Standard deviation in JavaScript

stdDev.js

function stdDev(arr) {

  var avg = mean(arr);

  var squareDiffs = arr.map(function (value) {

    var diff = value - avg;
    var sqrDiff = diff * diff;
    return sqrDiff;
  });

  return Math.sqrt(mean(squareDiffs));
}

function mean(arr) {

  return arr.reduce(function (previous, current) {

    return previous + current;
  }, 0) / arr.length;
}

stdDev([251, 360, 210, 900, 425, 12]);
// => 274.25940680717264