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)

Check out Date.js

Date.today().previous().monday()

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 dayOfTheWeek = date.getDay();
  const newDate = date.setDate(today - dayOfTheWeek + 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 dayOfTheWeek = date.getDay();
  const newDate = date.setDate(today - (dayOfTheWeek || 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()

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;
}

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,

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

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);

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)

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.

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

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