Standard deviation in js

stddev.js

function stddev(values) {
  return Math.sqrt(mean(values.map(square)) - square(mean(values)));
}

function mean(values) {
  return values.reduce(add, 0) / values.length;
}

function add(x, y) {
  return x + y;
}

function square(x) {
  return x * x;
}