gistfile1.txt

const randomPercentile = () => {
    return Math.floor(Math.random() * 100);
};

const events = [
    {
        name:       'A',
        percentage: 33
    },
    {
        name:       'B',
        percentage: 33
    },
    {
        name:       'C',
        percentage: 34
    }
];

const results = {};

for (let i = 0; i < 100_000; i++) {
    for (event of events) {
        if (randomPercentile() < event.percentage || event.name === 'C') {
            results[event.name] ??= 0;
            results[event.name]++;
            break;
        }
    }
}

console.log(results);