Project Milestone: The Edge Interceptor

Previously in this course, we covered Handling HTTP Requests: A Developer’s Guide to Workers and Dynamic Header Manipulation: Securing Apps with Cloudflare Workers. We also learned the Introduction to Wrangler CLI: Setup and Project Initialization to manage our deployments.

In this lesson, we are bringing these concepts together to build the "Edge Interceptor"—a middleware layer that sits in front of your static site. Instead of simply serving files, we will now intercept every incoming request, evaluate its context, and dynamically decorate the response before it reaches the user.

The Middleware Concept

In a traditional server architecture, middleware runs after the request reaches your origin. In the serverless paradigm, the "Edge Interceptor" pattern allows you to run this logic at the Cloudflare edge—milliseconds away from the user—before the request ever touches your origin server.

By routing traffic through a Worker, you gain total control over the request lifecycle. This is perfect for:

• Injecting security headers that your static host might not support.
• Redirecting users based on geography or device type.
• Adding "debug" headers to trace requests through your infrastructure.

Worked Example: Building the Interceptor

We will create a Worker that intercepts requests to our site, adds a custom X-Edge-Interceptor header, and ensures our site's security policy is always enforced.

First, ensure you have initialized your project using the steps from Writing Your First Worker: A Practical Guide to Cloudflare Deployment.

The Code

Update your index.js (or index.ts) file with the following logic to act as a transparent proxy that modifies the response:

JAVASCRIPT
export default {
  async fetch(request, env, ctx) {
    // 1. Fetch the original response from the origin
    const response = await fetch(request);

    // 2. Clone the response to modify headers
    const newResponse = new Response(response.body, response);

    // 3. Inject our custom security and metadata headers
    newResponse.headers.set("X-Edge-Interceptor", "Active");
    newResponse.headers.set("X-Content-Type-Options", "nosniff");
    newResponse.headers.set("Strict-Transport-Security", "max-age=31536000; includeSubDomains");

    // 4. Return the modified response
    return newResponse;
  },
};

Routing the Traffic

For this code to run, you must tell Cloudflare which requests to intercept. Open your wrangler.toml file and define a route:

TOML
routes = [
	{ pattern = "example.com/*", zone_name = "example.com" }
]

Hands-On Exercise

1. Deploy: Run npx wrangler deploy to push your interceptor to the edge.
2. Verify: Open your browser's Developer Tools (Network tab).
3. Inspect: Navigate to your site and click on the primary HTML document request.
4. Confirm: Check the "Response Headers" section. You should see X-Edge-Interceptor: Active. If you see it, your middleware is successfully intercepting and modifying the traffic flow.

Common Pitfalls

• Infinite Loops: If your Worker makes a request back to the same domain without careful handling, you can trigger an infinite loop where the Worker calls itself. Always ensure your Worker fetches from a different origin or uses an internal bypass if necessary.
• Caching Issues: Remember that modifying headers on a cached response can sometimes be tricky. If you aren't seeing your headers, try purging the cache or checking if the response is coming from the edge cache rather than the origin.
• Blocking Headers: Some headers are "restricted" by Cloudflare (like cf-connecting-ip). You cannot overwrite these from a standard Worker; the system will ignore your attempt.

FAQ

Q: Does this replace my origin server? A: No, this is an interceptor. It acts as a transparent layer (middleware) that processes the request and response in transit between the user and your origin.

Q: Can I use this to block users? A: Absolutely. You can add logic at the top of the fetch function to check headers like cf-ipcountry and return a new Response("Forbidden", { status: 403 }) before the request even reaches your origin.

Recap

You have successfully deployed an Edge Interceptor. You learned how to:

1. Intercept traffic using a Cloudflare Worker.
2. Modify response headers dynamically.
3. Configure routing to ensure the Worker runs for your specific domain traffic.

This milestone ensures that every request to your site is now processed by your own custom logic, laying the foundation for the backend features we will implement in the coming lessons.

Up next: We move from purely dynamic logic to persistent storage with Introduction to R2 Storage.