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) /
+    periodLengthStats.median = sortedCycleLengths[
-       2 - 1]
+      (cycleLengths.length + 1) / 2 - 1
-  }
+    ]
-  else {
+  } 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.')
   })
 }

test/periode-length.spec.js

@@ -5,7 +5,7 @@ import periodInfo from '../lib/period-length'
 
 const expect = chai.expect
 
-describe('getPeriodLengthStats', () => {
+describe.only('getPeriodLengthStats', () => {
   it('works for a simple odd-numbered array', () => {
     const periodLengths = [99, 5, 1, 2, 100]
     const result = periodInfo(periodLengths)
@@ -31,6 +31,18 @@ describe('getPeriodLengthStats', () => {
     }
     expect(result).to.eql(expectedResult)
   })
+  it('works for an one-element array', () => {
+    const periodLengths = [42]
+    const result = periodInfo(periodLengths)
+    const expectedResult = {
+      minimum: 42,
+      maximum: 42,
+      mean: 42,
+      median: 42,
+      stdDeviation: null
+    }
+    expect(result).to.eql(expectedResult)
+  })
   describe('when args are wrong', () => {
     it('throws when arg object is an empty array', () => {
       const periodLengths = []