little improvements and test for array of one element

lib/period-length.js

@@ -9,25 +9,28 @@ export default function getPeriodLengthStats(cycleLengths) {
   periodLengthStats.minimum = sortedCycleLengths[0]
   periodLengthStats.maximum = sortedCycleLengths[cycleLengths.length - 1]
   periodLengthStats.mean = Math.round(
-    cycleLengths.reduce(getSum) / cycleLengths.length * 100) / 100
+    cycleLengths.reduce(getSum) / cycleLengths.length * 100
+  ) / 100
   // median
   if (cycleLengths.length % 2 == 1) {
-    periodLengthStats.median = sortedCycleLengths[(cycleLengths.length + 1) /
-       2 - 1]
-  }
-  else {
+    periodLengthStats.median = sortedCycleLengths[
+      (cycleLengths.length + 1) / 2 - 1
+    ]
+  } else {
     const middle = cycleLengths.length / 2
     periodLengthStats.median = (sortedCycleLengths[middle - 1] +
        sortedCycleLengths[middle]) / 2
   }
   // corrected standard deviation (based on unbiased sample variance)
-  periodLengthStats.stdDeviation = null // for case t
   if (cycleLengths.length > 1) {
     const sumOfSquares = cycleLengths.map(cycleLength => {
       return Math.pow(cycleLength - periodLengthStats.mean, 2)
     }).reduce(getSum)
     periodLengthStats.stdDeviation = Math.round(
-      Math.sqrt(sumOfSquares / (cycleLengths.length - 1 )) * 100) / 100
+      Math.sqrt(sumOfSquares / (cycleLengths.length - 1 )) * 100
+    ) / 100
+  } else {
+    periodLengthStats.stdDeviation = null
   }
   return periodLengthStats
 }
@@ -37,10 +40,10 @@ function getSum(total, num) {
 }
 
 function throwIfArgsAreNotInRequiredFormat(cycleLengths) {
-  assert.ok(Array.isArray(cycleLengths), 'Function requires input to be an array.')
+  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.')
+    assert.ok(!isNaN(cycleLength), 'Elements of array should not be NaN.')
   })
 }