Reset input value after alert - Javascript

if(jj_input23 > 2) {
    alert("Making the scale higher than 2 will make the preview too big");
    document.getElementById('jj_input23').value = "";
}

If you're using jQuery then you almost got it:

$('#jj_input23').val("");

.value is a property, not a function.

You want .value = "";.