mean.js

const mean = (values) => {
  const sum = values.reduce((sum, value) => (sum + value), 0)
  const valuesCount = values.length
  return (sum / valuesCount)
}

standard-deviation.js

const standardDeviation = (values) => {
  const average = mean(values)
  const squaredDifferences = values.map((value) => {
    const difference = (value - average)
    return (difference * difference)
  })
  const squaredDifferencesAverage = mean(squaredDifferences)
  return Math.sqrt(squaredDifferencesAverage)
}