Java checkform

Hello,

I am using the function below and I would like to have it so that prop_desc needs to have more the 100 characters or there is an alert 'Please make your description a little longer'.

function checkform(form)
{
  if (form.userId.value == "" || form.firstname.value == "" || form.lastname.value == "" || form.phonenumber.value == "" || form.price.value == "" || form.zip.value == "" || form.title.value == "" || form.build_year.value == "" || form.sq_ft.value == "" || form.prop_desc.value == "") {
    alert( "Please complete all of the required* fields." );
    return false;
  }
  onSubmitButton();
  return true;
}