montyhall.js

var results, passes, tests;

results = {
    'switch': 0,
    'keep': 0
};
passes = 100000;
tests = ['switch', 'keep'];
    
tests.forEach( function (test) {
    var choice, correct;

    for (var i = 0; i < passes; i++) {
        choice = Math.floor(Math.random() * 3) + 1;
        correct = Math.floor(Math.random() * 3) + 1;
        
        if (test === 'switch') {
            if (correct !== choice) {
                results['switch']++;
            }
        } else {
            if (correct === choice) {
                results.keep++;
            }
        }
    }
} );

for (var test in results) {
    console.log(test, results[test] / passes * 100);
}