[Solution] Project euler challenge 19 with tests

Project-Euler/Problem019.js

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+/**
+ * Problem 19 - Counting Sundays
+ *
+ * @see {@link https://projecteuler.net/problem=19}
+ *
+ * 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)?
+ *
+ * @author ddaniel27
+ */
+
+// Check if a year is a leap year
+// A leap year occurs on any year evenly divisible by 4, but not on a century unless it is divisible by 400.
+function isLeapYear(year) {
+  return (year % 4 === 0 && year % 100 !== 0) || year % 400 === 0
+}
+
+function problem19() {
+  let sundaysCount = 0 // Count of Sundays
+  let dayOfWeek = 2 // 1st Jan 1900 was a Monday, so 1st Jan 1901 was a Tuesday
+
+  for (let year = 1901; year <= 2000; year++) {
+    // From 1901 to 2000
+    for (let month = 1; month <= 12; month++) {
+      // From January to December
+      if (dayOfWeek === 0) {
+        // If it's a Sunday (0 is Sunday, 1 is Monday, ..., 6 is Saturday)
+        sundaysCount++
+      }
+
+      let daysInMonth = 31
+      if (month === 4 || month === 6 || month === 9 || month === 11) {
+        // April, June, September, November
+        daysInMonth = 30
+      } else if (month === 2) {
+        // February
+        daysInMonth = isLeapYear(year) ? 29 : 28
+      }
+
+      dayOfWeek = (dayOfWeek + daysInMonth) % 7 // Calculate the day of the week
+    }
+  }
+
+  return sundaysCount
+}
+
+export { problem19 }

Project-Euler/test/Problem019.test.js

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+import { problem19 } from '../Problem019.js'
+
+describe('checking sundays during the twentieth century', () => {
+  // Project Euler Challenge Check
+  test('result should be 171', () => {
+    expect(problem19()).toBe(171)
+  })
+})