Linked options helper methods for Shopify. See this: http://docs.shopify.com/manual/configuration/store-customization/advanced-navigation/linked-product-options

## gistfile1.html
<script>
// (c) Copyright 2016 Caroline Schnapp. All Rights Reserved. Contact: mllegeorgesand@gmail.com
// See https://docs.shopify.com/themes/customization/navigation/link-product-options-in-menus

var Shopify = Shopify || {};

Shopify.optionsMap = {};

Shopify.updateOptionsInSelector = function(selectorIndex) {

  switch (selectorIndex) {
    case 0:
      var key = 'root';
      var selector = jQuery('.single-option-selector:eq(0)');
      break;
    case 1:
      var key = jQuery('.single-option-selector:eq(0)').val();
      var selector = jQuery('.single-option-selector:eq(1)');
      break;
    case 2:
      var key = jQuery('.single-option-selector:eq(0)').val();
      key += ' / ' + jQuery('.single-option-selector:eq(1)').val();
      var selector = jQuery('.single-option-selector:eq(2)');
  }

  var initialValue = selector.val();
  selector.empty();
  var availableOptions = Shopify.optionsMap[key];
  for (var i=0; i<availableOptions.length; i++) {
    var option = availableOptions[i];
    var newOption = jQuery('<option></option>').val(option).html(option);
    selector.append(newOption);
  }
  jQuery('.swatch[data-option-index="' + selectorIndex + '"] .swatch-element').each(function() {
    if (jQuery.inArray($(this).attr('data-value'), availableOptions) !== -1) {
      $(this).removeClass('soldout').show().find(':radio').removeAttr('disabled','disabled').removeAttr('checked');
    }
    else {
      $(this).addClass('soldout').hide().find(':radio').removeAttr('checked').attr('disabled','disabled');
    }
  });
  if (jQuery.inArray(initialValue, availableOptions) !== -1) {
    selector.val(initialValue);
  }
  selector.trigger('change');

};

Shopify.linkOptionSelectors = function(product) {
  // Building our mapping object.
  for (var i=0; i<product.variants.length; i++) {
    var variant = product.variants[i];
    if (variant.available) {
      // Gathering values for the 1st drop-down.
      Shopify.optionsMap['root'] = Shopify.optionsMap['root'] || [];
      Shopify.optionsMap['root'].push(variant.option1);
      Shopify.optionsMap['root'] = Shopify.uniq(Shopify.optionsMap['root']);
      // Gathering values for the 2nd drop-down.
      if (product.options.length > 1) {
        var key = variant.option1;
        Shopify.optionsMap[key] = Shopify.optionsMap[key] || [];
        Shopify.optionsMap[key].push(variant.option2);
        Shopify.optionsMap[key] = Shopify.uniq(Shopify.optionsMap[key]);
      }
      // Gathering values for the 3rd drop-down.
      if (product.options.length === 3) {
        var key = variant.option1 + ' / ' + variant.option2;
        Shopify.optionsMap[key] = Shopify.optionsMap[key] || [];
        Shopify.optionsMap[key].push(variant.option3);
        Shopify.optionsMap[key] = Shopify.uniq(Shopify.optionsMap[key]);
      }
    }
  }
  // Update options right away.
  Shopify.updateOptionsInSelector(0);
  if (product.options.length > 1) Shopify.updateOptionsInSelector(1);
  if (product.options.length === 3) Shopify.updateOptionsInSelector(2);
  // When there is an update in the first dropdown.
  jQuery(".single-option-selector:eq(0)").change(function() {
    Shopify.updateOptionsInSelector(1);
    if (product.options.length === 3) Shopify.updateOptionsInSelector(2);
    return true;
  });
  // When there is an update in the second dropdown.
  jQuery(".single-option-selector:eq(1)").change(function() {
    if (product.options.length === 3) Shopify.updateOptionsInSelector(2);
    return true;
  });
};

{% if product.available and product.options.size > 1 %}
  var $addToCartForm = $('form[action="/cart/add"]');
  if (window.MutationObserver && $addToCartForm.length) {
    if (typeof observer === 'object' && typeof observer.disconnect === 'function') {
      observer.disconnect();
    }
    var config = { childList: true, subtree: true };
    var observer = new MutationObserver(function() {
      Shopify.linkOptionSelectors({{ product | json }});
      observer.disconnect();
    });
    observer.observe($addToCartForm[0], config);
  }
{% endif %}

</script>