Javascript while loop weird behavior with dates

Question

I am currently getting weird behavior from the following while loop in javascript.

var i = 0;
var endDate = new Date();
var startDate = new Date();
startDate.setDate(startDate.getDate() - 6);

while (start <= end){
  alert (start.getDate());
  start.setDate(start.getDate() + i);
  i++;
}

The output I would expect (assuming today is Sept. 26) would be :

20 21 22 23 24 25 26

However I get the following output

20 20 21 23 26

I don't think I am handling the scope of the startDate variable correctly but I am not sure how to fix this.

Any help is greatly appreciated

You're setting the date, which modifies it. On top of that, you're adding i to the date, which is incrementing. As such, the date that is set increases from the previous sets and the current i. Either create a date that remains unmodified, and add i to that, or just add 1 instead of i. jsfiddle.net/gnmvao4n — user1106925
– user1106925, Commented Sep 26, 2014 at 23:00
I assume start and end are really just typos for startDate and endDate — Krease
– Krease, Commented Sep 26, 2014 at 23:02

Johan Karlsson · Accepted Answer · 2014-09-26 23:03:39Z

1

Assuming start and end are your date variables:

while (start <= end){
  alert (start.getDate());
  start.setDate(start.getDate() + i);
  i++;
}

Loop iteration 1: date is 20 and you add 0 => 20

Loop iteration 2: date is 20 and you add 1 => 21

Loop iteration 3: date is 21 and you add 2 => 23

Loop iteration 4: date is 23 and you add 3 => 26

To get what you expect, try this:

while (start <= end){
  alert (start.getDate());
  start.setDate(start.getDate() + 1);
  i++;
}

answered Sep 26, 2014 at 23:03

Johan Karlsson

6,4761 gold badge22 silver badges28 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

edbert Over a year ago

Thanks for your help. Everybody basically gave the same answer but I think yours was the clearest.

Charles Smartt · Accepted Answer · 2014-09-26 23:02:49Z

1

Your iterator is not correct. You don't really even need an iterator. Just add 1 instead of i to you second line of the while loop and delete var i = 0 and i++.

var endDate = new Date();
var startDate = new Date();
startDate.setDate(startDate.getDate() - 6);

while (startDate <= endData){
  alert (start.getDate());
  start.setDate(start.getDate() + 1);
}

The reason why you are getting 20 20 21 23 26 is because on the first loop you are six days back, and add zero; the second loop you are six days back, and add one; the third loop you are five days back and add 2; the fourth loop you are now three days back, and adding 3; etc.

answered Sep 26, 2014 at 23:02

Charles Smartt

3492 silver badges8 bronze badges

Comments

Krease · Accepted Answer · 2014-09-26 23:02:31Z

0

If you debug through your code, you'll likely see each iteration of the loop you're modifying the date by a different value - i is changing each time through the loop. First time it updates the date by 0, then by 1, then 2, etc. That's what you're seeing in the output.

Replace i with 1 and you'll get your expected output. In fact, you don't even need i.

answered Sep 26, 2014 at 23:02

Krease

16.3k8 gold badges58 silver badges92 bronze badges

Collectives™ on Stack Overflow

Javascript while loop weird behavior with dates

3 Answers 3

1 Comment

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

1 Comment

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related