Add a companion product to cart page

companion-product

{% comment %}
  To add a companion product to the cart automatically if a primary product is in cart:
  1. Create a new link list under your Navigation tab.
  2. In that link list, make the first link point to companion product.
  3. Copy your link list handle where indicated at line 8:
{% endcomment %}

{% assign linklist = linklists['put-your-link-list-handle-here'] %}

{% comment %}
  You're done. Include this code in cart.liquid at the top or bottom.
{% endcomment %}

{% if linklist.links.size > 0 %}

  <script>!window.jQuery && document.write('<script src="http://ajax.googleapis.com/ajax/libs/jquery/1/jquery.min.js"><\/script>')</script>
  
  <script>
  if (typeof Shopify === 'undefined') var Shopify = {};
  Shopify.cart = {{ cart | json }};
  Shopify.toAdd = {{ linklist.links.first.object.variants.first.id }};
  var pleaseAdd = false;
  Shopify.idsInCart = [];
  for (var i=0; i<Shopify.cart.items.length; i++) {
    Shopify.idsInCart.push(Shopify.cart.items[i].id);
    if (Shopify.cart.items[i].id !== Shopify.toAdd) {
      pleaseAdd = true;
    }
  }
  if (pleaseAdd && (jQuery.inArray(Shopify.toAdd, Shopify.idsInCart) === -1)) {
    var params = {
      type: 'POST',
      url: '/cart/add.js',
      data: 'quantity=1&id=' + Shopify.toAdd,
      dataType: 'json',
      success: function(line_item) { 
        window.location.href = '/cart';
      }
    };
    jQuery.ajax(params);
  }
  </script>
  
{% endif %}