resolves some of the suggested improvements, renames the term period with cycle

lib/cycle-length.js

@@ -0,0 +1,49 @@
+import assert from 'assert'
+
+export default function getCycleLengthStats(cycleLengths) {
+  throwIfArgsAreNotInRequiredFormat(cycleLengths)
+  const cycleLengthStats = {}
+  const sortedCycleLengths = cycleLengths.sort((a, b) => {
+    return a - b
+  })
+  cycleLengthStats.minimum = sortedCycleLengths[0]
+  cycleLengthStats.maximum = sortedCycleLengths[cycleLengths.length - 1]
+  cycleLengthStats.mean = Math.round(
+    cycleLengths.reduce(getSum) / cycleLengths.length * 100
+  ) / 100
+  // median
+  if (cycleLengths.length % 2 == 1) {
+    cycleLengthStats.median = sortedCycleLengths[
+      (cycleLengths.length + 1) / 2 - 1
+    ]
+  } else {
+    const middle = cycleLengths.length / 2
+    cycleLengthStats.median = (sortedCycleLengths[middle - 1] +
+       sortedCycleLengths[middle]) / 2
+  }
+  // corrected standard deviation (based on unbiased sample variance)
+  if (cycleLengths.length > 1) {
+    const sumOfSquares = cycleLengths.map(cycleLength => {
+      return Math.pow(cycleLength - cycleLengthStats.mean, 2)
+    }).reduce(getSum)
+    cycleLengthStats.stdDeviation = Math.round(
+      Math.sqrt(sumOfSquares / (cycleLengths.length - 1 )) * 100
+    ) / 100
+  } else {
+    cycleLengthStats.stdDeviation = null
+  }
+  return cycleLengthStats
+}
+
+function getSum(total, num) {
+  return total + num
+}
+
+function throwIfArgsAreNotInRequiredFormat(cycleLengths) {
+  assert.ok(Array.isArray(cycleLengths), 'Input should be an array.')
+  assert.ok(cycleLengths.length > 0, 'Input array should not be empty.')
+  cycleLengths.forEach(cycleLength => {
+    assert.equal(typeof cycleLength, 'number', 'Elements in the array should be of type number.')
+    assert.ok(!isNaN(cycleLength), 'Elements of array should not be NaN.')
+  })
+}