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

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

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

tests:
  - text: <code>countingSundays(1943, 1946)</code> should return a number.
    testString: assert(typeof countingSundays(1943, 1946) === 'number');
  - text: <code>countingSundays(1943, 1946)</code> should return 6.
    testString: assert.strictEqual(countingSundays(1943, 1946), 6);
  - text: <code>countingSundays(1995, 2000)</code> should return 10.
    testString: assert.strictEqual(countingSundays(1995, 2000), 10);
  - text: <code>countingSundays(1901, 2000)</code> should return 171.
    testString: assert.strictEqual(countingSundays(1901, 2000), 171);

Challenge Seed

function countingSundays(firstYear, lastYear) {

  return true;
}

countingSundays(1943, 1946);

Solution

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;
}