30-seconds-of-code/snippets/standard-deviation.md

Standard deviation

Use Array.reduce() to calculate the mean of the values, the variance of the values, and the sum of the variance of the values to determine the standard deviation of an array of numbers.

NOTE: This is population standard deviation. Use / (arr.length - 1) at the end to calculate sample standard deviation.

const standardDeviation = arr =>
  Math.sqrt(
    arr.reduce((acc, val) => acc.concat(Math.pow(val - arr.reduce((acc, val) => acc + val, 0) / arr.length, 2)), [])
       .reduce((acc, val) => acc + val, 0) 
    / arr.length
  );
// standardDeviation([10,2,38,23,38,23,21]) -> 12.298996142875