Develop Reference

How can I get start and end of a week using dateFNS depeogn on the current date and custom start day?

I want to get the custom start and end day of week from current date. For example
If current date is 10/01/2022 and my start of the week start form Wednesday then I want to get a week range between 29/12/2021 to 04/01/2022. Till now I have Monday as start of the week and Sunday as the end of the week. For that I am using below code to get the week range. How Can I get the custom week range?
const date = new Date();
const start = dateFNS.sub(dateFNS.startOfWeek(date, { weekStartsOn: 1 }), { days: 7 });
const end = dateFNS.sub(dateFNS.endOfWeek(date, { weekStartsOn: 1 }), { days: 7 });

new Date() behaving inconsistently in JavaScript

I have a JavaScript control consisting of 3 dropdowns to collect date of birth from users (don't ask me why we're not using a date picker)
A dropdown for day, month, and year. When all 3 dropdowns have a value selected, I collect the values and construct a date:
var d = new Date(Date.UTC(this.selectedYear, this.selectedMonth - 1, this.selectedDay));
I'm not sure why you need to subtract 1 from the month, I just copied that bit from existing codebase and it seems to work. I guess months have a zero-based index for whatever reason.
The obvious problem with these dropdowns is that the user can select an invalid date. There's nothing stopping them to select 31 for day and then select February. This will result in the date being shifted forward by a number of days, i.e. values 1999, 2, 31, would give me March 3rd, 1999. So I have a check in place:
if (d.getDate() != this.selectedDay) {
d = new Date(Date.UTC(this.selectedYear, this.selectedMonth - 1, 0));
this.selectedDay = d.getDate();
If the day of month is different then my original value, it means the date has shifted, so I change the date to the last day of last month (hence the 0 for day) and then I update the value in the 'days' dropdown.
The strange thing is, I can't do this.selectedMonth - 1 in this second conversion. This would bring me all the way back to January 31st. So I have to do the following:
var d = new Date(Date.UTC(this.selectedYear, this.selectedMonth - 1, this.selectedDay));
if (d.getDate() != this.selectedDay) {
d = new Date(Date.UTC(this.selectedYear, this.selectedMonth, 0));
this.selectedDay = d.getDate();
}
This works, but it's just mind-boggling to me. Why do months behave differently if I use 0 as a value for days instead of a valid day of month? Why would anyone implement it this way? Does anyone have a logical explanation?

Actually, it's not the month field that's any different from the others: it's the day part that is the only one that is 1-indexed, as you can find in the docs on Date:
Given at least a year and month, this form of Date() returns a Date object whose component values (year, month, day, hour, minute, second, and millisecond) all come from the following parameters. Any missing fields are given the lowest possible value (1 for the day and 0 for every other component).
(emphasis mine)
The incorrect assumption you made is that using 0 for the day part would represent the last day of the month. There's no such quirks to JS Dates!
Picking the day 0, instead of the day 1, merely offsets the Date you picked by one day. Just one full day before the first day of the month...
That would be the last day of the previous month.
If the month you want to select is this.selectedMonth - 1, its last day is obtained by doing the following:
select the following month — that's + 1, so: (this.selectedMonth - 1) + 1
go back one day before the first — that's day 0, one before the day 1.
This is exactly what you're doing when you go:
// ┌───── year ────┐ ┌───── month ────┐ day
new Date(Date.UTC(this.selectedYear, this.selectedMonth, 0))
// │ │ │
// ┌─────────────────────┘ │ │
// the month after (this.selectedMonth - 1) │
// │
// ┌──────────────────────────────┤
// the day before the first day (1)
Sometimes, even an ASCII picture is worth more than a thousand words 🙂.

As others have pointed out, months are zero indexed hence you need to subtract 1 from the calendar month number. You can make life easier by setting the values of the month options to their month index (0 to 11) rather than calendar month number (1 to 12). The displayed text can be the calendar month number or name.
To validate the input date values, you can just check that the month hasn't changed when creating the date.
I can't see why you're using Date.UTC, it will change the date for users east of Greenwich as for anyone with a positive timezone offset (or negative ECMAScript offset) 2019-10-01 UTC is 2019-09-30 local.
let i;
let ySel = document.getElementById('inYear');
for (i=0; i<10; i++) {
ySel.appendChild(new Option(2010+i, 2010+i));
}
let mSel = document.getElementById('inMonth');
'January February March April May June July August September October November December'
.split(' ')
.forEach((m, i) => mSel.appendChild(new Option(m, i)));
let dSel = document.getElementById('inDay');
for (i=1; i<32; i++) {
dSel.appendChild(new Option(i, i));
}
[ySel, mSel, dSel].forEach(el => el.addEventListener('change', validateDate, false));
function validateDate(){
let y = document.getElementById('inYear').value;
let m = document.getElementById('inMonth').value;
let d = document.getElementById('inDay').value;
let date = new Date(y, m, d); // m is already zero indexed
let errSpan = document.getElementById('dErr');
if (date.getMonth() != m) {
errSpan.textContent = 'Invalid date';
} else {
errSpan.textContent = date.toString();
}
}
Year: <select id="inYear"></select>
Month: <select id="inMonth"></select>
Day: <select id="inDay"></select><span id="dErr"></span>

I would suggest to make year input first and required, after it has value enable month picker and then create new date object from where you would display day picker

get week number from date in whole week base in javascript

I have searched the web and found the script to get the week number in year. However my counting is difference. The below image is the week number I want to get. When I tested using '1/5/2015', my code got week number is 2, but the week number should be 1 in my requirement. Would someone can help me. Thanks in advance.
I found the javascript at IamSilviu/Get week number
There is my code:
function myWeekNumber(thisDate) {
var dt = new Date(thisDate)
var onejan=new Date(dt.getFullYear(), 0, 2);
return Math.ceil((((dt - onejan) / 86400000) + onejan.getDay() + 1) / 7); }

The algorithm you're trying to implement seems to be that:
Weeks start on Sunday
The first week of the year is the one that has any days in the year, e.g. 1 Jan 2016 was a Friday, so the first week of 2016 started on Sunday 27 December 2015
In this case, it's best to use UTC methods to avoid daylight saving issues:
function getWeekNumberNonISO(d) {
// Create UTC equivalent for 23:59:59.999 on the passed in date
var sat = new Date(Date.UTC(d.getFullYear(), d.getMonth(), d.getDate(),23,59,59,999));
// Set to Saturday at end of week
sat.setUTCDate(sat.getUTCDate() + 6 - sat.getUTCDay());
// Get first day of year
var firstDay = new Date(Date.UTC(sat.getUTCFullYear(), 0, 1));
// Set to Sunday on or before, i.e. first day of first week in year
firstDay = firstDay.setUTCDate(firstDay.getUTCDate() - firstDay.getUTCDay());
// Week number is difference in dates divided by ms/week rounded
return Math.round((sat - firstDay)/(6.048e8));
}
// Get week number for Mon 5 Jan 2015
console.log(getWeekNumberNonISO(new Date(2015,0,5))); // 2
// Get week number for Sat 31 Dec 2011
console.log(getWeekNumberNonISO(new Date(2011,11,31))); //53
// Get week number for Sat 1 Jan 2011
console.log(getWeekNumberNonISO(new Date(2011,0,1))); // 1
// Get week number for Sun 2 Jan 2011
console.log(getWeekNumberNonISO(new Date(2011,0,2))); // 2

Js has function inbulid function which can be used to fetch the date from the given date of the week getweek().
var week=date.getWeek()

How to use moment to find first monday 18 month ago

I am writing some javascript and came across moment library.I would like to use the moment to get the first Monday of 18 Month ago. How would I do it?
I know 18 months ago it is
moment().subtract(18, 'months');
But how would I know the first Monday of that time.
Thanks ahead of time!

You can do this in one line.
moment().subtract(18,'months').startOf('month').add(6 - moment().day("Monday").day() ,'days').startOf('week').day(1);
Find the moment 18 months ago, get the start date of that month, add 6 days since in worst case the first of that month is a tuesday(results in a date which is in the week that includes the first monday) and then get the start of that week.
Voila, all in one line.
1st edit: corrected the calculation to be locale independent and always return a Monday and not beginning of week since that is locale dependent.
2nd edit: locales makes it a bit more complicated. Adding 6 days to a Monday with a locale where the week ends on Saturday will move into another week thus we need to adapt the algorithm to take the Monday week number into consideration, i.e. subtract moment().day("Monday").day() which gives the locale dependent week day number of a Monday(0 or 1). Changed in the code above.

cleaner and more reusable IMHO ;)
var monday = moment().day('Monday').weekday();
var searchFirstMonday = moment().subtract(18, 'months').startOf('month');
while (searchFirstMonday.weekday() !== monday){
searchFirstMonday.add(1, 'day');
}

If you go back 18 months, get the first day of that month, then the Monday of that week, you'll get close:
moment().subtract(18, 'months').startOf('month').day(1)
To make sure the Monday is in the right month, you need to break that down a bit:
var month = moment().subtract(18, 'months');
var firstMonday = month.startOf('month').day(1);
if (firstMonday.month() != month.month()) {
firstMonday = firstMonday.add(7, 'days');
}

If anyone wants to do the same without any libraries.
Not sure if this is the best way to do it :P
var now = new Date();
//Get year and month
var month = now.getMonth();
var year = now.getFullYear();
//Months to go back
var monthsIntoPast = 18;
//Day of week we're looking for
var firstWeekDay= 2;
//Loop for one week
for(var x = 0; x < 7; x++) {
//Go back 18 months to first day + x of the month
var past = new Date(year, month - monthsIntoPast, 2 + x);
//First monday
if(past.getDay() == firstWeekDay) {
//First monday 18 months ago
alert(past.toUTCString())
break;
}
}

You use the following code to get what you need:
// Subtract 18 months from the current date, then get the first day of the month
var date = moment().subtract(18, 'months').startOf('month');
// Get current month
var month = date.month();
// Get monday of the first week of the month
var firstMonday = moment(date).weekday(0);
// Check if the first day of the month is not Monday
// and first monday of first week is in the desired month
if( date.weekday() != 0 && firstMonday.month() != month ){
// Add 1 week if needed
firstMonday.add(1, 'weeks');
}
The example above consideres Monday as the first day of the week. If Monday is not the first day of the week (day with index 0), you can get the correct index using moment.weekdays()

var moment = require('moment');
date = moment().subtract(18, 'months').startOf('month');
First we get the the current date and subtract 18 months. Then we find the start of the month, which as of today is
_d: Tue Jul 01 2014 00:00:00 GMT-0800 (AKDT),
So now we need to adjust for the next Monday. We get the current day of the week with date.day() which will give us a number between 0-7 with 0 being last Sunday and 7 being next Sunday. In this case, the first is on a Tuesday.
>date.day();
2
So we take the offset of the day of the week and subtract it from 8. Why 8 you say? Because if you count every number including 0 and 7 there are 8 numbers. (Think arrays)
//since it's not monday we add the offset
date = date.add(8-date.day(), 'days');
In your code you'd want to check if the First day returned actually IS a Monday so as to not get the second Monday of the month. So the whole thing might look like
date = moment().subtract(18, 'months').startOf('month');
if(date.day() > 1){ //check if it's Monday
//since it's not monday we add the offset
date = date.add(8-date.day(), 'days');
}
returns
_d: Mon Jul 07 2014 00:00:00 GMT-0800 (AKDT),

MomentJS - how to select all days of the week by name?

How to sellect all same weekdays by using momentJS? Ex: All Mondays of current month. If the day is 15th day of the month, there are two Mondays in that month, from 0 to 15th day of the month. Maybe it is some kind of looping, because momenthJS should return two dates-one for first Monday and second for second Monday in the given range and so on.
Or is there any other way to do this by using any other JavaScript library?

As you said a loop can solve this problem
move 4 weeks before from the date you have e.g. if we were on the last monday of the month we move to the first monday of the month or a previous date (if we move to the previous month we can still get the correct dates by doing more iterations)
get the month of the date you have (all the dates you get should have the same month)
move forward from the initial date computed above a week each time and check if the new date has the same month as the input date
function weekdaysFromMonth (date) {
// all the weekdays previous/next to date should be
// 7 days apart and also have the same month
var month = date.get('month')
// make sure that the start date is 4 weeks before now
// we might go to the previous month but anyway we'll
// get the correct dates by doing more iterations
var start = moment(date).subtract(4 * 7, 'days')
var dates = []
for (var i = 0; i < 10; i += 1) {
start.add(7, 'days')
if (start.get('month') === month) {
dates.push(moment(start))
}
}
return dates
}
document.write(
JSON.stringify(
weekdaysFromMonth(moment()), null, ' '
)
)
<script src="http://momentjs.com/downloads/moment.js"></script>