brad43.html

<script>
let delay = 3000, i = 1;

function doThing(){
    if (delay <= 10) {
        doThing2();
        return;
    }
    console.log(i++);
    setTimeout(doThing, delay);
    delay -= delay/2;
}

function doThing2(){
    if (delay >= 3000){
        return;
    }
    console.log(i--);
    setTimeout(doThing2, delay);
    delay *= 1.5;
}

doThing();

</script>