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Logging headers

Examine the contents of a Headers object by logging to console with a Map.

async function handleRequest(request) {  console.log(new Map(request.headers))
  return new Response("Hello world")}
addEventListener("fetch", event => {  return event.respondWith(handleRequest(event.request))})

Console-logging headers

TL;DR: Use a Map if you just 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])

Or use ES2019 Object.fromEntries to convert it to an object:

let requestHeaders = Object.fromEntries(request.headers)

The problem

When debugging Worker scripts, we often want to examine the headers on a request or response. A common pitfall 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 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’s not an empty object per se, but rather an opaque object.

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

Pass headers through a Map

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 an enumerable JavaScript property, so the developer console can see into it.

Spread headers into an array

The Map trick works great for simple calls to console.log(), but if we need to actually stringify our headers, we quickly find out that stringifying a Map yields nothing more than [object Map], which isn’t very helpful.

The JSON stringifier can’t help us, either: even though our Map stores its data in an enumerable property, that property is Symbol-keyed, and JSON.stringify() ignores Symbol-keyed properties — we end up with an empty {} again.

Instead, we 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}`)

Convert headers into an object with Object.fromEntries (ES2019)

ES2019 provides Object.fromEntries, so it's just a simple call to convert the headers into an object:

let requestHeaders = JSON.stringify(Object.fromEntries(request.headers), 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",  // ...}"