Weighted random sample from a vector in JavaScript

sample.js

// Weighted random sample from a vector
//
// By default, the `weights` are set to 1. This equates to equal weighting.
// Loosely based on http://codereview.stackexchange.com/a/4265
//
// If any weight is `null`, revert to default weights (i.e., all 1).
//
// A random-number generator (RNG) seed is optionally set via seedrandom.js.
// NOTE: The JS file is loaded via jQuery.
// Details: https://github.com/davidbau/seedrandom
//
// @param vector to sample
// @param weights optional array of weights
// @param seed optional RNG seed to reproduce
// @return random selection from `vector`
var sample = function(vector, weights, seed) {
  weights_undefined = typeof weights === 'undefined';

  has_null_weights = false;
  if (!weights_undefined) {
      has_null_weights = weights.some(function(x) {return x === null;})
  }

  if (weights_undefined || has_null_weights) {
      weights = Array(vector.length);
      for (i = 0; i < weights.length; i++) {
          weights[i] = 1;
      }
  }
  // Normalize weights to sum to 1
  // For vector sum, source: http://stackoverflow.com/a/3762623/234233
  weights_sum = weights.reduce(function(x, y) { return x + y; }, 0);
  for (i = 0; i < weights.length; i++) {
      weights[i] = weights[i] / weights_sum;
  }

  // Set RNG seed (if given)
  var random_draw = null;
  if (!(typeof seed === 'undefined')) {
      $.getScript('http://cdnjs.cloudflare.com/ajax/libs/seedrandom/2.4.0/seedrandom.min.js')
          .done(function() {
              console.log('seedrandom.js loaded properly with seed: ' + seed);
              Math.seedrandom(seed);
              random_draw = Math.random();
          }).fail(function() {
              console.log('seedrandom.js failed to load');
          });
  }
  if (!random_draw) {
    random_draw = Math.random();
  }
  var cum_weights = 0;

  for (i = 0; i < vector.length; i++) {
      cum_weights += weights[i];
      if (random_draw < cum_weights) {
          return(vector[i]);
      }
  }
}

The RNG seed is set via seedrandom.js. NOTE: This file is loaded using jQuery's $.getScript.