Showing or hiding a back-order message

gistfile1.js

var selectCallback = function(variant, selector) {
  if (variant && variant.available) {               
    if (variant.inventory_management != '' && variant.inventory_quantity < 0) {
      $('.backorder').show();
    }
    else {
      $('.backorder').hide();
    }
    $("input#add").removeClass("disabled").removeAttr("disabled");
    $("h2#price span").html(Shopify.formatMoney(variant.price, "{{ shop.money_format }}"));
    if (variant.compare_at_price) {
      $("h2#price del").html(Shopify.formatMoney(variant.compare_at_price, "{{ shop.money_format }}"));
    }
    else {
      $("h2#price del").empty();
    }
  } 
  else {
    $('.backorder').hide();
    $("input#add").addClass("disabled").attr("disabled", "disabled");
    var message = variant ? "Sold Out" : "Unavailable";
    $("h2#price span").text(message);
    $("h2#price del").empty();
  }
};

There are like 3 situations here: is unavailable/sold out (1), is back-order (2) and is in stock (3).
The difference between (2) and (3) is ONLY the backorder message - for it to be shown or hidden.
Everything else is the same, that is, in both (2) and (3) we do NOT disable the add to cart button, we DO show the price, etc. It's business as usual, only difference is showing or not the backorder message.