standardDeviation

Calculates the standard deviation of an array of numbers.

• Use Array.prototype.reduce() to calculate the mean, variance and the sum of the variance of the values and determine the standard deviation.
• Omit the second argument, usePopulation, to get the sample standard deviation or set it to true to get the population standard deviation.

const standardDeviation = (arr, usePopulation = false) => {
  const mean = arr.reduce((acc, val) => acc + val, 0) / arr.length;
  return Math.sqrt(
    arr
      .reduce((acc, val) => acc.concat((val - mean) ** 2), [])
      .reduce((acc, val) => acc + val, 0) /
      (arr.length - (usePopulation ? 0 : 1))
  );
};

standardDeviation([10, 2, 38, 23, 38, 23, 21]); // 13.284434142114991 (sample)
standardDeviation([10, 2, 38, 23, 38, 23, 21], true);
// 12.29899614287479 (population)