feat(isYearLeapYear): Add isLeapYear function (#173)

* feat(isYearLeapYear): Add isLeapYear function

* Update index.js

Author: FollowJack

src/index.js

@@ -60,6 +60,7 @@ import swapElements from './swapElements'
 import reverse from './reverse'
 import removeAccents from './remove-accents'
 import getQueryStringValue from './get-query-string-value'
+import isLeapYear from './leap-year'
 
 export {
   reverseArrayInPlace,
@@ -124,4 +125,5 @@ export {
   removeAccents,
   getQueryStringValue,
   lessThan,
+  isLeapYear,
 }

src/leap-year.js

@@ -0,0 +1,12 @@
+export default isLeapYear
+
+/**
+ * This method will check if the given year is a leap year.
+ * More information on http://www.youtube.com/watch?v=xX96xng7sAE
+ *
+ * @param {Number} year - year to check if leap year
+ * @return {Bool} - Resul if year is leap year
+ */
+function isLeapYear(year) {
+  return year % 4 === 0 && (year % 100 !== 0 || year % 400 === 0)
+}

test/leap-year.test.js

@@ -0,0 +1,30 @@
+import test from 'ava'
+import leapYear from '../src/leap-year'
+
+test('leap year is every 4 years to adjust about a day', t => {
+  const year = 2016
+  const expected = true
+  const actual = leapYear(year)
+  t.deepEqual(actual, expected)
+})
+
+test('Leap year is skipped every 100 years to remove an extra day', t => {
+  const year = 1900
+  const expected = false
+  const actual = leapYear(year)
+  t.deepEqual(actual, expected)
+})
+
+test('Leap year is reintroduced every 400 years to adjust another day', t => {
+  const year = 2000
+  const expected = true
+  const actual = leapYear(year)
+  t.deepEqual(actual, expected)
+})
+
+test('A year that is not a leap year', t => {
+  const year = 1978
+  const expected = false
+  const actual = leapYear(year)
+  t.deepEqual(actual, expected)
+})