Weighted median in JavaScript

weightedMedian.js

function weightedMedian(values, weights) {

  var midpoint = 0.5 * sum(weights);

  var cumulativeWeight = 0;
  var belowMidpointIndex = 0;

  var sortedValues = [];
  var sortedWeights = [];

  values.map(function (value, i) {

    return [value, weights[i]];
  }).sort(function (a, b) {

    return a[0] - b[0];
  }).map(function (pair) {

    sortedValues.push(pair[0]);
    sortedWeights.push(pair[1]);
  });

  if (sortedWeights.some(function (value) { return value > midpoint; })) {

    return sortedValues[sortedWeights.indexOf(Math.max.apply(null, sortedWeights))];
  }

  while (cumulativeWeight <= midpoint) {

    belowMidpointIndex++;
    cumulativeWeight += sortedWeights[belowMidpointIndex - 1];
  }

  cumulativeWeight -= sortedWeights[belowMidpointIndex - 1];

  if (cumulativeWeight - midpoint < Number.EPSILON) {

    var bounds = sortedValues.slice(belowMidpointIndex - 2, belowMidpointIndex);
    return sum(bounds) / bounds.length;
  }

  return sortedValues[belowMidpointIndex - 1];
}

function sum(arr) {

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

    return previous + current;
  });
}

weightedMedian([251, 360, 210], [0.1, 0.5, 0.7]);
// => 210

weightedMedian([251, 360, 210, 340], [0.1, 0.5, 0.7, 0.6]);
// => 295.5

Nice! Going to try this out in my finance app. Or something similar...