freeCodeCamp/curriculum/challenges/english/10-coding-interview-prep/project-euler/problem-19-counting-sundays.md

id, title, challengeType, forumTopicId, dashedName

id	title	challengeType	forumTopicId	dashedName
5900f37f1000cf542c50fe92	Problem 19: Counting Sundays	5	301827	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)?

--hints--

countingSundays(1943, 1946) should return a number.

assert(typeof countingSundays(1943, 1946) === 'number');

countingSundays(1943, 1946) should return 6.

assert.strictEqual(countingSundays(1943, 1946), 6);

countingSundays(1995, 2000) should return 10.

assert.strictEqual(countingSundays(1995, 2000), 10);

countingSundays(1901, 2000) should return 171.

assert.strictEqual(countingSundays(1901, 2000), 171);

--seed--

--seed-contents--

function countingSundays(firstYear, lastYear) {

  return true;
}

countingSundays(1943, 1946);

--solutions--

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

  for (let year = firstYear; year <= lastYear; year++) {
    for (let month = 0; month <= 11; month++) {
      const thisDate = new Date(year, month, 1);
      if (thisDate.getDay() === 0) {
        sundays++;
      }
    }
  }
  return sundays;
}