Weighted Sampling

weighted.js

function rand (max) { return Math.random() * max; }

function randomSample (array) {
    var answer = array[0];
    var totalWeight = array[0].w;
    var left = array.slice(1);

    left.forEach(function (element) {
        var w = element.w;
        var r = rand(totalWeight + w);

        if (r > totalWeight) {
            answer = element;
        }

        totalWeight += w;
    });

    return answer;
}

var testElements = [{n: 'fe', w: 5.33},{n: 'fi', w: 1.33},{n: 'fo', w: 10}, {n: 'fum', w: 3.33}];

var runs = 100000;
var num = runs;
var results = {fe:0, fi:0, fo:0, fum:0};

while (num--) {
    var answer = randomSample(testElements);
    results[answer.n]++;
}

Object.keys(results).forEach(function (key) {
    results[key] = results[key] / runs * 100;
});

console.log(results);