> ## Documentation Index
> Fetch the complete documentation index at: https://mintlify.com/cloudflare/cloudflare-typescript/llms.txt
> Use this file to discover all available pages before exploring further.

# Logging and middleware

> Customize the fetch function to inspect and modify requests and responses

You can provide a custom `fetch` function when instantiating the client, which can be used to inspect or alter the `Request` or `Response` before/after each request.

## Custom fetch function

Provide a custom `fetch` implementation to intercept all HTTP requests:

```ts theme={null}
import { fetch } from 'undici'; // as one example
import Cloudflare from 'cloudflare';

const client = new Cloudflare({
  fetch: async (url: RequestInfo, init?: RequestInit): Promise<Response> => {
    console.log('About to make a request', url, init);
    const response = await fetch(url, init);
    console.log('Got response', response);
    return response;
  },
});
```

<Tip>
  This approach is useful for logging, request signing, response transformation, or integrating with custom HTTP clients.
</Tip>

## Logging requests and responses

You can use the custom `fetch` function to log all API interactions:

```ts theme={null}
import Cloudflare from 'cloudflare';

const client = new Cloudflare({
  fetch: async (url: RequestInfo, init?: RequestInit): Promise<Response> => {
    const startTime = Date.now();
    console.log(`[${new Date().toISOString()}] ${init?.method || 'GET'} ${url}`);
    
    const response = await fetch(url, init);
    const duration = Date.now() - startTime;
    
    console.log(`[${new Date().toISOString()}] ${response.status} ${url} (${duration}ms)`);
    return response;
  },
});
```

## Debug mode

If you set a `DEBUG=true` environment variable, this library will log all requests and responses automatically:

```bash theme={null}
DEBUG=true node your-script.js
```

<Warning>
  Debug mode is intended for debugging purposes only and may change in the future without notice.
</Warning>

## Request and response inspection

Use the custom `fetch` function to inspect headers, modify requests, or handle responses:

```ts theme={null}
import Cloudflare from 'cloudflare';

const client = new Cloudflare({
  fetch: async (url: RequestInfo, init?: RequestInit): Promise<Response> => {
    // Inspect request headers
    console.log('Request headers:', init?.headers);
    
    // Make the request
    const response = await fetch(url, init);
    
    // Inspect response headers
    console.log('Response headers:', Object.fromEntries(response.headers.entries()));
    console.log('Rate limit remaining:', response.headers.get('x-ratelimit-remaining'));
    
    return response;
  },
});
```

## Using alternative fetch implementations

By default, this library uses `node-fetch` in Node, and expects a global `fetch` function in other environments.

If you would prefer to use a global, web-standards-compliant `fetch` function even in a Node environment (for example, if you are running Node with `--experimental-fetch` or using NextJS which polyfills with `undici`), add the following import before your first import from `"cloudflare"`:

```ts theme={null}
// Tell TypeScript and the package to use the global web fetch instead of node-fetch.
// Note, despite the name, this does not add any polyfills, but expects them to be provided if needed.
import 'cloudflare/shims/web';
import Cloudflare from 'cloudflare';
```

To do the inverse, add `import "cloudflare/shims/node"` (which does import polyfills).

<Note>
  This can also be useful if you are getting the wrong TypeScript types for `Response`.
</Note>