Array of Date with for loop

Question

I have problem with this code. If I print item of daysArray in for loop (console.log(daysArray[i]);) it returns right date, but after for loop returns last date at all daysArray items. Please some could help me.

public getFullMonth(date: Date): Array<Date> {
    var firstDay = this.getSunday(date);
    var daysArray: Array<Date> = Array<Date>();

    for (var i = 0; i < 43; i++) {
        firstDay.setDate(firstDay.getDate() + 1);
        daysArray[i] = firstDay;
        console.log(daysArray[i]);
    }

    console.log(daysArray.length);
    console.log(daysArray[0]);
    console.log(daysArray[30]);
    return daysArray;
}

What exactly you want ? and could you share fiddle for this ? — Manjuboyz
– Manjuboyz, Commented Sep 9, 2016 at 7:49

Nitzan Tomer · Accepted Answer · 2016-09-09 07:58:51Z

The problem is that you always use the same instance of Date, you never create a new one, so all of the items in daysArray are the same exact instance.

Because of that, when you do:

firstDay.setDate(firstDay.getDate() + 1);

Then you actually change the value for all items.
You should create a new instance for every item:

public getFullMonth(date: Date): Array<Date> {
    var firstDay = this.getSunday(date);
    var daysArray: Array<Date> = Array<Date>();

    for (var i = 0; i < 43; i++) {
        firstDay = new Date(firstDay.getTime());
        firstDay.setDate(firstDay.getDate() + 1);
        daysArray[i] = firstDay;
        console.log(daysArray[i]);
    }

    console.log(daysArray.length);
    console.log(daysArray[0]);
    console.log(daysArray[30]);
    return daysArray;
}

Notice that each iteration now does:

firstDay = new Date(firstDay.getTime());

Which creates a new instance of Date which is a "clone" of the previous one.

iberbeu · Accepted Answer · 2016-09-09 08:02:27Z

0

Your problem is easy to solve. You are just copying the same item in each position of the array. These means you are overriding the value you set in each step of the for loop.

Just create a new object for each position of the array and you won't have the error any more. Something like this

for (var i =0; i < 43; i++) {
  var nDay = new Date();
  nDay.setDate(firstDay.getDate() + i + 1);
  daysArray[i] = nDay;
  console.log(daysArray[i]);
}

edited Sep 9, 2016 at 8:02

answered Sep 9, 2016 at 7:57

iberbeu

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