Converted https://gist.github.com/3171981 to JavaScript. Used a few functions from http://phpjs.org/ to simplify refactoring.

voseAlias.js

/*
* Implementation of Vose's Alias Method
* Pass in an array of probabilities such as [.1, .2, .4, .7],
* probabilities do not have to add up to 1.
* Returned will be the selected index based on the probabilities.
*
* show statistics with
* aliasMethod(yourArray, true);
*
* use with
* aliasMethod(yourArray);
* @author Russell Dempsey
*       Original code by Tyler Stroud for PHP
* @see https://gist.github.com/3171981
* @param array probabilities An array of probabilities
*
* @return int
*
* @version 1
*/
function aliasMethod(probabilities, test)
{
    //we need to ensure the probabilities are scaled properly.
    probabilities = scaleForAliasMethod(probabilities);

    if (test === true) {
        var wins = [];
        var maxPicks = 100000;
        for (i = 0; i < probabilities.length; i++)
        {
            wins[i] = 0;
        }
        for (i = 0; i < maxPicks; i++)
        {
            wins[aliasMethod(probabilities)]++;
        }
        console.log("Probabilities: " + arr + "\n Number of picks out of "
            + maxPicks + ": " + wins);
        return;
    }

    var small = [];
    var large = [];
    var prob = [];
    var alias = [];
    var n = probabilities.length;

    probabilities = array_map(function(p) {
        return p * n; // Scale each probability by n
    }, probabilities);

    for (i = 0; i < n; ++i) {
        if (probabilities[i] < 1) {
            small.push(i);
        } else {
            large.push(i);
        }
    }

    while ((small.length !== 0) && (large.length !== 0)) {
        var l = small.shift();
        var g = large.shift();

        prob[l] = probabilities[l];
        alias[l] = g;

        probabilities[g] = (probabilities[g] + probabilities[l]) - 1;
        if (probabilities[g] < 1) {
            small.push(g);
        } else {
            large.push(g);
        }
    }

    while (large.length !== 0) {
        prob[large.shift()] = 1;
    }

    while (small.length !== 0) {
        prob[small.shift()] = 1;
    }

    // Coin Toss and die roll
    var i = mt_rand(0, n-1);
    coinToss = 1 * mt_rand() / mt_getrandmax() < prob[i];

    return coinToss ? i : alias[i];
}

function array_map (callback) {
    // http://kevin.vanzonneveld.net
    // +   original by: Andrea Giammarchi (http://webreflection.blogspot.com)
    // +   improved by: Kevin van Zonneveld (http://kevin.vanzonneveld.net)
    // +   improved by: Brett Zamir (http://brett-zamir.me)
    // %        note 1: Takes a function as an argument, not a function's name
    // %        note 2: If the callback is a string, it can only work if the function name is in the global context
    // *     example 1: array_map( function (a){return (a * a * a)}, [1, 2, 3, 4, 5] );
    // *     returns 1: [ 1, 8, 27, 64, 125 ]
    var argc = arguments.length,
        argv = arguments;
    var j = argv[1].length,
        i = 0,
        k = 1,
        m = 0;
    var tmp = [],
        tmp_ar = [];

    while (i < j) {
        while (k < argc) {
            tmp[m++] = argv[k++][i];
        }

        m = 0;
        k = 1;

        if (callback) {
            if (typeof callback === 'string') {
                callback = this.window[callback];
            }
            tmp_ar[i++] = callback.apply(null, tmp);
        } else {
            tmp_ar[i++] = tmp;
        }

        tmp = [];
    }

    return tmp_ar;
}


function mt_rand (min, max) {
    var argc = arguments.length;
    if (argc === 0) {        min = 0;
        max = mt_getrandmax();
    } else if (argc === 1) {
        throw new Error('Warning: mt_rand() expects exactly 2 parameters, or none, 1 given');
    }
    var score = Math.floor(Math.random() * (max - min + 1));
    score = parseInt(score) + parseInt(min);
    return score;
}

function mt_getrandmax () {
    // http://kevin.vanzonneveld.net
    // +   original by: Onno Marsman
    // *     example 1: mt_getrandmax();
    // *     returns 1: 2147483647
    return 2147483647;
}

function scaleForAliasMethod (probabilities)
{
    var total = 0;
    var newProbabilities = probabilities;
    for (var i = 0; i < newProbabilities.length; i++)
    {
        total += newProbabilities[i];
    }

    if (total > 1 )
    {
        for (i = 0; i < newProbabilities.length; i++)
        {
            newProbabilities[i] = newProbabilities[i] / total;
        }
    }
    return newProbabilities;
}