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How to get the first Monday of a month with MomentJS?

I'm using MomentJS and fullcalendar.
I want to get the first Monday of a month.
I tried the following code but it doesn't work.
let date = new Date(year, month, 1)
moment(date).isoWeekday(1)

I believe #xenteros's answer doesn't work for months that begin on a Sunday, because Monday would be the 9th.
Here is a simple fix:
let date = moment().year(y).month(m).date(1).day(8);
if (date.date() > 7)
date.day(-6);

The following code have solved my problem:
let date = moment().set('year', y).set('month', m).set('date', 1).isoWeekday(8)
if(date.date() > 7) { //
date = date.isoWeekday(-6)
}

Here are the steps to get the first monday
Create a day (any day in that specific month)
Get the start of the month, this will return a date
There is two cases for first day, it could be Monday or not Monday
We add 6 days to the first day of the month, if it is Monday, we will get Sunday (same week), else we get a date in the next week that has a Monday (that occure in the same month)
calling startOf('isoWeek') will return the first Monday of that month
let date = new Date(year, month, 1);
const firstMondayOfTheMonth = date
.startOf('month')
.add(6, 'day')
.startOf('isoWeek');

Use moment js.
You can pass as parameter any year and month.
import moment from 'moment';
const startOfMonth = moment().year(2021).month(0).startOf('month').isoWeekday(8);
console.log(startOfMonth.format('dddd DD-MM-YYYY')); // Monday 04-01-2021
Month: 0-11.

Moment.js : Get first day of first week by given year

Im trying to get the first day of first week by given day
Correct results :
2017 = 2 Jan (Monday)
2016 = 4 Jan (Monday)
2015 = 1 Jan (Thursday)
2014 = 1 Jan (Wednesday)
2013 = 1 Jan (Tuesday)
I can get the first day of the year by :
moment().year(year).startOf('year') // All result is 1. jan XXXX
Then I tried : (correct)
moment().day(1).year(2017).week(1) // 2 Jan (Monday) (correct)
But when changed to 2016 : (wrong)
moment().day(1).year(2016).week(1) // 10 Jan (Sunday) (wrong)
Any know how to get correct results ? (Im open for pure JS date() also)
PS: Week number is based in Norway
Playground : https://jsfiddle.net/6sar7eb4/

There is no need to use loop, you can simply test if the week number is not 1 using week() that gives localized output
The output of moment#week will depend on the locale for that moment.
You have to set the locale to moment using locale() method to get start of week locale aware. If you have to show result in english you can change locale locally.
Here a working sample:
function getFirstDayOfFirstWeekByYear( year ) {
// Create moment object for the first day of the given year
let func = moment({year:year})
// Check if 1st of January is in the first week of the year
if( func.week() !== 1 ){
// If not, add a week to the first day of the current week
func.startOf('week').add(1, 'week');
}
// Return result using english locale
return func.locale('en').format('D. MMMM YYYY (dddd)')
}
// Set Norwegian Bokmål locale globally
moment.locale('nb');
// Tester
[2017, 2016, 2015, 2014, 2013].forEach(function ( year ) {
let s = '<li>' + getFirstDayOfFirstWeekByYear( year ) + '</li>'
document.getElementsByTagName('BODY')[0].innerHTML += s
})
<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.18.1/moment-with-locales.min.js"></script>

Anyway if your happy to do it within a loop this works
function getFirstDayOfFirstWeekByYear( year ){
let D = 1;
x = 0;
while(x !== 1){
x = moment(`${year}-01-${D}`).isoWeek();
if(x === 1 ){
return moment(`${year}-01-${D}`).format('D. MMMM YYYY (dddd)')
}
D++;
};
}
results
2. January 2017 (Monday)
4. January 2016 (Monday)
1. January 2015 (Thursday)
1. January 2014 (Wednesday)
1. January 2013 (Tuesday)

Im trying to get the first day of first week by given day
Using moment.js to get the first day of the first week of the year based on the ISO 8601 scheme, you can just use the first day of the first week of the year since the ISO week starts on Monday:
// Return a moment.js object for the first weekday of the first week
// of the year using ISO 8601 scheme
function firstDay(year) {
return moment(year + '011', 'GGGGWWE');
}
// Examples
['2015', '2016', '2017', '2018', '2019', '2020']
.forEach(function(year) {
console.log('ISO First day of ' + year + ': ' +
firstDay(year).format('dddd D MMMM, YYYY'));
});
<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.18.1/moment.min.js"></script>
However you seem to not want to use the ISO scheme, so if you just want to get the first day of the year that isn't a Saturday or Sunday (i.e. ignore the ISO 8601 scheme), you can do (without moment.js):
// Return Date for first day of year that is one of
// Monday to Friday
function firstDay(year) {
var d = new Date(year, 0, 1);
var day = d.getDay();
d.setDate(d.getDate() + (day%6? 0 : (++day%4)));
return d;
}
// Examples
['2015', '2016', '2017', '2018', '2019', '2020']
.forEach(function(year) {
console.log('First weekday of ' + year + ': ' + firstDay(year).toString());
});
Discussion
There are many different schemes for first day of the week and first day of the year. A commonly used scheme is ISO 8601, where the first day of the week is Monday and the first week of the year is the one in which the first Thursday appears (or the first that contains at least 4 days of the week, it's the same result).
According to ISO 8601 rules, the first day of the first week of 2019 is Monday 31 December, 2018.
If you decide to support different schemes (e.g. in Saudi Arabia the week starts on Saturday and ends on Friday, with work days Saturday to Wednesday and in the US a week is typically Sunday to Saturday with work days Monday to Friday), then you should clearly state which scheme you support and not simply assume a scheme based on the language the user has chosen. Schemes may also be based on other cultural factors such as religion, so are not necessarily region or language-based.
Which scheme should be used for an English person in Saudi Arabia? An Islamic person in Bhutan? In Saudi the official language for commerce is French, but most international discussion is in English, while the national language is Arabic. But they use the Hijri calendar, which does not align with the Georgian calendar commonly used elsewhere and is not defined by language but by religion.
There may also be differences even in the same place, with departments of the same government choosing different schemes so even locale (in the correct meaning of the term) is not a definitive indicator of which scheme to use.
If you decide to support different schemes, far better to adopt a standard scheme (like ISO 8601) and let the user change it to the one they prefer if they wish (and which the preferences in my computer and calendar application allow me to do).

How to check if javascript date is from a specified week ago?

I am trying to make a function that can check if a given date is in a specified week ago.
For example, if the input is <1, date object>, then it asks, if the given date is from last week. If the input is <2, date object>, then it asks if the given date is from 2 weeks ago, etc.. (0 is for current week).
Week is Sun-Sat.
this.isOnSpecifiedWeekAgo = function(weeks_ago, inputDate) {
return false;
};
But I don't want to use any libraries, and also I am not sure how to change the week of a date object. Does anyone know how to begin?
Thanks

If you want to find out a date that was a week ago, you can simply subtract 7 days from the current date:
var weekAgo = new Date();
weekAgo.setDate(weekAgo.getDate() - 7);
console.log(weekAgo.toLocaleString());
If you want to find out if a date is in a specific week, you'll need to:
Work out the start date for that week
Work out the end date for that week
See if the date is on or after the start and on or before the end
Since your weeks are Sunday to Saturday, you can get the first day of the week from:
var weekStart = new Date();
weekStart.setDate(weekStart.getDate() - weekStart.getDay());
console.log(weekStart.toLocaleString());
The time should be zeroed, then a new date created for 7 days later. That will be midnight at the start of the following Sunday, which is identical to midnight at the end of the following Saturday. So a function might look like:
function wasWeeksAgo(weeksAgo, date) {
// Create a date
var weekStart = new Date();
// Set time to 00:00:00
weekStart.setHours(0,0,0,0);
// Set to previous Sunday
weekStart.setDate(weekStart.getDate() - weekStart.getDay());
// Set to Sunday on weeksAgo
weekStart.setDate(weekStart.getDate() - 7*weeksAgo)
// Create date for following Saturday at 24:00:00
var weekEnd = new Date(+weekStart);
weekEnd.setDate(weekEnd.getDate() + 7);
// See if date is in that week
return date >= weekStart && date <= weekEnd;
}
// Test if dates in week before today (1 Nov 2016)
// 1 Oct 24 Oct
[new Date(2016,9,1), new Date(2016,9,24)].forEach(function(date) {
console.log(date.toLocaleString() + ' ' + wasWeeksAgo(1, date));
});

Use moment.js http://momentjs.com/docs/#/manipulating/subtract/
We use it a lot and its a great lib.

Get previous business day with moment.js

What's the best way to get the previous business day's date with moment.js? A business day is Monday through Friday.
Some expectations:
If today is Satuday, Sunday or Monday, return last Friday's date
If today is Tuesday, return last Monday's date (yesterday)

function getPreviousWorkday(){
let workday = moment();
let day = workday.day();
let diff = 1; // returns yesterday
if (day == 0 || day == 1){ // is Sunday or Monday
diff = day + 2; // returns Friday
}
return workday.subtract(diff, 'days');
}

Updated Approach (without looping)
You could actually take advantage of the day() function that would allow you to set the current day of the week in moment.js to find the previous Friday based on certain days :
function getPreviousWorkday(){
// Based on the current day, handle accordingly
switch(moment().day())
{
// If it is Monday (1),Saturday(6), or Sunday (0), Get the previous Friday (5)
// and ensure we are on the previous week
case 0:
case 1:
case 6:
return moment().subtract(6,'days').day(5);
// If it any other weekend, just return the previous day
default:
return moment().day(today - 1);
}
}
which can be seen here and demonstrated below :
Looping Approach
You could simply subtract days from your current moment instance via the subtract() function from the current day until you reached a non-weekend day:
function getPreviousWorkday(){
// Get today
var today = new moment().subtract(-1,'days');;
// If today isn't a weekend, continue iterating back until you hit a non-weekend
while([0,6].indexOf(today.day()) !== -1){
today = today.subtract(1, 'days');
}
// Return the non-weekend day
return today;
}
You can see an example of this in action here and demonstrated below :

There is a npm module for that!
https://github.com/kalmecak/moment-business-days
From documentation:
prevBusinessDay() : Will retrieve the previous business date as moment date object:
//Previous busines day of Monday 02-02-2015
moment('02-02-2015', 'DD-MM-YYYY').prevBusinessDay()._d // Fri Jan 30 2015 00:00:00 GMT-0600 (CST)
//Previous busines day of Tuesday 03-02-2015
moment('03-02-2015', 'DD-MM-YYYY').prevBusinessDay()._d //Mon Feb 02 2015 00:00:00 GMT-0600 (CST)
P.S: Important node. Why to use dependency? If all you need is prev business day - no reason to use 3-d party lib. But for real application all that stuff will be useful for more complex operations like importing dates, formatting and calculations based on calendars, holidays, etc.

function getPreviousWorkday() {
return [1, 2, 3, 4, 5].indexOf(moment().subtract(1, 'day').day()) > -1 ?
moment().subtract(1, 'day') : moment(moment().day(-2));
}
If the previous day is a weekday, return the previous day / weekday. Otherwise return the previous Friday since, if the previous day is a Saturday or Sunday it should return the previous Friday.

If you need to check only for Saturday or Sunday, this snippet can be used. I'm getting last day of month first then checking for business day (so basically last business day of a month)
getLastDayofMonth() {
let date = moment(this.selectedDate).endOf("month").format("MM-DD-YYYY");
// console.log(moment(date).day());
// check if last day of selected month is Saturday or Sunday, assign last friday if true
if (moment(date).day() === 0 || moment(date).day() === 6)
this.selectedDate = moment(date)
.subtract(6, "days")
.day(5)
.format("MM-DD-YYYY");
else this.selectedDate = date;
},

Moment js getting next date given specified week day

I seem to have a bit of a problem getting the previous Monday given a particular date. I'm trying to use Moment js for the task. Obviously, I can do it by hand, but found it curious that I couldn't get it to work using the example in the moment.js documentation on their website: http://momentjs.com/docs/#/get-set/day/.
I was trying something like:
moment([2013, 08, 15, 15, 20]).day(-1).format('ddd, MMM DD')
which results in the 'two days ago' date, that being September 13 instead of the expected September 9th.
Does anybody have a clue here? Thanks.

Here is how it works:
moment().day(1) // this monday
moment().day(-6) // last monday, think of it as this monday - 7 days = 1 - 7 = -6
Same applies in other direction:
moment().day(8) // next monday, or this monday + 7 days = 1 + 7 = 8
Your code moment().day(-1) can be explained as this Sunday - 1 day = 0 - 1 = -1
or this Saturday - 7 days = 6 - 7 = -1

The accepted answer only works if you already know whether the day in question is in this week or next week. What if you don't know? You simply need the next available Thursday following some arbitrary date?
First, you want to know if the day in question is smaller or bigger than the day you want. If it's bigger, you want to use the next week. If it's smaller, you can use the same week's Monday or Thursday.
const dayINeed = 4; // for Thursday
if (moment().isoWeekday() <= dayINeed) {
return moment().isoWeekday(dayINeed);
} else...
If we're past the day we want already (if for instance, our Moment is a Friday, and we want the next available Thursday), then you want a solution that will give you "the Thursday of the week following our moment", regardless of what day our moment is, without any imperative adding/subtracting. In a nutshell, you want to first go into the next week, using moment().add(1, 'weeks'). Once you're in the following week, you can select any day of that week you want, using moment().day(1).
Together, this will give you the next available day that meets your requirements, regardless of where your initial moment sits in its week:
const dayINeed = 4; // for Thursday
// if we haven't yet passed the day of the week that I need:
if (moment().isoWeekday() <= dayINeed) {
// then just give me this week's instance of that day
return moment().isoWeekday(dayINeed);
} else {
// otherwise, give me next week's instance of that day
return moment().add(1, 'weeks').isoWeekday(dayINeed);
}
See also: https://stackoverflow.com/a/27305748/800457

function nextWeekday (day, weekday) {
const current = day.day()
const days = (7 + weekday - current) % 7
return day.clone().add(days, 'd')
}
// example: get next Friday starting from 7 Oct 2019
nextWeekday(moment('2019-10-07'), 5)) // 2019-10-11

I think the point is that using day() or isoWeekday() you get a date in the current week, no matter which day of the week is today. As a consequence, the date you get can be past, or still to come.
Example:
if today is Wednesday, moment().isoWeekday(5).format() would return the date of the upcoming Friday.
While
moment().isoWeekday(1).format() would return the previous Monday.
So when you say you want the date of, let's say, "last Tuesday", this date could belong to the current week or to the previous week, depending on which day is today.
A possible function to get the date of the last dayOfTheWeek is
function getDateOfPreviousDay(dayOfTheWeek) {
currentDayOfTheWeek = moment().isoWeekday();
if ( currentDayOfTheWeek >= dayOfTheWeek ) {
return moment().isoWeekday(dayOfTheWeek).format(); // a date in the current week
}
else {
return moment().add(-1,'weeks').isoWeekday(dayOfTheWeek).format(); // a date in the previous week
}
}

const upcomingDay = (dayIndex, format = "DD MMMM YYYY") => {
if (
Number(moment().format("D")) >= Number(moment().day(dayIndex).format("D"))
) {
return moment()
.day(7 + dayIndex)
.format(format);
}
return moment().day(dayIndex).format(format);
};