Logging headers to console

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This creates a repository in your GitHub account and deploys the application to Cloudflare Workers.

export default {
	async fetch(request) {
		console.log(new Map(request.headers));
		return new Response("Hello world");
	},
};

export default {
	async fetch(request): Promise<Response> {
		console.log(new Map(request.headers));
		return new Response("Hello world");
	},
} satisfies ExportedHandler;

from workers import WorkerEntrypoint, Response

class Default(WorkerEntrypoint):
    async def fetch(self, request):
        print(dict(request.headers))
        return Response('Hello world')

use worker::*;

#[event(fetch)]
async fn fetch(req: HttpRequest, _env: Env, _ctx: Context) -> Result<Response> {
    console_log!("{:?}", req.headers());
    Response::ok("hello world")
}

import { Hono } from 'hono';

const app = new Hono();

app.get('*', (c) => {
  // Different ways to log headers in Hono:

  // 1. Using Map to display headers in console
  console.log('Headers as Map:', new Map(c.req.raw.headers));

  // 2. Using spread operator to log headers
  console.log('Headers spread:', [...c.req.raw.headers]);

  // 3. Using Object.fromEntries to convert to an object
  console.log('Headers as Object:', Object.fromEntries(c.req.raw.headers));

  // 4. Hono's built-in header accessor (for individual headers)
  console.log('User-Agent:', c.req.header('User-Agent'));

  // 5. Using c.req.headers to get all headers
  console.log('All headers from Hono context:', c.req.header());

  return c.text('Hello world');
});

export default app;

Use a Map if you need to log a Headers object to the console:

console.log(new Map(request.headers));

Use the spread operator if you need to quickly stringify a Headers object:

let requestHeaders = JSON.stringify([...request.headers]);

Use Object.fromEntries to convert the headers to an object:

let requestHeaders = Object.fromEntries(request.headers);

When debugging Workers, examine the headers on a request or response. A common mistake is to try to log headers to the developer console via code like this:

console.log(request.headers);

Or this:

console.log(`Request headers: ${JSON.stringify(request.headers)}`);

Both attempts result in what appears to be an empty object — the string "{}" — even though calling request.headers.has("Your-Header-Name") might return true. This is the same behavior that browsers implement.

The reason this happens is because Headers ↗ objects do not store headers in enumerable JavaScript properties, so the developer console and JSON stringifier do not know how to read the names and values of the headers. It is not actually an empty object, but rather an opaque object.

Headers objects are iterable, which you can take advantage of to develop a couple of quick one-liners for debug-printing headers.

The first common idiom for making Headers console.log()-friendly is to construct a Map object from the Headers object and log the Map object.

console.log(new Map(request.headers));

This works because:

• Map objects can be constructed from iterables, like Headers.

• The Map object does store its entries in enumerable JavaScript properties, so the developer console can see into it.

The Map approach works for calls to console.log(). If you need to stringify your headers, you will discover that stringifying a Map yields nothing more than [object Map].

Even though a Map stores its data in enumerable properties, those properties are Symbol ↗-keyed. Because of this, JSON.stringify() will ignore Symbol-keyed properties ↗ and you will receive an empty {}.

Instead, you can take advantage of the iterability of the Headers object in a new way by applying the spread operator ↗ (...) to it.

let requestHeaders = JSON.stringify([...request.headers], null, 2);
console.log(`Request headers: ${requestHeaders}`);

ES2019 provides Object.fromEntries ↗ which is a call to convert the headers into an object:

let headersObject = Object.fromEntries(request.headers);
let requestHeaders = JSON.stringify(headersObject, null, 2);
console.log(`Request headers: ${requestHeaders}`);

This results in something like:

Request headers: {
  "accept": "text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8",
  "accept-encoding": "gzip",
  "accept-language": "en-US,en;q=0.9",
  "cf-ipcountry": "US",
  // ...
}"