Программа для проверки парадокса Монти Холла (https://ru.wikipedia.org/wiki/Парадокс_Монти_Холла)

monty.js

function rnd(a, b) {
    return Math.floor(Math.random() * (b - a)) + a;
}

function randBool() {
    return Math.random() >= .5;
}

var success = 0;
var tries = 1000000;

for (var i = 0; i < tries; i++) {
    var won = rnd(1, 4);
    var chosen = rnd(1, 4);
    var opened = 0;
    if (chosen == won) switch (chosen) {
        case 1:
            opened = randBool() ? 2 : 3;
            break;
        case 2:
            opened = randBool() ? 1 : 3;
            break;
        case 3:
            opened = randBool() ? 1 : 2;
    } else
        opened = 6 - (chosen + won);

    if (chosen != won) success++;
}

console.log((success / tries * 100) + '%');