CF Worker example: KV lookup and adding a customer header to a request

worker.js

async function handleRequest(request) {
  // Is the visitor in the VIP list?
  let data = request.cf
  let visitor_asn = data.asn
  
  // Query the KV database
  let network_name = await VIP_API_ASN.get(visitor_asn)
  
  // Create the HTTP header value
  let is_vip = (network_name !== null)
  let msg = 'vip: ' + (is_vip ? 'yes' : 'no ') + '; ' + data.asn.toString().padStart(7) + ' - ' + data.asOrganization
  console.log(msg);

  // Clone the request so that it's no longer immutable
  const newRequest = new Request(request);

  // Add a custom header with the VIP message
  newRequest.headers.append("x-vip", msg)  
  
  // Send the request on its way
  let response = await fetch(newRequest);

  // Return the response to the user
  return response
}


addEventListener('fetch', event => {
  event.respondWith(handleRequest(event.request));
});