standardDeviation.js

let orders = [26, 3, 5, 7, 8, 5, 0, 8, 4]

let arrSum = arr => (arr.reduce((x, accumulate) => accumulate + x, 0))

let avgOrders = arrSum(orders)/orders.length
console.log(avgOrders)

let differences = orders.map(x => x - avgOrders).map(x => x * x)
console.log(differences)

let variance = arrSum(differences)/differences.length // orders.length === differences.length
console.log(variance)

let standardDeviation = Math.sqrt(variance)
console.log(standardDeviation)

let isOutlier = orders.map(x => Math.abs(x - avgOrders) >  standardDeviation)
console.log(isOutlier)