Test the weightRandom function by calling 1000 times and counting each result.

testWeightedRandom.js

// array of items. Weight must add up to 100
// video1 = 20%, vieo2 = 50%, video3 = 10%, video4 = 20%
var items = ['Video1', 'Video2', 'Video3', 'Video4'];
var weights = [20, 50, 10, 20];

/**
 * Return a random value from aItems based on aWeights
 * @param  {array} aItems   Array of you items.
 * @param  {array} aWeights Array of weights that match items.
 * @return {String}         Selected item.
 */
var weightedRandom = function(aItems, aWeights){
    var sum = 0;
    var rand = Math.floor(Math.random() * 100);

    for (var i = aWeights.length - 1; i >= 0; i--) {sum += aWeights[i]};
    if( sum > 100 ) { throw new Error("WeightedRandom:WeightsExceed100")};

    // Create array with
    var weightedList = [];
    for (var i = aItems.length - 1; i >= 0; i--) {
        for (var x = aWeights[i] - 1; x >= 0; x--) {
            weightedList.push(aItems[i]);
        };
    };
    return weightedList[rand];
};

var testRandom = function(iterations, items, weights){
    // Create base object to store results.
    var results = {};
    var item;
    for (var i = 0; i < items.length; i++) {
        results[items[i]] = 0;
    };

    // Call weightedRandom iterations times
    while( iterations > 0 ){
        item = weightedRandom(items, weights);
        results[item]++;
        iterations--;
    }

    return results;
};
console.log(testRandom(1000, items, weights));

Run code in web dev console to see results of weightedRandom() over 1000 iterations.