gistfile1.js

<script src="https://code.jquery.com/jquery-1.12.0.min.js"></script>

<script>

var getQueryVariable = function(variable)
{
       var query = window.location.search.substring(1);
       var vars = query.split("&");
       for (var i=0;i<vars.length;i++) {
               var pair = vars[i].split("=");
               if(pair[0] == variable){return pair[1];}
       }
       return(false);
}


var addItemToCart = function (variant_id, quantity, shipping_interval_frequency, shipping_interval_unit_type, subscription_id) {
  data = {
      	"quantity": quantity,
      	"id": variant_id,
      	"properties[shipping_interval_frequency]": shipping_interval_frequency, 
		"properties[shipping_interval_unit_type]": shipping_interval_unit_type,            
		"properties[subscription_id]": subscription_id
    }
  console.log('clicked',data);
  jQuery.ajax({
      type: 'POST',
      url: '/cart/add.js',
      data: data,
      dataType: 'json',
      success: function() { 
          window.location.href = '/cart'; 
      }
  });
}


// http://www.example.com/index.php?id=1&image=awesome.jpg

var product_id = getQueryVariable("id"); // where id is the query param


if (product_id.length) {
    addItemToCart(product_id, 1, "1", "Months", "12599");
} else {
	window.history.go(-1) // should redirect back to old page if product id is wrong
}

</script>