fetch-headers

The Headers class for NodeJS Executed against wpt test suits so it follows the spec correctly.

Install

fetch-headers is an ESM-only module - you are not able to import it with require. If you are unable to use ESM in your project you can use the async import('fetch-headers') from CommonJS to load fetch-headers asynchronously.
npm install fetch-headers

Use

import { Headers, bag } from 'fetch-headers'

const headers = new Headers({
  'content-type': 'text/plain'
})

// Turn headers to become immutable.
bag.get(headers).guard = 'immutable'
headers.set('content-type', 'text/html') // Throws

Regards to Set-Cookie and values joined by comma

The new norm is that all headers with the same key should be joined by a comma value. but set-cookies Can still contain a comma value for historical reasons. (It's best to avoid using it in any header value). All other headers are not allowed to have it.

Browser don't expose Set-Cookies headers in any way. That's why there is no issue with headers.get(name).split(',') that should always return a string joined by comma value, This header class will apply to this rule as well. meaning headers.get('set-cookie') will return a string with every Set-Cookie joined together.

So parsing it can be tricky, that's why iterating over the headers can be the preferred way, this is the least way we could expose all set-cookie headers individually without deviating from the spec by adding a custom getAll() or raw() method that don't exist in the spec.

const header = new Headers()
headers.append('xyz', 'abc')
headers.append('xyz', 'abc')
headers.append('Set-Cookie', 'a=1')
headers.append('Set-Cookie', 'b=2')

for (const [name, value] of headers) {
  if (name === 'set-cookie') {
    // Could happen twice
  } else {
    // Will never see the same `name` twice
  }
}

console.log([...headers])
// yields [[ "set-cookie", "a=1" ], ["set-cookie", "b=2"], ["xyz", "abc, abc"]]

This matches the same way Deno handles headers in core as well. It also looks like we might be getting a getSetCookie method soon.