add to cart prerequisite validation

validate-cart-prerequisites.js

jQuery(function($) {

  function validatePrerequisites() {

    // <select /> DOM ID to item label map
    var selectDOMIdToLabelMap = {
      "free-t-shirt-size": "Free T Shirt Size",
      "power-protein-flavor": "Power Protein Flavor",
      "pump-powder": "Pump Powder"
    };

    // check if each item has a selected value
    var itemsNotSelected = [];
    for (var selectDOMId in selectDOMIdToLabelMap) {
      var $select = $("select#" + selectDOMId);
      var $tr = $select.parent().parent();

      // clear warning if exists
      $tr.find("td.warn").remove();

      if ($select.val() === "") { // empty, so add it to the list of items without a selection
        itemsNotSelected.push(selectDOMIdToLabelMap[selectDOMId]);
        $tr.append('<td class="warn" style="color: red; font-size: small;">Please choose an option</td>');
      }
    }

    // return true if all items have a selected value; otherwise false
    return itemsNotSelected.length === 0;
  }

  // button handler
  $("button.placeholder_button").click(function(e) {
    var valid = validatePrerequisites();

    // don't submit for if not valid
    if (!valid) {
      e.preventDefault();
    }
  });


});