Autofill Discount Code from Shopify URL

getDiscountCode.html

<script>
  /* Put this in theme.liquid, preferably right before "</body>" */
  (function() {
    var discountParam = document.location.search.match(/discount=(\w+)/);
    if (discountParam && discountParam.length > 1) {
      document.cookie = discountParam[0];
    }
  })();
</script>

setDiscountCode.html

<script>
  /*
    Put this in cart.liquid, preferably at the bottom of the file.
    Also, make sure your cart's "<form>" has an ID of "cartform".
  */
  (function() {
    var discountCookie = document.cookie.match(/discount=(\w+)/);
    if (discountCookie && discountCookie.length > 1) {
      document.getElementById('cartform').action = '/cart?' + discountCookie[0];
    }
  })();
</script>

To make this work on all pages, you need to add ;path=/ to the cookie string so that the cookie can apply to all paths under the domain.

That is, use the following snippet. Works nicely on our Shopify store.

<script>
  /* Put this in theme.liquid, preferably right before "</body>" */
  (function() {
    var discountParam = document.location.search.match(/discount=(\w+)/);
    if (discountParam && discountParam.length > 1) {
      document.cookie = discountParam[0] + ";path=/";
    }
  })();
</script>