set.js

Array.prototype.shuffle =  function() {
    var c = this.length, temp, i;
    while (c > 0) {
        i = Math.floor(Math.random() * c);
        c--;
        temp = this[c];
        this[c] = this[i];
        this[i] = temp;
    }
    return this;
};

var cards = (function(n) {
    var list = [];
    for (var i = 0; i < n; i++) {
        list.push([
            ~~(i / 27) % 3,
            ~~(i /  9) % 3,
            ~~(i /  3) % 3,
            ~~(i /  1) % 3
        ]);
    }
    return list;
}(81));

function isSet(a, b, c) {
    var result = true;
    for (var i = 0; i < 4; i++) {
        if (!(a[i] === b[i] && b[i] === c[i]) &&
            !(a[i] !== b[i] && a[i] !== c[i] && b[i] !== c[i])) {
            result = false;
            break;
        }
    }
    return result;
}

function hasSet(cards) {
    var len = cards.length, result = false;
    for (var i = 0; !result && i < len - 2; i++) {
        for (var j = i + 1; !result && j < len - 1; j++) {
            for (var k = j + 1; !result && k < len - 0; k++) {
                result = isSet(cards[i], cards[j], cards[k]);
            }
        }
    }
    return result;
}

function monteCarlo(count, n) {
    var contains = 0;
    for (var i = 0; i < n; i++) {
        contains += hasSet(cards.shuffle().slice(0, count)) ? 1 : 0;
    }
    return contains / n;
}