JavaScript - Get the First Day of the Week from Current Date

How to get first and last day of the current week in JavaScript

var curr = new Date; // get current date
var first = curr.getDate() - curr.getDay(); // First day is the day of the month - the day of the week
var last = first + 6; // last day is the first day + 6

var firstday = new Date(curr.setDate(first)).toUTCString();
var lastday = new Date(curr.setDate(last)).toUTCString();

firstday
"Sun, 06 Mar 2011 12:25:40 GMT"
lastday
"Sat, 12 Mar 2011 12:25:40 GMT"

This works for firstday = sunday of this week and last day = saturday for this week. Extending it to run Monday to sunday is trivial.

Making it work with first and last days in different months is left as an exercise for the user

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

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

How to get first and last day of current week when days are in different months?

The getDay method returns the number of the day in the week, with Sunday as 0 and Saturday as 6. So if your week starts on Sunday, just subtract the current day number in days from the current date to get the start, and add 6 days get the end, e.g.

function getStartOfWeek(date) {    // Copy date if provided, or use current date if not  date = date? new Date(+date) : new Date();  date.setHours(0,0,0,0);    // Set date to previous Sunday  date.setDate(date.getDate() - date.getDay());    return date;}
function getEndOfWeek(date) {  date = getStartOfWeek(date);  date.setDate(date.getDate() + 6);  return date; }  document.write(getStartOfWeek());
document.write('<br>' + getEndOfWeek())
document.write('<br>' + getStartOfWeek(new Date(2016,2,27)))
document.write('<br>' + getEndOfWeek(new Date(2016,2,27)))

How can I get the first and last day of the week crossing months in javascript

You can use date-fns library for that:

const start = startOfWeek(date);
const end = endOfWeek(date);

Check out these threads for more solutions:

How to get first and last day of current week when days are in different months?
How to get first and last day of the week in JavaScript

How do I get the first day of the first week in a given year, in javascript?

so I have two solutions to offer
1 quite simple (making use of the Date object - basically doing what barrycarter suggested in a comment)

Creating a Date object for the first of our desired year ... and then based on which day of the week that is, adjusting the date to end up with the first monday of the first week of the year

As you might notice when running the code. The answers are off by (mostly 1 hour) - I don't know why, and so far I only encountered this when running my code on here - in VS Code and several Online JS Playgrounds the code returned exactly what I intended it to return. So yeah there's that.

(date.getDay() + 6) % 7 // 0 - 6 for Mon - Sun

function getDateOfFirstDayOfFirstWeek(year) {
  const date = new Date(year, 0, 1)
  if ((date.getDay() + 6) % 7 === 0) return date
  if ((date.getDay() + 6) % 7 < 4) {
    date.setDate(date.getDate() - (date.getDay() + 6) % 7)
    return date
  } else {
    date.setDate(8 - (date.getDay() + 6) % 7)
    return date
  }
}

console.log(getDateOfFirstDayOfFirstWeek(2020))
console.log(getDateOfFirstDayOfFirstWeek(2021))
console.log(getDateOfFirstDayOfFirstWeek(2022))

console.log(getDateOfFirstDayOfFirstWeek(2100))
console.log(getDateOfFirstDayOfFirstWeek(3567))
console.log(getDateOfFirstDayOfFirstWeek(2002))
console.log(getDateOfFirstDayOfFirstWeek(1845))

Javascript : Get the first day of the week passing the current date

function getStartOfWeek(index) {  var d = new Date();  var day = d.getDay();  var diff = index - day;  if(index > day) {    diff -= 7;   }  return new Date(d.setDate(d.getDate() + diff));}
document.write(getStartOfWeek(0).toLocaleDateString()+"<br/>");document.write(getStartOfWeek(1).toLocaleDateString()+"<br/>");document.write(getStartOfWeek(2).toLocaleDateString()+"<br/>");document.write(getStartOfWeek(3).toLocaleDateString()+"<br/>");document.write(getStartOfWeek(4).toLocaleDateString()+"<br/>");document.write(getStartOfWeek(5).toLocaleDateString()+"<br/>");document.write(getStartOfWeek(6).toLocaleDateString()+"<br/>");