Question 51456 in Javascript Question posted in
A very good W3school tutorial can be found here.

Get the first day of every week until infinity in JavaScript

I want a program that can generate the date of first date of every week, so that I can insert it into the code below

const dateOfTheFirstDay = null;

//preferred format: "April 9, 2023 00:00:01"

const d = new Date(dateOfTheFirstDay);
  setNewWeekTime(d.getTime());

From the code above, I can then get the milliseconds between 1 January 1970 00:00 and the first date of the week.

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3

Answers


  1. 0

    Here could be the code to achieve your request in javascript

    const startDate = new Date('January 1, 2023');
const endDate = new Date('December 31, 2023');
const days = ['Sunday'];
const sundayDates = [];

for (let date = startDate; date <= endDate; date.setDate(date.getDate() + 1)) {
   if (days.includes(date.toLocaleString('en-us', { weekday: 'long' }))) {
        sundayDates.push(new Date(date));
   }
}

    sundayDates array will give you the dates you wish

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  2. 0

    Since any given day of the week is guaranteed to be 7 days apart you really just need to find an initial reference date and then increment by multiples of 7 either forward or backward. Keep in mind that you will need to decide how to handle timezone issues.

    Here is a generator based solution that will yield any given day forwards from 1583 (to avoid wierdness of dates before adoption of the Gregorian calendar, see proleptic Gregorian calendar).

    function* dayGenerator(day) {
  const addDays = (date, days) => date.setUTCDate(date.getUTCDate() + days);

  const ref = new Date(Date.UTC(1583, 0, 1, 12, 0, 0));
  while (ref.toLocaleString('en-us', { weekday: 'long' }) !== day) {
    addDays(ref, 1);
  }

  yield new Date(ref);

  while (true) {
    yield new Date(addDays(ref, 7));
  }
}


/* Example output */

// The first 10,000 Sundays from Jan 1, 1583
console.log('Sundays:');
const sundayGenerator = dayGenerator('Sunday');
for (let i = 0; i < 10000; i++) {
  const next = sundayGenerator.next().value;
  if (i <= 3 || i === 9999) {
    console.log(
      `Sunday ${i + 1}:`,
      next,
      next.toLocaleString('en-us', { weekday: 'long' })
    );
  }
}

// A few Tuesdays
console.log('Tuesdays:');
const tuesdayGenerator = dayGenerator('Tuesday');
for (let i = 0; i < 3; i++) {
  const next = tuesdayGenerator.next().value;
  console.log(next, next.toLocaleString('en-us', { weekday: 'long' }));
}
    .as-console-wrapper { max-height: 100% !important; top: 0; }
.as-console-row::after { display: none !important; }

    You can make it generic by accepting a startDate and endDate and could further customize to increment backwards if start > end. Here using an alternative arithmetic based method for determining the first matching day after the passed start date.

    function* dayGenerator(
  day,
  startDateISO = '1583-01-01T12:00:00.000Z',
  endDateISO = '9999-12-31T12:00:00.000Z'
) {
  const days = ['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday',];

  const ref = new Date(startDateISO);
  ref.setDate(ref.getDate() + ((7 + days.indexOf(day) - ref.getDay()) % 7));

  yield new Date(ref);

  const endDate = new Date(endDateISO);
  while (ref < endDate) {
    yield new Date(ref.setDate(ref.getDate() + 7));
  }
}

// Spread the full output of the iterator into an array
const sundays2023 = [
  ...dayGenerator('Sunday', '2023-01-01T12:00:00Z', '2023-12-31T12:00:00Z'),
];

console.log(
  'First: ',
  sundays2023[0],
  sundays2023[0].toLocaleString('en-us', { weekday: 'long' })
);
console.log(
  'Last: ',
  sundays2023.at(-1),
  sundays2023.at(-1).toLocaleString('en-us', { weekday: 'long' })
);

    (curiously 2023 starts and ends on a Sunday)

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  3. 0

    An efficient algorithm to get an array of Sundays between two dates is to get the Sunday on or before a reference date, then sequentially add 7 days until the end date is reached.

    The following gets all Sundays for the year of the supplied date, with the current date (and hence year) as the default.

    E.g.

    // Return an array of all Sundays in year of
// supplied date
function getSundaysInYear(date = new Date()){
  // Get first Sunday of year
  let year = date.getFullYear();
  let start = new Date(year, 0, 7);
  start.setDate(start.getDate() - start.getDay());
  // Set last date of year
  let end = new Date(year, 11, 31);
  // Array of Sundays
  let sundays = [];

  while (start <= end) {
    sundays.push(new Date(start));
    start.setDate(start.getDate() + 7);
  }
  return sundays
}

// Example
let d = new Date()
let sundays = getSundaysInYear(d);
console.log(`${d.getFullYear()} has ${sundays.length} Sundays.` +
  `nFirst: ${sundays[0].toString().substr(0,10)} ` +
  `nLast : ${sundays[sundays.length - 1].toString().substr(0,10)}`
);
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