Adds a month

addMonths.js

// Based on: https://stackoverflow.com/questions/5645058/how-to-add-months-to-a-date-in-javascript

const isLeapYear = ( year ) => { 
    return (((year % 4 === 0) && (year % 100 !== 0)) || (year % 400 === 0)); 
};

const getDaysInMonth = (d) => { 
    const year = d.getFullYear();
		const month = d.getMonth();
    return [31, (isLeapYear(year) ? 29 : 28), 31, 30, 31, 30, 31, 31, 30, 31, 30, 31][month];
};

const addMonths = (d, value) => {
    var n = d.getDate();
    d.setDate(1);
    d.setMonth(d.getMonth() + value);
    d.setDate(Math.min(n, getDaysInMonth(d)));
    return d;
};


const test = new Date(2021, 0, 31);

console.log(addMonths(test, 1));