Ah, you beat me to the minute (game comes with a function is_prime):

    var yes = document.getElementById('yes'),
        no = document.getElementById('no'),
        n = document.getElementById('n'),
        start = document.getElementById('start');


    function tick() {
        console.log('tick');
        if (is_prime(n.innerHTML) === 'prime') {
            yes.click();
        } else {
            no.click();
        }
        window.setTimeout(tick, 500);
    }

    start.click();
    tick();