freeCodeCamp/curriculum/challenges/en/08-coding-interview-prep/project-euler/problem-19-counting-sundays.en.md

id, challengeType, title

id	challengeType	title
5900f37f1000cf542c50fe92	5	Problem 19: Counting Sundays

Description

You are given the following information, but you may prefer to do some research for yourself.

• 1 Jan 1900 was a Monday.
• Thirty days has September,
  April, June and November.
  All the rest have thirty-one,
  Saving February alone,
  Which has twenty-eight, rain or shine.
  And on leap years, twenty-nine.
• A leap year occurs on any year evenly divisible by 4, but not on a century unless it is divisible by 400.
How many Sundays fell on the first of the month during the twentieth century (1 Jan 1901 to 31 Dec 2000)?

Instructions

Tests

- text: '<code>countingSundays(1943, 1946)</code> should return 6.'
  testString: 'assert.strictEqual(countingSundays(1943, 1946), 6, ''<code>countingSundays(1943, 1946)</code> should return 6.'');'
- text: '<code>countingSundays(1995, 2000)</code> should return 9.'
  testString: 'assert.strictEqual(countingSundays(1995, 2000), 9, ''<code>countingSundays(1995, 2000)</code> should return 9.'');'
- text: '<code>countingSundays(1901, 2000)</code> should return 171.'
  testString: 'assert.strictEqual(countingSundays(1901, 2000), 171, ''<code>countingSundays(1901, 2000)</code> should return 171.'');'

Challenge Seed

function countingSundays(firstYear, lastYear) {
  // Good luck!
  return true;
}

countingSundays(1943, 1946);

Solution

function countingSundays(firstYear, lastYear) {
  let sundays = 0;

  for (let year = firstYear; year <= lastYear; year++) {
    for (let month = 1; month <= 12; month++) {
      const thisDate = new Date(year, month, 1);
      if (thisDate.getDay() === 0) {
        sundays++;
      }
    }
  }
  return sundays;
}