly after the end of a UTC day. – user4020527 Jan 12 '18 at 02:48

score 1 · Answer 2 · answered Jan 11 '18 at 22:42

1

As example - calculate time delta in milliseconds

var today = new Date();
var lastSunday = new Date(today - today.getDay()*3600*24*1000);
console.log('Last Sunday: ' + lastSunday)
for(var i=2; i <= 8; i++) {
  lastSunday = new Date(lastSunday - 7 * 3600*24*1000);
  console.log('Sunday ' + i + ': ' + lastSunday);
}

answered Jan 11 '18 at 22:42

newman

2,689
15
23

1

Not all days are 24 hours long where daylight saving is observed, so much better to use the *set* and *get* Date methods rather than adjusting the time value by 24hrs. – RobG Jan 11 '18 at 22:45

score 1 · Answer 3 · answered Jan 11 '18 at 22:45

you are setting the date each time to equal the Sunday you are referencing in the line of code; so you are increasing each line the number of days you are subtracting where they should all just be 7 try changing all the "- number" to "- 7" because each line you are setting the date and subtracting 7 * n ; where n = number of weeks; You should also get used to using console.log() statements or cli debug to step through your code line by line to check the value variables currently hold.

How to get the previous 8 Sundays in JavaScript?

3 Answers3