Result of getting next 12 months in Javascript is messed up

Question

I have the following code for generating a list of the next 12 months (inclusive), starting from today's date:

function DateUtilFunctions() {
    var self = this;

    var monthNames = new Array();

    monthNames[0] = "January";
    monthNames[1] = "February";
    monthNames[2] = "March";
    monthNames[3] = "April";
    monthNames[4] = "May";
    monthNames[5] = "June";
    monthNames[6] = "July";
    monthNames[7] = "August";
    monthNames[8] = "September";
    monthNames[9] = "October";
    monthNames[10] = "November";
    monthNames[11] = "December";

    self.getNext12MonthNamesWithYear = function () {
        var months = new Array();
        var today = new Date(Date());

        var loopDate = new Date();
        loopDate.setTime(today.valueOf());

        var todayPlus12Months = new Date(today.setMonth(today.getMonth() + 12));

        while (loopDate.valueOf() < todayPlus12Months.valueOf()) {
            alert(loopDate);
            alert(loopDate.getMonth());
            var month = monthNames[loopDate.getMonth()];


            months.push(month + ' ' + loopDate.getFullYear());
            loopDate.setMonth(loopDate.getMonth() + 1);
        }

        return months;
    };
}

The result of calling getNext12MonthNamesWithYear() is:

• "May 2012"
• "July 2012"
• "August 2012"
• "May 2012"
• "July 2012"
• "August 2012"
• "September 2012"
• "October 2012"
• "November 2012"
• "December 2012"
• "January 2013"
• "February 2013"
• "March 2013"
• "April 2013"
• "May 2013"

As you can, the start of the list is a bit weird, in that "June" is missing, plus "May", "July" and "August" appear twice.

Naturally I;m doing something very wrong here; could someone please help me out?

EDIT:

Based on micadelli's comment, here is the solution I used:

function DateUtilFunctions() {
    var self = this;

    var monthNames = new Array();

    monthNames[0] = "January";
    monthNames[1] = "February";
    monthNames[2] = "March";
    monthNames[3] = "April";
    monthNames[4] = "May";
    monthNames[5] = "June";
    monthNames[6] = "July";
    monthNames[7] = "August";
    monthNames[8] = "September";
    monthNames[9] = "October";
    monthNames[10] = "November";
    monthNames[11] = "December";

    self.getNext12MonthNamesWithYear = function () {
        var months = new Array();
        var today = new Date();
        var tmpDate = new Date();
        var tmpYear = tmpDate.getFullYear();
        var tmpMonth = tmpDate.getMonth();
        var monthLiteral;

        for (var i = 0; i < 12; i++) {
            tmpDate.setMonth(tmpMonth + i);
            tmpDate.setFullYear(tmpYear);
            monthLiteral = monthNames[tmpMonth];

            months.push(monthLiteral + ' ' + tmpYear);

            tmpYear = (tmpMonth == 11) ? tmpYear + 1 : tmpYear;
            tmpMonth = (tmpMonth == 11) ? 0 : tmpMonth + 1;
        }

        return months;
    };
}

Answer 1

Don't try to manipulate the Date object - just use it to get the initial values for month and year and then use simple arithmetic on the results:

function getNext12MonthNamesWithYear() {
    var now = new Date();
    var month = now.getMonth();
    var year = now.getFullYear();

    var names = ['January', 'February', 'March', 'April', 'May', 'June',
                 'July', 'August', 'September', 'October', 'November', 'December'];

    var res = [];
    for (var i = 0; i < 13; ++i) {
        res.push(names[month] + ' ' + year);
        if (++month === 12) {
            month = 0;
            ++year;
        }
    }
    return res;
}

working demo at http://jsfiddle.net/alnitak/SQQdg/

Answer 2

Here is my solution using Moment.js

Next 12 Months

let months = [];
let monthsRequired = 12

for (let i = 1; i <= monthsRequired; i++) {
  months.push( moment().add(i, 'months').format('MMMM YYYY') )
}

console.log(months)

<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.18.1/moment.min.js"></script>

Just in case if you need previous 12 Months

let months = [];
let monthsRequired = 12

for (let i = monthsRequired; i >= 1; i--) {
  months.push( moment().subtract(i, 'months').format('MMMM YYYY') )
}

console.log(months)

<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.18.1/moment.min.js"></script>

Answer 3

I dont see any reason why this wouldn't work

function DateUtilFunctions() {
   var self = this;

   var monthNames = new Array();

   monthNames[0] = "January";
   monthNames[1] = "February";
   monthNames[2] = "March";
   monthNames[3] = "April";
   monthNames[4] = "May";
   monthNames[5] = "June";
   monthNames[6] = "July";
   monthNames[7] = "August";
   monthNames[8] = "September";
   monthNames[9] = "October";
   monthNames[10] = "November";
   monthNames[11] = "December";

   self.getNext12MonthNamesWithYear = function () {
     var months = new Array();
     var today = new Date();
     var tmpDate = new Date();
     var tmpYear = tmpDate.getFullYear();
     var tmpMonth = tmpDate.getMonth();
     var monthLiteral;

     for (var i = 0 ; i < 12 ; i++) {
        tmpDate.setMonth(tmpMonth + i);
        tmpDate.setFullYear(tmpYear);
        monthLiteral = monthNames[tmpMonth];
        months.push(monthLiteral + ' ' + tmpYear);
        tmpMonth = (tmpMonth == 11) ? 0 : tmpMonth+1;
        tmpYear = (tmpMonth == 11) ? tmpYear+1 : tmpYear;
     }

     return months;
   };
}

JS Bin
http://jsbin.com/aqezom

Answer 4

Here is a working version. You can not perform setMonth directly if the value is more than 12. Also, there is no 31st for some of the months which results in unexpected results. JS fiddle here FIDDLE

    var monthNames = new Array();

    monthNames[0] = "January";
    monthNames[1] = "February";
    monthNames[2] = "March";
    monthNames[3] = "April";
    monthNames[4] = "May";
    monthNames[5] = "June";
    monthNames[6] = "July";
    monthNames[7] = "August";
    monthNames[8] = "September";
    monthNames[9] = "October";
    monthNames[10] = "November";
    monthNames[11] = "December";

var today = new Date();
var currentMonth = today.getMonth();
var i;
for (i = 0 ; i < 12 ; i++) {
    var newMonth = currentMonth + i;
    var newYear = newMonth > 11 ? today.getFullYear() + 1: today.getFullYear();
    newMonth = newMonth > 11? (newMonth - 12): newMonth;

    var newDate = new Date(newYear, newMonth, '1');

    console.log(monthNames[newDate.getMonth()] + ' ' + newDate.getFullYear());
}

​

Answer 5

I also needed a list of the last 12 months this is what I did:

var theMonths = new Array("January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December");
var today = new Date();
var aMonth = today.getMonth();
var i;
for (i=0; i<12; i++) {
    document.writeln(theMonths[aMonth] + '<br>'); //here you can do whatever you want...
    aMonth++;
    if (aMonth > 11) {
        aMonth = 0;
    }
}