JavaScript Examples - Functions on Firewall

Explore JavaScript-based Functions for robust firewall interactions, featuring event handling, HTTP header adjustments, and custom responses.

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A general-purpose example for functions that run on the firewall, combining several request-handling actions in one handler. Use it as a starting point when you need to inspect incoming headers and metadata and then decide whether to block, drop, modify, or answer a request directly on Azion’s global network. The sample code exemplifies the following capabilities:

  • event.deny()
  • event.drop()
  • event.addRequestHeader()
  • event.addResponseHeader()
  • event.respondWith()
  • event.continue()
async function firewallHandler(event) {
// Access the value of x-deny, one of the headers
let condition = event.request.headers.get("x-deny");
if (condition) {
// event.deny() triggers a default 403 Response
event.deny();
}
let anotherCondition = event.request.headers.get("x-drop"); // Access the value of x-drop, , one of the headers
if (anotherCondition) {
// event.drop() drops the connection without returning a Response
event.drop();
}
// Access the value of x-third, one of the headers
let aThirdCondition = event.request.headers.get("x-third");
if (aThirdCondition) {
// event.addRequestHeader() adds a new header to the Request object, passing key:"value".
event.addRequestHeader("X-New-Request-Header", "Hello");
}
// Access the value of x-fourth, one of the headers
let aFourthCondition = event.request.headers.get("x-fourth");
if (aFourthCondition) {
// event.addResponseHeader() adds a new header to the Response object
// that will be delivered after accessing the origin.
event.addResponseHeader("X-New-Response-Header", "Bye");
}
// The metadata object contains the attributes of the request
if (event.request.metadata["remote_addr"] == "127.0.0.1") {
// event.respondWith(new Response()) allows you to serve your own Response
event.respondWith(new Response('{"my_custom_response": true}', {
status: 599,
headers: { "content-type": "application/json" }
}));
}
// If the request was not denied or dropped and did not have a Response returned,
// it is necessary to continue the processing, and to do so use use -> event.continue()
// Do not forget this command, otherwise the request won't continue!
event.continue();
}
addEventListener("firewall", (event) => event.waitUntil(firewallHandler(event)));

How it works

The handler runs on the firewall event inside event.waitUntil() and reads several request headers with event.request.headers.get() to drive its decisions. Depending on which header is present, it can call event.deny() to return a 403, event.drop() to close the connection without a response, or event.addRequestHeader() and event.addResponseHeader() to inject custom headers. It can also short-circuit with event.respondWith(new Response(...)) to serve a custom JSON body, and it ends with event.continue() so that any request not already handled proceeds normally.