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JavaScript application for showing the weekend dates?

I thought a lot - I tried but I could not solve it. I need a JavaScript application that shows the nearest weekend dates in the current date.
If it's a weekend now, give it the dates of this weekend, if not, then next weekend's dates.
I'm waiting for your help.
Respects.

You can use the built-in Date constructor.
var date = new Date();
var day = date.getDay();
var saturday;
var sunday;
if(day === 0 || day === 6){ //0 for Sunday, 6 for Saturday
saturday = date;
sunday = new Date(saturday.getTime());
sunday.setDate(saturday.getDate() + (day === 0 ? -1 : 1));
if(day === 0){
var temp = saturday;
saturday = sunday; //Confusing, but they are actually the wrong dates, so we are switching the dates
sunday = temp;
temp = null; //Free up some memory!
}
}
else{
//This is the complicated part, we need to find when is the next Saturday
saturday = new Date(date.getFullYear(), date.getMonth(), (date.getDate() + 6) - day);
sunday = new Date(saturday.getTime());
sunday.setDate(saturday.getDate() + (saturday.getDay() === 0 ? -1 : 1));
}
date = day = null; //Free up some memory!
document.body.innerText = [saturday, sunday];
To get the date, use saturday.getDate() or sunday.getDate().Remember that Date months are 0-based. See here for more info.

var chosenDay = new Date();
var box = [];
var counting = 0;
for (var i = 0; i < 7; i++) {
chosenDay.setDate(chosenDay.getDate() + counting);
var day = chosenDay.getDate();
var dayy = chosenDay.getDay();
var month = chosenDay.getMonth()+1;
var year = chosenDay.getFullYear();
box.push({day: day, dayy: dayy});
counting = 1;
};
Now to find Saturday and Sunday
box.map(function(obj) {
if (obj.dayy === 6) {
console.log('Saturday found');
alert(obj.day);
};
if (obj.dayy === 0) {
console.log('Sunday found');
alert(obj.day);
};
});

I interpret the "nearest" weekend as being the previous weekend for Monday and Tuesday, and the next weekend for Thursday and Friday. You didn't provide any information on what to do with Wednesday.
However, from other answers it seems you want either the current weekend for Saturday and Sunday and or the next weekend for weekdays.
The following is a little more concise than other answers:
/* Get nearest weekend to the provided date
** #param {Date} date - date to get weekends nearst to
** #returns {Array} array of Dates [Saturday, Sunday]
*/
function getNearestWeekend(date) {
// Copy date so don't mess with provided date
var d = new Date(+date);
// If weekday, move d to next Saturday else to current weekend Saturday
if (d.getDay() % 6) {
d.setDate(d.getDate() + 6 - d.getDay());
} else {
d.setDate(d.getDate() - (d.getDay()? 0 : 1));
}
// Return array with Dates for Saturday, Sunday
return [new Date(d), new Date(d.setDate(d.getDate() + 1))]
}
// Some tests
[new Date(2017,0,7), // Sat 7 Jan
new Date(2017,0,8), // Sun 8 Jan
new Date(2017,0,9), // Mon 9 Jan
new Date(2017,0,12) // Thu 12 Jan
].forEach(function(d) {
var opts = {weekday:'short', day:'numeric', month:'short'};
console.log('Date: ' + d.toLocaleString('en-GB',opts) + ' | Next weekend: ' +
getNearestWeekend(d).map(d =>d.toLocaleString('en-GB',opts)).join(' and ')
);
});

Is there an easy way to find the last date a day of week occurs in the current month

I am trying to display the date of the last Wednesday in the current month... so that it will automatically change to the correct date when the next month occurs. (So instead of having to say: "Performing the last wednesday of every month", I can dymanmically give the actual date.)
For example, I would want the date to show on the webpage as Wednesday, Sept 25th for this month, and then appear as Wednesday, Oct 30th next month.
A bonus additional solution would be if I could get the next month's date to display after the previous date has past. In my above example, when the current date is Sept 26-30 (any date after that last wednesday, but still in the same month).. the date would show the next performance date of Oct 30th.
It would be great if the solution was through html, javascript/jquery or asp.
Thanks,
SunnyOz

It depends on your criteria for "easy". Here's a simple function to do as required, it's 5 lines of working code that can be reduced to 4, but will lose a bit of clarity if that's done:
function lastDayInMonth(dayName, month, year) {
// Day index map - modify to suit whatever you want to pass to the function
var dayNums = {Sunday: 0, Monday:1, Tuesday:2, Wednesday:3,
Thursday:4, Friday:5, Saturday:6};
// Create a date object for last day of month
var d = new Date(year, month, 0);
// Get day index, make Sunday 7 (could be combined with following line)
var day = d.getDay() || 7;
// Adjust to required day
d.setDate(d.getDate() - (7 - dayNums[dayName] + day) % 7);
return d;
}
You can change the map to whatever, just determine what you want to pass to the function (day name, abbreviation, index, whatever) that can be mapped to an ECMAScript day number.
Edit
So in the case of always wanting to show the last Wednesday of the month or next month if it's passed:
function showLastWed() {
var now = new Date();
var lastWedOfThisMonth = lastDayInMonth('Wednesday', now.getMonth()+1, now.getFullYear());
if (now.getDate() > lastWedOfThisMonth().getDate()) {
return lastDayInMonth('Wednesday', now.getMonth()+2, now.getFullYear());
} else {
return lastWedOfThisMonth;
}
}
Note that the function expects the calendar month number (Jan = 1, Feb = 2, etc.) whereas the getMonth method returns the ECMAScript month index (Jan = 0, Feb = 1, etc.) hence the +1 and +2 to get the calendar month number.

You could use a javascript library such as moment.js:
http://momentjs.com/
and then get it with this:
moment().add('months', 1).date(1).subtract('days', 1).day(-4)

Here is an approach in JS:
var monthLengths = [ 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 ];
function getLastWednesday() {
var d = new Date();
var month = d.getMonth();
var lastDay = monthLengths[month];
// mind leap years
if (month == 1) {
var year = d.getFullYear();
var isLeapYear = ((year % 4 == 0 && year % 100 > 0) || year % 400 == 0);
if (isLeapYear) lastDay++;
}
// get the weekday of last day in the curent mont
d.setDate(lastDay);
var weekday = d.getDay();
// calculate return value (wednesday is day 3)
if (weekday == 3) {
return lastDay;
}
else {
var offset = weekday - 3;
if (offset < 0) offset += 7;
return lastDay - offset;
}
}

I prefer to use an abstraction like moment.js as #Aralo suggested. To do it in raw JavaScript, however, you can use some code like this... create a function that gets all the days in a month. Then reverse-traverse the list to find the last day number. Wednesday is 3.
function getDaysInMonth(date) {
var dayCursor = new Date(today.getFullYear(), today.getMonth()); // first day of month
var daysInMonth = [];
while(dayCursor.getMonth() == date.getMonth()) {
daysInMonth.push(new Date(dayCursor));
dayCursor.setDate(dayCursor.getDate() + 1);
}
return daysInMonth;
}
function findLastDay(date, dayNumber) {
var daysInMonth = getDaysInMonth(date);
for(var i = daysInMonth.length - 1; i >= 0; i--) {
var day = daysInMonth[i];
if(day.getDay() === dayNumber) return day;
}
}
Then, to get the last Wednesday in the current month:
var today = new Date();
var lastWednesday = findLastDay(today, 3);

javascript function return date array for all Sundays in a year

i am writing a function in javascript that will return date array of all sundays.
below you can see my code :
function getDefaultOffDays(year){
var offdays=new Array();
i=0;
for(month=1;month<12;month++)
{
tdays=new Date(year, month, 0).getDate();
for(date=1;date<=tdays;date++)
{
smonth=(month<10)?"0"+month:month;
sdate=(date<10)?"0"+date:date;
dd=year+"-"+smonth+"-"+sdate;
day=new Date();
day.setDate(date);
day.setMonth(month);
day.setFullYear(year);
if(day.getDay() == 0 )
{
offdays[i++]=dd;
}
}
}
return offdays;
}
the issue is that the returned array is giving random dates not the only dates for sunday :(
m i missing some thing?

If you examine the result, you can see that it's actually not random. It returns the dates for january that are sundays in february, and so on.
The month property of the Date object is zero based, not one based. If you change this line, the function will return the correct dates:
day.setMonth(month - 1);
Also, the loop only runs from 1 to 11, you need to include december too:
for (month=1; month <= 12; month++)
Another way to do this would be to find the first sunday, then just step forward seven days at a time:
function getDefaultOffDays2(year) {
var date = new Date(year, 0, 1);
while (date.getDay() != 0) {
date.setDate(date.getDate() + 1);
}
var days = [];
while (date.getFullYear() == year) {
var m = date.getMonth() + 1;
var d = date.getDate();
days.push(
year + '-' +
(m < 10 ? '0' + m : m) + '-' +
(d < 10 ? '0' + d : d)
);
date.setDate(date.getDate() + 7);
}
return days;
}

One bug:
for(month=1;month<12;month++)
That's just 11 months.
If you want for the whole year you need:
for(month=0;month<12;month++)
Because there are 12 months in a year. You can even combine that with Guffa's answer.

Month is zero based as like day, so if month is 0 then it Jan, So changed your code as below,
function getDefaultOffDays(year){
var offdays=new Array();
i=0;
for(month=0;month<12;month++) {
tdays=new Date(year, month, 0).getDate();
for(date=1;date<=tdays;date++) {
smonth=(month<10)?"0"+(month+1):(month+1);
sdate=(date<10)?"0"+date:date;
dd=year+"-"+smonth+"-"+sdate;
var day=new Date(year,month,date);
if(day.getDay() == 0 ) {
offdays[i++]=dd;
}
}
}
return offdays;
}

This function has error
error code:
tdays=new Date(year, month, 0).getDate();
replace to:
tdays=new Date(year, month, 1).getDate();
becouse 0(day) return prev month

JavaScript - get the first day of the week from current date

I need the fastest way to get the first day of the week. For example: today is the 11th of November, and a Thursday; and I want the first day of this week, which is the 8th of November, and a Monday. I need the fastest method for MongoDB map function, any ideas?

Using the getDay method of Date objects, you can know the number of day of the week (being 0=Sunday, 1=Monday, etc).
You can then subtract that number of days plus one, for example:
function getMonday(d) {
d = new Date(d);
var day = d.getDay(),
diff = d.getDate() - day + (day == 0 ? -6:1); // adjust when day is sunday
return new Date(d.setDate(diff));
}
getMonday(new Date()); // Mon Nov 08 2010

Not sure how it compares for performance, but this works.
var today = new Date();
var day = today.getDay() || 7; // Get current day number, converting Sun. to 7
if( day !== 1 ) // Only manipulate the date if it isn't Mon.
today.setHours(-24 * (day - 1)); // Set the hours to day number minus 1
// multiplied by negative 24
alert(today); // will be Monday
Or as a function:
# modifies _date_
function setToMonday( date ) {
var day = date.getDay() || 7;
if( day !== 1 )
date.setHours(-24 * (day - 1));
return date;
}
setToMonday(new Date());

CMS's answer is correct but assumes that Monday is the first day of the week.
Chandler Zwolle's answer is correct but fiddles with the Date prototype.
Other answers that add/subtract hours/minutes/seconds/milliseconds are wrong because not all days have 24 hours.
The function below is correct and takes a date as first parameter and the desired first day of the week as second parameter (0 for Sunday, 1 for Monday, etc.). Note: the hour, minutes and seconds are set to 0 to have the beginning of the day.
function firstDayOfWeek(dateObject, firstDayOfWeekIndex) {
const dayOfWeek = dateObject.getDay(),
firstDayOfWeek = new Date(dateObject),
diff = dayOfWeek >= firstDayOfWeekIndex ?
dayOfWeek - firstDayOfWeekIndex :
6 - dayOfWeek
firstDayOfWeek.setDate(dateObject.getDate() - diff)
firstDayOfWeek.setHours(0,0,0,0)
return firstDayOfWeek
}
// August 18th was a Saturday
let lastMonday = firstDayOfWeek(new Date('August 18, 2018 03:24:00'), 1)
// outputs something like "Mon Aug 13 2018 00:00:00 GMT+0200"
// (may vary according to your time zone)
document.write(lastMonday)

First / Last Day of The Week
To get the upcoming first day of the week, you can use something like so:
function getUpcomingSunday() {
const date = new Date();
const today = date.getDate();
const currentDay = date.getDay();
const newDate = date.setDate(today - currentDay + 7);
return new Date(newDate);
}
console.log(getUpcomingSunday());
Or to get the latest first day:
function getLastSunday() {
const date = new Date();
const today = date.getDate();
const currentDay = date.getDay();
const newDate = date.setDate(today - (currentDay || 7));
return new Date(newDate);
}
console.log(getLastSunday());
* Depending on your time zone, the beginning of the week doesn't has to start on Sunday, it can start on Friday, Saturday, Monday or any other day your machine is set to. Those methods will account for that.
* You can also format it using toISOString method like so: getLastSunday().toISOString()

Check out Date.js
Date.today().previous().monday()

var dt = new Date(); // current date of week
var currentWeekDay = dt.getDay();
var lessDays = currentWeekDay == 0 ? 6 : currentWeekDay - 1;
var wkStart = new Date(new Date(dt).setDate(dt.getDate() - lessDays));
var wkEnd = new Date(new Date(wkStart).setDate(wkStart.getDate() + 6));
This will work well.

I'm using this
function get_next_week_start() {
var now = new Date();
var next_week_start = new Date(now.getFullYear(), now.getMonth(), now.getDate()+(8 - now.getDay()));
return next_week_start;
}

Returns Monday 00am to Monday 00am.
const now = new Date()
const startOfWeek = new Date(now.getFullYear(), now.getMonth(), now.getDate() - now.getDay() + 1)
const endOfWeek = new Date(now.getFullYear(), now.getMonth(), startOfWeek.getDate() + 7)

This function uses the current millisecond time to subtract the current week, and then subtracts one more week if the current date is on a monday (javascript counts from sunday).
function getMonday(fromDate) {
// length of one day i milliseconds
var dayLength = 24 * 60 * 60 * 1000;
// Get the current date (without time)
var currentDate = new Date(fromDate.getFullYear(), fromDate.getMonth(), fromDate.getDate());
// Get the current date's millisecond for this week
var currentWeekDayMillisecond = ((currentDate.getDay()) * dayLength);
// subtract the current date with the current date's millisecond for this week
var monday = new Date(currentDate.getTime() - currentWeekDayMillisecond + dayLength);
if (monday > currentDate) {
// It is sunday, so we need to go back further
monday = new Date(monday.getTime() - (dayLength * 7));
}
return monday;
}
I have tested it when week spans over from one month to another (and also years), and it seems to work properly.

Good evening,
I prefer to just have a simple extension method:
Date.prototype.startOfWeek = function (pStartOfWeek) {
var mDifference = this.getDay() - pStartOfWeek;
if (mDifference < 0) {
mDifference += 7;
}
return new Date(this.addDays(mDifference * -1));
}
You'll notice this actually utilizes another extension method that I use:
Date.prototype.addDays = function (pDays) {
var mDate = new Date(this.valueOf());
mDate.setDate(mDate.getDate() + pDays);
return mDate;
};
Now, if your weeks start on Sunday, pass in a "0" for the pStartOfWeek parameter, like so:
var mThisSunday = new Date().startOfWeek(0);
Similarly, if your weeks start on Monday, pass in a "1" for the pStartOfWeek parameter:
var mThisMonday = new Date().startOfWeek(1);
Regards,

a more generalized version of this... this will give you any day in the current week based on what day you specify.
//returns the relative day in the week 0 = Sunday, 1 = Monday ... 6 = Saturday
function getRelativeDayInWeek(d,dy) {
d = new Date(d);
var day = d.getDay(),
diff = d.getDate() - day + (day == 0 ? -6:dy); // adjust when day is sunday
return new Date(d.setDate(diff));
}
var monday = getRelativeDayInWeek(new Date(),1);
var friday = getRelativeDayInWeek(new Date(),5);
console.log(monday);
console.log(friday);

Simple solution for getting the first day of the week.
With this solution, it is possible to set an arbitrary start of week (e.g. Sunday = 0, Monday = 1, Tuesday = 2, etc.).
function getBeginOfWeek(date = new Date(), startOfWeek = 1) {
const result = new Date(date);
while (result.getDay() !== startOfWeek) {
result.setDate(result.getDate() - 1);
}
return result;
}
The solution correctly wraps on months (due to Date.setDate() being used)
For startOfWeek, the same constant numbers as in Date.getDay() can be used

setDate() has issues with month boundaries that are noted in comments above. A clean workaround is to find the date difference using epoch timestamps rather than the (surprisingly counterintuitive) methods on the Date object. I.e.
function getPreviousMonday(fromDate) {
var dayMillisecs = 24 * 60 * 60 * 1000;
// Get Date object truncated to date.
var d = new Date(new Date(fromDate || Date()).toISOString().slice(0, 10));
// If today is Sunday (day 0) subtract an extra 7 days.
var dayDiff = d.getDay() === 0 ? 7 : 0;
// Get date diff in millisecs to avoid setDate() bugs with month boundaries.
var mondayMillisecs = d.getTime() - (d.getDay() + dayDiff) * dayMillisecs;
// Return date as YYYY-MM-DD string.
return new Date(mondayMillisecs).toISOString().slice(0, 10);
}

Here is my solution:
function getWeekDates(){
var day_milliseconds = 24*60*60*1000;
var dates = [];
var current_date = new Date();
var monday = new Date(current_date.getTime()-(current_date.getDay()-1)*day_milliseconds);
var sunday = new Date(monday.getTime()+6*day_milliseconds);
dates.push(monday);
for(var i = 1; i < 6; i++){
dates.push(new Date(monday.getTime()+i*day_milliseconds));
}
dates.push(sunday);
return dates;
}
Now you can pick date by returned array index.

An example of the mathematically only calculation, without any Date functions.
const date = new Date();
const ts = +date;
const mondayTS = ts - ts % (60 * 60 * 24 * (7-4) * 1000);
const monday = new Date(mondayTS);
console.log(monday.toISOString(), 'Day:', monday.getDay());
const formatTS = v => new Date(v).toISOString();
const adjust = (v, d = 1) => v - v % (d * 1000);
const d = new Date('2020-04-22T21:48:17.468Z');
const ts = +d; // 1587592097468
const test = v => console.log(formatTS(adjust(ts, v)));
test(); // 2020-04-22T21:48:17.000Z
test(60); // 2020-04-22T21:48:00.000Z
test(60 * 60); // 2020-04-22T21:00:00.000Z
test(60 * 60 * 24); // 2020-04-22T00:00:00.000Z
test(60 * 60 * 24 * (7-4)); // 2020-04-20T00:00:00.000Z, monday
// So, what does `(7-4)` mean?
// 7 - days number in the week
// 4 - shifting for the weekday number of the first second of the 1970 year, the first time stamp second.
// new Date(0) ---> 1970-01-01T00:00:00.000Z
// new Date(0).getDay() ---> 4

It is important to discern between local time and UTC. I wanted to find the start of the week in UTC, so I used the following function.
function start_of_week_utc(date, start_day = 1) {
// Returns the start of the week containing a 'date'. Monday 00:00 UTC is
// considered to be the boundary between adjacent weeks, unless 'start_day' is
// specified. A Date object is returned.
date = new Date(date);
const day_of_month = date.getUTCDate();
const day_of_week = date.getUTCDay();
const difference_in_days = (
day_of_week >= start_day
? day_of_week - start_day
: day_of_week - start_day + 7
);
date.setUTCDate(day_of_month - difference_in_days);
date.setUTCHours(0);
date.setUTCMinutes(0);
date.setUTCSeconds(0);
date.setUTCMilliseconds(0);
return date;
}
To find the start of the week in a given timezone, first add the timezone offset to the input date and then subtract it from the output date.
const local_start_of_week = new Date(
start_of_week_utc(
date.getTime() + timezone_offset_ms
).getTime() - timezone_offset_ms
);

I use this:
let current_date = new Date();
let days_to_monday = 1 - current_date.getDay();
monday_date = current_date.addDays(days_to_monday);
// https://stackoverflow.com/a/563442/6533037
Date.prototype.addDays = function(days) {
var date = new Date(this.valueOf());
date.setDate(date.getDate() + days);
return date;
}
It works fine.

Accepted answer won't work for anyone who runs the code in UTC-XX:XX timezone.
Here is code which will work regardless of timezone for date only. This won't work if you provide time too. Only provide date or parse date and provide it as input. I have mentioned different test cases at start of the code.
function getDateForTheMonday(dateString) {
var orignalDate = new Date(dateString)
var modifiedDate = new Date(dateString)
var day = modifiedDate.getDay()
diff = modifiedDate.getDate() - day + (day == 0 ? -6:1);// adjust when day is sunday
modifiedDate.setDate(diff)
var diffInDate = orignalDate.getDate() - modifiedDate.getDate()
if(diffInDate == 6) {
diff = diff + 7
modifiedDate.setDate(diff)
}
console.log("Given Date : " + orignalDate.toUTCString())
console.log("Modified date for Monday : " + modifiedDate)
}
getDateForTheMonday("2022-08-01") // Jul month with 31 Days
getDateForTheMonday("2022-07-01") // June month with 30 days
getDateForTheMonday("2022-03-01") // Non leap year February
getDateForTheMonday("2020-03-01") // Leap year February
getDateForTheMonday("2022-01-01") // First day of the year
getDateForTheMonday("2021-12-31") // Last day of the year

Extending answer from #Christian C. Salvadó and information from #Ayyash (object is mutable) and #Awi and #Louis Ameline (set hours to 00:00:00)
The function can be like this
function getMonday(d) {
var day = d.getDay(),
diff = d.getDate() - day + (day == 0 ? -6:1); // adjust when day is sunday
d.setDate(diff);
d.setHours(0,0,0,0); // set hours to 00:00:00
return d; // object is mutable no need to recreate object
}
getMonday(new Date())

Check out: moment.js
Example:
moment().day(-7); // last Sunday (0 - 7)
moment().day(7); // next Sunday (0 + 7)
moment().day(10); // next Wednesday (3 + 7)
moment().day(24); // 3 Wednesdays from now (3 + 7 + 7 + 7)
Bonus: works with node.js too

Detect last week of each month with javascript

what would be a way in javascript to detect the last week of each (current) month. Or last monday of the month?

I would suggest to get the number of days in the month and then loop from the last day until getDay() gives back a Monday (1) or Sunday(0) .. based on when does your week start. Once you get your start date ... end date would be startDate + 7 so something along these lines
I found this helpful :
//Create a function that determines how many days in a month
//NOTE: iMonth is zero-based .. Jan is 0, Feb is 2 and so on ...
function daysInMonth(iMonth, iYear)
{
return 32 - new Date(iYear, iMonth, 32).getDate();
}
Then the loop:
//May - should return 31
var days_in_month = daysInMonth(4, 2010);
var weekStartDate = null;
var weekEndDate = null;
for(var i=days_in_month; i>0; i--)
{
var tmpDate = new Date(2010, 4, i);
//week starting on sunday
if(tmpDate.getDay() == 0)
{
weekStartDate = new Date(tmpDate);
weekEndDate = new Date(tmpDate.setDate(tmpDate.getDate() + 6));
//break out of the loop
break;
}
}

Playing with the date object and its methods you can do the following..
update
the complete calculations to get to last monday of the month could be compacted to
var d = new Date();
d.setMonth( d.getMonth() + 1 );
d.setDate(0);
lastmonday = d.getDate() - (d.getDay() - 1);
alert(lastmonday);
verbose example..
var now = new Date(); // get the current date
// calculate the last day of the month
if (now.getMonth() == 11 ) // if month is dec then go to next year and first month
{
nextmonth = 0;
nextyear = now.getFullYear() + 1;
}
else // otherwise go to next month of current year
{
nextmonth = now.getMonth() + 1;
nextyear = now.getFullYear();
}
var d = new Date( nextyear , nextmonth , 0); // setting day to 0 goes to last date of previous month
alert( d.getDay() ); // will alert the day of the week 0 being sunday .. you can calculate from there to get the first day of that week ..

Use getDay() to get the day of week of the last day in month and work from that (substracting the value from the number of days of the month should probably do the trick. +/- 1).

To determine whether it is a Monday, use .getDay() == 1. To determine if it is the last of the month, add seven days and compare months: nextMonday.setDate(monday.getDate()+7);
nextMonday.getMonth() == monday.getMonth();

The Javascript "Date" object is your friend.
function lastOfThisMonth(whichDay) {
var d= new Date(), month = d.getMonth();
d.setDate(1);
while (d.getDay() !== whichDay) d.setDate(d.getDate() + 1);
for (var n = 1; true; n++) {
var nd = new Date(d.getFullYear(), month, d.getDate() + n * 7);
if (nd.getMonth() !== month)
return new Date(d.getFullYear(), month, d.getDate() + (n - 1) * 7).getDate();
}
}
That'll give you the date (in the month, like 30) of the last day of the month that's the chosen day of the week (0 through 7).
Finding the last week of the month will depend on what you mean by that. If you mean the last complete week, then (if you mean Sunday - Saturday) find the last Saturday, and subtract 6. If you mean the last week that starts in the month, find the last Sunday.

You may also like to find the third Monday or the first Tuesday before or after a given date,
or flag every Wednesday between two dates.
Date.prototype.lastweek= function(wd, n){
n= n || 1;
return this.nextweek(wd, -n);
}
Date.prototype.nextweek= function(wd, n){
if(n== undefined) n= 1;
var incr= (n<0)? 1: -1,
D= new Date(this),
dd= D.getDay();
if(wd=== undefined) wd= dd;
if(dd!= wd) while(D.getDay()!= wd) D.setDate(D.getDate()+incr);
D.setDate(D.getDate()+7*n);
D.setHours(0, 0, 0, 0);
return D;
}
function lastMondayinmonth(month, year){
var day= new Date();
if(!month) month= day.getMonth()+1;
if(!year) year= day.getFullYear();
day.setFullYear(year, month, 0);
return day.lastweek(1);
}
alert(lastMondayinmonth())

i found such example that detects last monday of each week but it wont detect last monday of the month. maybe it will help to find better solution, that code looks short.
var dif, d = new Date(); // Today's date
dif = (d.getDay() + 6) % 7; // Number of days to subtract
d = new Date(d - dif * 24*60*60*1000); // Do the subtraction
alert(d); // Last monday.

OK, so far i came up with such solution making it a bit of my own way and getting a few things mentioned here. It works correct and always returns the last monday of current month.
//function that will help to check how many days in month
function daysInMonth(iMonth, iYear)
{
return 32 - new Date(iYear, iMonth, 32).getDate();
}
var dif = null;
d = new Date(); // Today's date
countDays = daysInMonth(d.getMonth(),d.getFullYear()); //Checking number of days in current month
d.setDate(countDays); //setting the date to last day of the month
dif = (d.getDay() + 6) % 7; // Number of days to subtract
d = new Date(d - dif * 24*60*60*1000); // Do the subtraction
alert(d.getDate()); //finally you get the last monday of the current month

Get the last day of the month:
/**
* Accepts either zero, one, or two parameters.
* If zero parameters: defaults to today's date
* If one parameter: Date object
* If two parameters: year, (zero-based) month
*/
function getLastDay() {
var year, month;
var lastDay = new Date();
if (arguments.length == 1) {
lastDay = arguments[0];
} else if (arguments.length > 0) {
lastDay.setYear(arguments[0]);
lastDay.setMonth(arguments[1]);
}
lastDay.setMonth(lastDay.getMonth() + 1);
lastDay.setDate(0);
return lastDay;
}
Get the last Monday:
/**
* Accepts same parameters as getLastDay()
*/
function getLastMonday() {
var lastMonday = getLastDay.apply(this, arguments);
lastMonday.setDate(lastMonday.getDate() - (lastMonday.getDay() == 0 ? 6 : (lastMonday.getDay() - 1)));
return lastMonday;
}
Get week of the year for a given day:
/**
* Accepts one parameter: Date object.
* Assumes start of week is Sunday.
*/
function getWeek(d) {
var jan1 = new Date(d.getFullYear(), 0, 1);
return Math.ceil((((d - jan1) / (24 * 60 * 60 * 1000)) + jan1.getDay() + 1) / 7);
}
Putting them together (assuming you're using Firebug):
// Get the last day of August 2006:
var august2006 = new Date(2006, 7);
var lastDayAugust2006 = getLastDay(august2006);
console.log("lastDayAugust2006: %s", lastDayAugust2006);
// ***** Testing getWeek() *****
console.group("***** Testing getWeek() *****");
// Get week of January 1, 2010 (Should be 1):
var january12010Week = getWeek(new Date(2010, 0, 1));
console.log("january12010Week: %s", january12010Week);
// Get week of January 2, 2010 (Should still be 1):
var january22010Week = getWeek(new Date(2010, 0, 2));
console.log("january22010Week: %s", january22010Week);
// Get week of January 3, 2010 (Should be 2):
var january32010Week = getWeek(new Date(2010, 0, 3));
console.log("january32010Week: %s", january32010Week);
console.groupEnd();
// *****************************
// Get the last week of this month:
var lastWeekThisMonth = getWeek(getLastDay());
console.log("lastWeekThisMonth: %s", lastWeekThisMonth);
// Get the last week of January 2007:
var lastWeekJan2007 = getWeek(getLastDay(2007, 0));
console.log("lastWeekJan2007: %s", lastWeekJan2007);
// Get the last Monday of this month:
var lastMondayThisMonth = getLastMonday();
console.log("lastMondayThisMonth: %s", lastMondayThisMonth);
// Get the week of the last Monday of this month:
var lastMondayThisMonthsWeek = getWeek(lastMondayThisMonth);
console.log("lastMondayThisMonthsWeek: %s", lastMondayThisMonthsWeek);