perform a weighted random selection in JS

weightedRandomSelector2.js

// weightedRandomSelector.js
//
// This is a simple weighted random selector, works in Rhino, can use within an
// Apigee JavaScript step.
//
// last saved: <2021-December-02 13:11:16>
//
// --------------------------------------------------------

(function (){

  function WeightedRandomSelector(a) {
    // This implements a weighted random selector, over an array. The argument a
    // must be an array of elements, each of which has a weight property. The
    // items need not be sorted in any particular order, and the weights need
    // not sum to any particular value.  Example usage:
    //
    // var routingTable = [
    //   { "name" : "A", "url": "https://a.example.com", "weight": 6 },
    //   { "name" : "B", "url": "https://b.example.com", "weight": 16 },
    //   { "name" : "C", "url": "https://c.example.com", "weight": 50 },
    //   { "name" : "D", "url": "https://d.example.com", "weight": 2 }
    // ];
    //
    // var wrs = new WeightedRandomSelector(routingTable);
    // var selected = wrs.select();
    // var selectedUrl = selected.url;
    //

    var i, L;
    this.totalWeight = 0;
    this.a = a;
    this.selectionCounts = [];
    this.weightThreshold = [];
    // initialize
    for (i = 0, L = a.length; i<L; i++) {
      this.totalWeight += a[i].weight;
      this.weightThreshold[i] = this.totalWeight;
      this.selectionCounts[i] = 0;
    }
  }

  WeightedRandomSelector.prototype.select = function() {
    // 1. select a random value
    var R = Math.floor(Math.random() * this.totalWeight),
        i, L;

    // 2. find the bucket that R value falls into.
    for (i = 0, L = this.a.length; i < L; i++) {
      if (R < this.weightThreshold[i]) {
        this.selectionCounts[i]++;
        return(this.a[i]);
      }
    }
    // 3. return the selected item
    return this.a[L - 1];
  };


  // export into the global namespace
  if (typeof exports === "object" && exports) {
    // works for nodejs
    exports.WeightedRandomSelector = WeightedRandomSelector;
  }
  else {
    // works in rhino
    var globalScope = (function(){ return this; }).call(null);
    globalScope.WeightedRandomSelector = WeightedRandomSelector;
  }

}());