How to add months to a date in JavaScript?

Corrected as of 25.06.2019:

var newDate = new Date(date.setMonth(date.getMonth()+8));

Old From here:

var jan312009 = new Date(2009, 0, 31);
var eightMonthsFromJan312009  = jan312009.setMonth(jan312009.getMonth()+8);

Split your date into year, month, and day components then use Date:

var d = new Date(year, month, day);
d.setMonth(d.getMonth() + 8);

Date will take care of fixing the year.

I took a look at the datejs and stripped out the code necessary to add months to a date handling edge cases (leap year, shorter months, etc):

Date.isLeapYear = function (year) { 
    return (((year % 4 === 0) && (year % 100 !== 0)) || (year % 400 === 0)); 
};

Date.getDaysInMonth = function (year, month) {
    return [31, (Date.isLeapYear(year) ? 29 : 28), 31, 30, 31, 30, 31, 31, 30, 31, 30, 31][month];
};

Date.prototype.isLeapYear = function () { 
    return Date.isLeapYear(this.getFullYear()); 
};

Date.prototype.getDaysInMonth = function () { 
    return Date.getDaysInMonth(this.getFullYear(), this.getMonth());
};

Date.prototype.addMonths = function (value) {
    var n = this.getDate();
    this.setDate(1);
    this.setMonth(this.getMonth() + value);
    this.setDate(Math.min(n, this.getDaysInMonth()));
    return this;
};

This will add "addMonths()" function to any javascript date object that should handle edge cases. Thanks to Coolite Inc!

Use:

var myDate = new Date("01/31/2012");
var result1 = myDate.addMonths(1);

var myDate2 = new Date("01/31/2011");
var result2 = myDate2.addMonths(1);

->> newDate.addMonths -> mydate.addMonths

result1 = "Feb 29 2012"

result2 = "Feb 28 2011"