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

Question

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 play with hour/minutes/seconds/milliseconds are wrong.

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()

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.

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()

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;
}