Discrete random variable standard deviation in JavaScript

weightedStdDev.js

function weightedStdDev(values, weights) {

  var avg = weightedMean(values, weights);

  var result = values.map(function (value, i) {

    var weight = weights[i];
    var diff = value - avg;
    var sqrDiff = weight * Math.pow(diff, 2);

    return [sqrDiff, weight];
  }).reduce(function (previous, current) {

    return [previous[0] + current[0], previous[1] + current[1]];
  }, [0, 0]);

  return Math.sqrt(result[0] / result[1]);
}

function weightedMean(values, weights) {

  var result = values.map(function (value, i) {

    var weight = weights[i];
    var sum = value * weight;

    return [sum, weight];
  }).reduce(function (previous, current) {

    return [previous[0] + current[0], previous[1] + current[1]];
  }, [0, 0]);

  return result[0] / result[1];
}

weightedStdDev([251, 360, 210, 900, 425, 12], [0.1, 0.5, 0.7, 0.3, 0.4, 0.1]);
// => 236.5988980294558