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Find first monday date of month

I have a date (usually the first day of a month but not necessary, it could be any date) and I want a new date corresponding to the first Monday of that month.
example:
findFirstMonday('1 jul 2021') -> 7 jul 2021
findFirstMonday('1 aug 2021') -> 2 aug 2021
findFirstMonday('13 aug 2021') -> 2 aug 2021
Here is my code that doesn't work:
const selectedDate = new Date();
const daysInSelectedDate = daysInMonth(selectedDate);
const lastDayPreviousMonth = addDays(selectedDate, daysInSelectedDate - selectedDate.getDate() + 1);
const firstDayPreviousMonth = removeDays(
lastDayPreviousMonth,
daysInMonth(lastDayPreviousMonth),
);
console.log('firstDayPreviousMonth: ', firstDayPreviousMonth);
let firstMonday = firstDayPreviousMonth;
while (firstDayPreviousMonth.getDay() !== 1) {
console.log('firstMonday: ', firstMonday, firstMonday.getDay());
firstMonday.setDate(firstMonday.getDate() + 1);
}
console.log('firstMonday: ', firstMonday, firstMonday.getDay());
function addDays(date, days) {
const result = new Date(date)
result.setDate(result.getDate() + days)
return result
}
function removeDays(date, days) {
const result = new Date(date)
result.setDate(result.getDate() - days)
return result
}
function daysInMonth(date) {
return new Date(date.getFullYear(), date.getMonth(), 0).getDate()
}
What am I wrong?
Thanks a lot

I came up with the following code. Some explanations about the general idea:
For a given date get the first date in the month of the given date. This is quite easy by generating a new Date object with day = 1 and the year and month of the given date.
Get the weekday of the first date.
Depending on the weekday of the first date, you must calculate which day number the first Monday has. This number is calculated by ((8 - firstWeekdayInMonth) % 7). You can easily verify yourself, that this always yields a Monday. The modulo is important for Sundays and Mondays, where you would otherwise add 8 and 7 respectively, which would not yield the first Monday anymore.
console.log(findFirstMonday('1 jul 21'))
console.log(findFirstMonday('1 aug 21'))
console.log(findFirstMonday('13 aug 21'))
function findFirstMonday(dateString) {
let targetDate = new Date(dateString);
let targetMonth = targetDate.getMonth();
let targetYear = targetDate.getFullYear();
let firstDateInMonth = new Date(targetYear, targetMonth, 1);
let firstWeekdayInMonth = firstDateInMonth.getDay();
let firstMondayDate = 1 + ((8 - firstWeekdayInMonth) % 7);
return new Date(targetYear, targetMonth, firstMondayDate).toLocaleDateString();
}

Edit:
console.log(findFirstMondayMonth('1 jul 2021').toLocaleDateString())
console.log(findFirstMondayMonth('1 aug 2021').toLocaleDateString())
console.log(findFirstMondayMonth('2 aug 2021').toLocaleDateString())
console.log(findFirstMondayMonth('13 aug 2021').toLocaleDateString())
function findFirstMonday(dateString) {
let date = new Date(dateString)
let diffDay = date.getDay() - 1
if (diffDay == -1) {
diffDay = 6
}
let mondayDate = new Date(dateString);
mondayDate.setHours(mondayDate.getHours() - diffDay*24)
return mondayDate
}
function findFirstMondayMonth(dateString) {
let date = new Date(dateString)
if (date.getMonth() == findFirstMonday(date).getMonth()) {
let dateOneWeekBefore = new Date(dateString)
dateOneWeekBefore.setHours(dateOneWeekBefore.getHours() - 24 * 7)
if (date.getMonth() == dateOneWeekBefore.getMonth()) {
return findFirstMondayMonth(dateOneWeekBefore)
} else {
return findFirstMonday(date)
}
} else {
let dateOneWeekAfter = new Date(dateString)
dateOneWeekAfter.setHours(dateOneWeekAfter.getHours() + 24 * 7)
return findFirstMonday(dateOneWeekAfter)
}
}
Sorry for the last answer, I think it was the first monday of week and I don't see Sunday.getMonth() == -1

How to find 2nd and 4th Saturday in js Fullcalender?

How I can find second and fourth Saturday in JavaScript fullcalender api, so that I can highlight those 2nd and 4th Saturday with "week Day Off" event name?
Fullcalendar_screenshot

You need to find all saturdays in current month. Then filter by odd index since 2th and 4th are 1 and 3 indexes.
Now you can highlight day that matched.
function getSaturdays(year, month) {
let day, date;
let saturdays = [];
day = 1;
date = new Date(year, month, day);
while (date.getMonth() === month) {
if (date.getDay() === 6) { // Sun=0, Mon=1, Tue=2, etc.
saturdays.push(new Date(year, month, day).getDate());
}
day += 1;
date = new Date(year, month, day);
}
return saturdays;
}
let saturdays = getSaturdays(2021, 5).filter((day, index) => index % 2 !== 0)
console.log(saturdays)
Original answer

//Here instead of arrayOfDaysInMonth variable you can pass javascript dates according to month here i have passed statically
var arrayOfDaysInMonth = [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28];//feb
var saturdays = [2,4] // 2nd and 4th saturday;
var secondWeekOfCount = [];
var whichDateIsSecondorFourthSat=[];
arrayOfDaysInMonth.map((i)=>{
var myDate = new Date();
myDate.setFullYear(2022);
myDate.setMonth(1); // Feb
myDate.setDate(i);
if(myDate.getDay() === 6){
secondWeekOfCount.push(i);
saturdays.map((d)=>{
if(d ===secondWeekOfCount.length ){
whichDateIsSecondorFourthSat.push(i)
}
})
}
})
console.log(whichDateIsSecondorFourthSat)// This will be your result

How to check if last day of month is on friday Javascript

I'm supposed to write a code for codewars to find out the number of times a month ends with a Friday within a range of years.
To start off, I did research and found out several solutions but I still couldn't figure out the results in the console.log.
The first solution is from this tutorial:
In this code, the solution is
let LastDay = new Date(1998, 5 + 1, 0).getDate();
I was able to get the date, but it wasn't clear which day the date falls upon.
Then I found another solution at w3schools. This solution also set the date to be the last day of this month:
var d = new Date();
d.setMonth(d.getMonth() +1, 0);
document.getElementById("demo").innerHTML = d;
However, it works if it displays it as innerHTML = Sat Nov 30 2019 00:57:09 GMT-0500 (Eastern Standard Time). However, when I tried to rewrite the code and console.log it like in this example:
let d = new Date();
let month = d.getMonth()+1;
let lastday = d.setMonth(month, 0);
console.log(lastday);
The result I got was 1575093343211. I don't understand how it displays those numbers instead of the dates I was expecting. I thought that if it does display the dates, starting with the day, I can convert the date to string or array and check if the first element is Friday and then add it to the counter in the code I'm writing. How do I get the code to display the way I want it to.

something like this will work...
function LastDayOfMonth(Year, Month) {
return new Date((new Date(Year, Month, 1)) - 1);
}
var d = LastDayOfMonth(new Date().getYear(), new Date().getMonth())
//var d = LastDayOfMonth(2009, 11)
var dayName = d.toString().split(' ')[0];
console.log(dayName)

The result I got was 1575093343211. I don't understand how it displays those numbers instead of the dates I was expecting
Because you console.log the output of the setMonth method, not the date object:
let lastday = d.setMonth(month, 0);
console.log(lastday);
According to the documentation, the setMonth method returns:
The number of milliseconds between 1 January 1970 00:00:00 UTC and the updated date.
Instead you should use that output to create a new instance of the date object:
let lastday = new Date(d.setMonth(month, 0));
console.log(lastday);

Algorithms to get the last day of the month are generally based on setting a date to day 0 of the following month, which ends up being the last day of the required month.
E.g. to get the last day for June, 2019 (noting that 6 is July, not June):
let endOfJune = new Date(2019, 6, 0):
Once you have the date, you can get the day where 0 is Sunday, 1 is Monday, etc. and 5 is Friday:
let endOfJuneDay = endOfJune.getDay();
The set* methods modify the Date they're called on and return the time value for the modified date. So you don't need to assign the result to anything:
let d = new Date();
let month = d.getMonth() + 1;
// Set date to the new month
d.setMonth(month, 0);
console.log(d);
So if you want to loop over the months for a range of years and get the number that end with a Friday (or any particular day), you might loop over the months something like:
/*
** #param {number} startYear - start year of range
** #param {number} endYear - end year of range
** #param {number} dat - day number, 0 = Sunday, 1 = Monday, etc.
** default is 0 (Sunday)
*/
function countEOMDay(startYear, endYear, day = 0) {
// startYear must be <= end year
if (startYear > endYear) return;
// Start on 31 Jan of start year
let start = new Date(startYear, 0, 31);
// End on 31 Dec of end year
let end = new Date(endYear, 11, 31);
let count = 0;
// Loop over months from start to end
while (start <= end) {
// Count matching days
if (start.getDay() == day) {
++count;
}
// Increment month to end of next month
start.setMonth(start.getMonth() + 2, 0);
}
return count;
}
console.log(countEOMDay(2019, 2019, 5)); // 1
console.log(countEOMDay(2018, 2019, 5)); // 3

You can use setMonth() method to set the month of a date object. The return value of setMonth() method is milliseconds between the date object and midnight January 1 1970. That's what you get from console.log(lastday);
Your return value,
1575093343211
is milliseconds between your date object (d) and midnight January 1 1970.
If you want to get the expected date, you have to console log your date object instead the lastday, as follows:
let d = new Date();
let month = d.getMonth()+1;
let lastday = d.setMonth(month, 0);
console.log(d);
output: Sat Nov 30 2019 00:02:47 GMT+0530 (India Standard Time)
This is an alternative solution I wrote to solve your problem. This will return the number of times a month ends with a Friday within a range of years. Hope this will help you :)
var days = [];
var count = 0;
function getLastFridaysCount(startYear, endYear) {
for (var year = startYear; year <= endYear; year++) {
days = [
31,
0 === year % 4 && 0 !== year % 100 || 0 === year % 400 ? 29 : 28,
31, 30, 31, 30, 31, 31, 30, 31, 30, 31
];
for (var month = 0; month <= 11; month++) {
var myDate = new Date();
myDate.setFullYear(year);
myDate.setMonth(month);
myDate.setDate(days[month]);
if(myDate.getDay() == 5)
{
count++;
}
}
}
return count;
}
console.log("count", getLastFridaysCount(2014, 2017));

this is the solution, in the code can find the comments "//" explaining of what happens in each iteration.
function lastDayIsFriday(initialYear, endYear) {
let count = 0;
//according to when the year ends starts the loop
if (endYear !== undefined) {
let start = new Date(initialYear, 0, 31);
let end = new Date(endYear, 11, 31);
while(start <= end) { //check if the start date is < or = to the end
//The getDay() method returns the day of the week (from 0 to 6) for the specified date.
if(start.getDay() === 5) { //if = to FriYAY!!!
count++; //count the day
}
start.setMonth(start.getMonth()+2, 0);// returns the month (from 0 to 11) .getMonth
} //& sets the month of a date object .setMonth
return count;
} else {
let start = new Date(initialYear, 0, 31);
console.log(start.toString());
for(let i = 0; i < 12; i++) {
if(start.getDay() === 5) {
count++;
}
start.setMonth(start.getMonth() + 2, 0);
// console.log(start.toString());
}
return count;
}
}

How to get a specify date in every month in Javascript?

I need to find this month, previous month and the next month of a specific date.
For example, date was set to 31 of every month, what I expect to get the date is
2018-02-28, 2018-03-31 and 2018-04-30. For those dates which has no 31, than it becomes the day before.
And finally generate 2 period, 2018-02-28 to 2018-03-29, 2018-03-30 to 2018-04-31.
I don't know how to handle feb and the month which less than 31.
var d = new Date();
var tyear = d.getFullYear(); //2018
var tmonth = d.getMonth(); //2
new Date(2018, tmonth-1, 31);//output 2018-03-02 not what I wanted

A simple algorithm is to add months to the original date, and if the new date is wrong, set it to the last day of the previous month. Keeping the original date values unmodified helps, e.g.
/* #param {Date} start - date to start
** #param {number} count - number of months to generate dates for
** #returns {Array} monthly Dates from start for count months
*/
function getMonthlyDates(start, count) {
var result = [];
var temp;
var year = start.getFullYear();
var month = start.getMonth();
var startDay = start.getDate();
for (var i=0; i<count; i++) {
temp = new Date(year, month + i, startDay);
if (temp.getDate() != startDay) temp.setDate(0);
result.push(temp);
}
return result;
}
// Start on 31 Jan in leap year
getMonthlyDates(new Date(2016,0,31), 4).forEach(d => console.log(d.toString()));
// Start on 31 Jan not in leap year
getMonthlyDates(new Date(2018,0,31), 4).forEach(d => console.log(d.toString()));
// Start on 30 Jan
getMonthlyDates(new Date(2018,0,30), 4).forEach(d => console.log(d.toString()));
// Start on 5 Jan
getMonthlyDates(new Date(2018,0,5), 4).forEach(d => console.log(d.toString()));

I think you're going to need an array with 12 numbers in it. Each number is the amount of days in each month and the numbers in the array go in order (first number is 31 because January has 31 days, second is 28 or 29 for Feb), etc. Then you'll get the month number from your input date and look in the array at the number corresponding to the month number +/- 1.
You'll then need to construct a date for the previous month and the next month based on the number of days in the current month.
See comments inline:
let daysInMonths = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];
document.getElementById("date").addEventListener("input", function(){
console.clear();
// Create new Date based on value in date picker
var selectedDate = new Date(this.value + 'T00:00');
var year = selectedDate.getYear();
// Determine if it is a leap year (Feb has 29 days) and update array if so.
if (((year % 4 === 0) && (year % 100 !== 0)) || (year % 400 === 0)) {
daysInMonths[1] = 29;
}
var selectedDateMonth = selectedDate.getMonth();
// Get previous month number (if current month is January, get December)
let prevMonth = selectedDateMonth > 0 ? selectedDateMonth - 1 : 11;
let prevMonthDate = null;
// If selected date is last day of month...
if(selectedDate.getDate() === daysInMonths[selectedDateMonth]){
// Create new date that takes the selected date and subtracts the correct amount of
// days from it based on a lookup in the array.
var newDate1 = new Date(selectedDate.getTime());
prevMonthDate =
new Date(newDate1.setDate(selectedDate.getDate() - daysInMonths[selectedDateMonth]));
} else {
// Create a new date that is last month and one day earlier
var newDate2 = new Date(selectedDate.getTime());
prevMonthDate =
new Date(new Date(newDate2.setDate(selectedDate.getDate() - 1))
.setMonth(selectedDate.getMonth() - 1));
}
// Get next month (if current month is December, get January
let nextMonth = selectedDateMonth < 11 ? selectedDateMonth + 1 : 0;
let nextMonthDate = null;
// Same idea for next month, but add instead of subtract.
// If selected date is last day of month...
if(selectedDate.getDate() === daysInMonths[selectedDateMonth]){
var newDate3 = new Date(selectedDate.getTime());
nextMonthDate =
new Date(newDate3.setDate(selectedDate.getDate() + daysInMonths[selectedDateMonth + 1]));
} else {
var newDate4 = new Date(selectedDate.getTime());
nextMonthDate = new Date(new Date(newDate4.setDate(selectedDate.getDate() + 1)).setMonth(selectedDate.getMonth() + 1));
}
console.log("Last month date: " + prevMonthDate.toLocaleDateString());
console.log("Next month date: " + nextMonthDate.toLocaleDateString());
});
<p>Pick a date: <input type="date" id="date"></p>

Use this approach:
Javascript Date Object – Adding and Subtracting Months
From the Author
There is a slight problem with the Javascript Date() Object when trying to advance to the next month or go back to the previous month.
For example, if your date is set to October 31, 2018 and you add one month, you'd probably expect the new date to be November 30, 2018 because November 31st doesn't exist. This, however, isn't the case.
Javascript automatically advances your Date object to December 1st. This functionality is very useful in most situations(i.e. adding days to a date, determining the number of days in a month or if it's a leap year), but not for adding/subtracting months. I've put together some functions below that extend the Date() object: nextMonth() and prevMonth().
function prevMonth() {
var thisMonth = this.getMonth();
this.setMonth(thisMonth - 1);
if (this.getMonth() != thisMonth - 1 && (this.getMonth() != 11 || (thisMonth == 11 && this.getDate() == 1)))
this.setDate(0);
}
function nextMonth() {
var thisMonth = this.getMonth();
this.setMonth(thisMonth + 1);
if (this.getMonth() != thisMonth + 1 && this.getMonth() != 0)
this.setDate(0);
}
Date.prototype.nextMonth = nextMonth;
Date.prototype.prevMonth = prevMonth;
var today = new Date(2018, 2, 31); //<----- March 31st, 2018
var prevMonth = new Date(today.getTime());
prevMonth.prevMonth();
console.log("Previous month:", prevMonth);
console.log("This month:", today)
var nextMonth = new Date(today.getTime());
nextMonth.nextMonth();
console.log("Next month:", nextMonth);
.as-console-wrapper { max-height: 100% !important; top: 0; }

Dates and time zones are a real pain in JS, so challenge accepted.
I broke it down in two steps:
- Count the days of prev and next month
- Compare with selected day and pick the lowest number
Testcases included
function createUTCDate(year, month, day) {
return new Date(Date.UTC(year, month, day));
}
function splitDate(date) {
return {
year: date.getUTCFullYear(),
month: date.getUTCMonth(),
day: date.getUTCDate()
};
}
function numberOfDaysInMonth(year, month) {
return new Date(year, month + 1, 0).getDate();
}
function dateNextMonth(dateObj) {
const daysNextMonth = numberOfDaysInMonth(dateObj.year, dateObj.month + 1);
const day = Math.min(daysNextMonth, dateObj.day);
return createUTCDate(dateObj.year, dateObj.month + 1, day);
}
function datePreviousMonth(dateObj) {
const daysPrevMonth = numberOfDaysInMonth(dateObj.year, dateObj.month - 1);
const day = Math.min(daysPrevMonth, dateObj.day);
return createUTCDate(dateObj.year, dateObj.month - 1, day);
}
const log = console.log;
function print(dateString) {
const date = new Date(dateString);
const dateObj = splitDate(date);
log("Previous: ", datePreviousMonth(dateObj).toISOString());
log("Selected: ", date.toISOString());
log("Next: ", dateNextMonth(dateObj).toISOString());
log("--------------");
}
const testCases = [
"2018-03-01 UTC",
"2018-03-31 UTC",
"2018-01-01 UTC",
"2018-12-31 UTC"
];
testCases.forEach(print);
Please note that the hack with new Date(xxx + " UTC") is not according to spec and is just there for testing purposes. Results may vary per browser.
You should choose an input format and construct your dates accordingly.

I handle it in a foolish way by concatenating string
let daysInMonths = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];
let months = ["01", "02", "03", "04", "05", "06", "07", "08", "09", "10", "11", "12"];
var target = nexttarget = lasttarget = "29"; //target day
if (((year % 4 === 0) && (year % 100 !== 0)) || (year % 400 === 0)) {
daysInMonths[1] = 29;
}
function findLastDay(target, month){
if(target > daysInMonths[month]){
target = daysInMonths[month];
}
return target;
}
then
var d = new Date();
var year = d.getFullYear();
var month = d.getMonth();
target = findLastDay(target, month);
var this_month = year+"-"+months[month]+"-"+target;
console.log(this_month);//2018-03-29
// next month
if(month == 11){
nextmonth = 0;
nextyear = year + 1;
}else{
nextmonth = month+1;
nextyear = year;
}
nexttarget = findLastDay(nexttarget, nextmonth);
var next_month = nextyear+"-"+months[nextmonth]+"-"+nexttarget;
console.log(next_month);//2018-04-29
//last month
if(month == 0){
lastmonth = 11;
lastyear = year - 1;
}else{
lastmonth = month - 1;
lastyear = year;
}
lasttarget = findLastDay(lasttarget, lastmonth);
var last_month = lastyear+"-"+months[lastmonth]+"-"+lasttarget;
console.log(last_month);//2018-02-28

Date handling is tricky at the best of times. Don't do this yourself. Use Moment.js.
var target = 31;
var today = moment().date(target).calendar();
// today == '03/31/2018'
var nextMonth = moment().date(target).add(1, 'month').calendar();
// nextMonth == '04/30/2018'
var lastMonth = moment().date(target).subtract(1, 'month').calendar()
// lastMonth == '02/28/2018'

Get next day, skip weekends

I want to generate next working day using JavaScript.
This is my code as of now
var today = new Date();
today.setDate(today.getDate());
var tdd = today.getDate();
var tmm = today.getMonth()+1;
var tyyyy = today.getYear();
var date = new Date();
date.setDate(date.getDate()+3);
Problem is, on Fridays it returns Saturday's date whereas I want it to be Monday

This will choose the next working day when a date is passed to it.
I suggest you normalise the date you pass, so you will not be surprised around summertime/wintertime change
Updated in 2023
const getNextWork = date => {
let day = date.getDay(), add = 1;
if (day === 6) add = 2; else
if (day === 5) add = 3;
date.setDate(date.getDate() + add); // will correctly handle 31+1 > 32 > 1st next month
return date;
};
// tests:
const dt = new Intl.DateTimeFormat("en-US", {
weekday: "short",
year: "numeric",
month: "long",
day: "numeric",
timeZone: "UTC",
timeZoneName: "short",
hour: "numeric",
minute: "numeric",
});
const aDay = 24 * 60 * 60 * 1000;
// 26th of March 2023 is daylight savings date in my country
let date = new Date(2023, 2, 24, 15, 0, 0, 0).getTime();
for (let i = 0; i < 7; i++) {
const d = new Date(date + i * aDay);
console.log(dt.format(d), "-->", dt.format(getNextWork(d)));
}
Older code:
var today = new Date(2016, 7, 26,12,0,0,0,0); // Friday at noon
console.log("today, Monday",today,"day #"+today.getDay());
var next = new Date(today.getTime());
next.setDate(next.getDate()+1); // tomorrow
while (next.getDay() == 6 || next.getDay() == 0) next.setDate(next.getDate() + 1);
console.log("no change ",next,"day #"+next.getDay());
console.log("-------");
// or without a loop:
function getNextWork(d) {
d.setDate(d.getDate()+1); // tomorrow
if (d.getDay()==0) d.setDate(d.getDate()+1);
else if (d.getDay()==6) d.setDate(d.getDate()+2);
return d;
}
next = getNextWork(today); // Friday
console.log("today, Friday",today);
console.log("next, Monday ",next);
console.log("-------");
today = new Date(2016, 7, 29,12,0,0,0); // Monday at noon
next = getNextWork(today); // Still Monday at noon
console.log("today, Monday",today);
console.log("no change ",next);
console.log("-------");
// Implementing Rob's comment
function getNextWork1(d) {
var day = d.getDay(),add=1;
if (day===5) add=3;
else if (day===6) add=2;
d.setDate(d.getDate()+add);
return d;
}
today = new Date(2016, 7, 26,12,0,0,0,0); // Friday at noon
next = getNextWork1(today); // Friday
console.log("today, Friday",today);
console.log("next, Monday ",next);
console.log("-------");
today = new Date(2016, 7, 26,12,0,0,0,0); // Monday at noon
next = getNextWork1(today); // Monday
console.log("today, Monday",today);
console.log("no change ",next);

You can add 1 day at at time until you get to a day that isn't Saturday or Sunday:
function getNextBusinessDay(date) {
// Copy date so don't affect original
date = new Date(+date);
// Add days until get not Sat or Sun
do {
date.setDate(date.getDate() + 1);
} while (!(date.getDay() % 6))
return date;
}
// today, Friday 26 Aug 2016
[new Date(), new Date(2016,7,26)].forEach(function(d) {
console.log(d.toLocaleString() + ' : ' + getNextBusinessDay(d).toLocaleString());
});
You can also test the day and add extra to get over the weekend:
// Classic Mon to Fri
function getNextWorkDay(date) {
let d = new Date(+date);
let day = d.getDay() || 7;
d.setDate(d.getDate() + (day > 4? 8 - day : 1));
return d;
}
for (let i=0, d=new Date(); i<7; i++) {
console.log(`${d.toDateString()} -> ${getNextWorkDay(d).toDateString()}`);
d.setDate(d.getDate() + 1);
}
Here is another approach where the work week can be specified using ECMAScript weekday numbers (Sun = 0, Mon = 1, etc.). Dates outside the range are shifted to the start of the next work week.
This is useful where the week is not the classic Mon to Fri, such as the Middle East where Sat to Wed is common or for some who might work Fri to Mon (or whatever).
function getNext(start, end, date) {
let d = new Date(+date);
d.setDate(d.getDate() + 1);
let day = d.getDay();
// Adjust end and day if necessary
// The order of tests and adjustment is important
if (end < start) {
if (day <= end) {
day += 7;
}
end += 7;
}
// If day is before start, shift to start
if (day < start) {
d.setDate(d.getDate() + start - day);
// If day is after end, shift to next start (treat Sunday as 7)
} else if (day > end) {
d.setDate(d.getDate() + 8 - (day || 7));
}
return d;
}
// Examples
let f = new Intl.DateTimeFormat('en-GB', {
weekday:'short',day:'2-digit', month:'short'});
let d = new Date();
[{c:'Work days Mon to Fri',s:1,e:5},
{c:'Work days Sat to Wed',s:6,e:3},
{c:'Work days Fri to Mon',s:5,e:1}
].forEach(({c,s,e}) => {
for (let i = 0; i < 7; i++) {
!i? console.log(`\n${c}`) : null;
console.log(`${f.format(d)} => ${f.format(getNext(s, e, d))}`);
d.setDate(d.getDate() + 1);
}
});

Check this out: https://jsfiddle.net/e9a4066r/
function get_next_weekday (date) {
var tomorrow = new Date(date.setDate(date.getDate() + 1))
return tomorrow.getDay() % 6
? tomorrow
: get_next_weekday(tomorrow)
}

The accepted answer will skip one day at a time, which answers the OPs question, but for anyone looking to add a variable number of days while still skipping weekends the function below may be helpful:
function addWorkDays(date, days) {
while (days > 0) {
date.setDate(date.getDate() + 1);
if (date.getDay() != 0 && date.getDay() != 6) {
days -= 1;
}
}
return date;
}

Thought I'd throw my hat in the ring here with:
function getNextBusinessDate(date) {
// Create date array [S, M, T, W, T, F, S]
const days = new Array(7);
let nextDate = date;
for(let i = 0; i < 7; i++) {
days[nextDate.getDay()] = new Date(nextDate);
nextDate.setDate(nextDate.getDate() + 1);
}
// Shift indices to index as though array was [M, T, W, T, F, S, S]
// Then truncate with min to make F, S, S all yield M for next date
return days[Math.min((date.getDay() + 6) % 7 + 1, 5) % 5 + 1];
}