JS To read a GET query string and redirect if it matches some value. Used for coupon codes.

coupon.js

<script>
// Function to get the GET parameters
const getGET = () => 
{
  // Read the URI the user arrived with 
   const loc = document.location.href;

   // Find out if it has a question mark
   if(loc.indexOf('?')>0)
   {
     // If does, so get the first chunk of text after the question mark
     const getString = loc.split('?')[1];
     // Now, get all the way to the ampersand into a string called GET
     const GET = getString.split('&');
     const get = {};
     // Loop through that string and break it into separate pieces by the equals sign
     for(let i = 0, l = GET.length; i < l; i++)
     {
       const tmp = GET[i].split('=');
       get[tmp[0]] = unescape(decodeURI(tmp[1]));
     }
     // Return this as a set of keys and values e.g. coupon, 12345
     return get;
   } 
}

// Listen for the page to load. When it does, run this code
document.addEventListener('DOMContentLoaded', () => 
{
    // Call our get function above and get the query strings in an array of key values
    const values = getGET();
    // If we got something, announce it
    if(values)
    {
      // For all the keys we found, pop an alert with the key and value
      for(const index in values)
      {
        if( index == 'couponcode' && values[index] == '10BUCKSOFF')
        {
          alert("You get $10 off");
          window.location = "https://branpaul.convertri.com/10bucksoff";
        }
      }
    }
    // If we didn't find a query string, announce that
    else
    {
      alert("You're Here Naked!");
      // Do nothing
    }
});
</script>