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>

For now, I updated the following code to update the "Add to Cart" button so that the discount gets added to the URL. This works for me on the District Theme. I'm sure there is a better way to implement it.
`<script>
/* Put this in theme.liquid, preferably right before "" */
(function() {
var discountParam = document.location.search.match(/discount=(\w+)/);
if (discountParam && discountParam.length > 1) {
document.cookie = discountParam[0];
}
var discountCookie = document.cookie.match(/discount=(\w+)/);
if (discountCookie && discountCookie.length > 1) {
document.getElementById('CartButton').href = '/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>