Add Days to Date in Loop Javascript

Question

I need to add days to a date in Javascript inside of a loop.

Currently, I have this -

var occurences = 2;
var start_date = "10/2/2020";
for(i=0; i < occurences; i++){
  var repeat_every = 2; //repeat every number of days/weeks/months
  var last = new Date(start_date);
  var day =last.getDate() + repeat_every;
  var month=last.getMonth()+1;
  var year=last.getFullYear();
  var fulldate = month + '/' + day + '/' + year;
  console.log(fulldate);
}

However, this outputs 10/4/2020 twice. I know the issue is because in the 2nd iteration of the loop it again simply adds 2 to the date 10/2/2020, is there a way on the subsequent iterations I can add 2 to the previous result?

Thank you!

Does this answer your question? [Add days to JavaScript Date](https://stackoverflow.com/questions/563406/add-days-to-javascript-date) — zcoop98, Oct 02 '20 at 18:59
@zcoop98 unfortunately it does not, I know how to add days to the date, my issue is stemming moreso from the loop itself, once I add the 2 days, I'm struggling to figure out a way to store this result, then on the 2nd iteration of the loop add 2 days to the previous result. — Cody Carmichael, Oct 02 '20 at 19:02

score 1 · Accepted Answer · answered Oct 02 '20 at 19:22

You can use a multiple of your interval and then use last.setDate( last.getDate() + repeat_every ) to add days and get the correct month and year:

var occurences = 20;
var start_date = "10/2/2020";
for(i=1; i <= occurences; i++){
  var repeat_every = 2*i; //repeat every number of days/weeks/months
  var last = new Date(start_date);
  last.setDate( last.getDate() + repeat_every );
  console.log( `${last.getDate()}/${last.getMonth()+1}/${last.getFullYear()}` );
}

score 0 · Answer 2 · answered Oct 02 '20 at 19:01

0

Make counter i a multiple of repeat_every:

/*<ignore>*/console.config({maximize:true,timeStamps:false,autoScroll:false});/*</ignore>*/
var occurences = 2;
var start_date = "10/2/2020";
for(i=1; i <= occurences; i++){
  var repeat_every = 2*i; //repeat every number of days/weeks/months
  var last = new Date(start_date);
  var day =last.getDate() + repeat_every;
  var month=last.getMonth()+1;
  var year=last.getFullYear();
  var fulldate = month + '/' + day + '/' + year;
  console.log(fulldate);
}

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answered Oct 02 '20 at 19:01

TheMaster

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This is perfect, exactly what I needed. Thank you! – Cody Carmichael Oct 02 '20 at 19:08
@CodyCarmichael Alternatively try storing the data outside the loop as a global variable. – TheMaster Oct 02 '20 at 19:09
@MT0 Ah good catch, this is going to be a more complicated script than I originally thought. – Cody Carmichael Oct 02 '20 at 19:21

score 0 · Answer 3 · answered Oct 02 '20 at 20:17

I would separate the generation of the days from their formatting / logging. Here we have a function that collects count instances of incrementally adding n days to a date, returning a collection of Date objects:

const everyNDays = (n, count, start = new Date()) => {
  const y = start.getFullYear(), m = start.getMonth(), d = start.getDate()
  return Array.from({length: count}, (_, i) => new Date(y, m, d + (i + 1) * n))
}

const formatDate = (date) =>
  date .toLocaleDateString ()

console .log (
  everyNDays (2, 20) .map (formatDate)
)
// or everyNDays (2, 20) .forEach (date => console .log (formatDate (date)))

.as-console-wrapper {max-height: 100% !important; top: 0}

(If you don't pass a start date, it uses the current date.)

We then map the simple formatDate function over these dates to get an array of strings.

If you would rather start with the current date, you can simply replace new Date(y, m, d + (i + 1) * n) with new Date(y, m, d + i * n).

Add Days to Date in Loop Javascript

3 Answers3