Bug: When headers is native Headers object, they won't get send

[FokkeZB]

Steps to Reproduce

Code: https://codesandbox.io/s/determined-cdn-7k3gx?file=/src/index.js
App: https://7k3gx.csb.app/

Theories

We do new Headers(options.headers):

https://github.com/github/fetch/blob/75d9455d380f365701151f3ac85c5bda4bbbde76/fetch.js#L367-L369

Which will use the local polyfill function Headers which is also what this line will check against:

https://github.com/github/fetch/blob/75d9455d380f365701151f3ac85c5bda4bbbde76/fetch.js#L86-L90

But if options.headers is the builtin Headers object, the instanceof check will fail, as well as the fallback case because getOwnPropertyNames() will return an empty object:

https://github.com/github/fetch/blob/75d9455d380f365701151f3ac85c5bda4bbbde76/fetch.js#L94-L98

Solution

Don't polyfill Headers when there's a builtin, native version.

[apisim]

I came across the problem, too. Before I saw this Issue, it took a few debugging sessions stepping through the code to discover that, indeed, the if (headers instanceof Headers) checks fails when unknowingly using the native Headers object.

Attempting to use an Array doesn't work neither because !(init.headers instanceof Headers) in
https://github.com/github/fetch/blob/a8aa427de0ed808ff26c0e3eb2e59c122c44488a/fetch.js#L573-L581
...evaluates to true and the execution goes through the if block, and the header elements end up being somethig like 0: accept, application/json (0 is the index in the array) for something that should be accept: application/json.

What worked was to alias polyfill's Headers. For example:

import { Headers as FetchPolyfillHeaders } from 'whatwg-fetch';

...and use FetchPolyfillHeaders everywhere in the code.

Hope this helps.

[JakeChampion]

@FokkeZB and/or @apisim -- Which browser or runtime has Headers defined but does not have fetch defined?

[JakeChampion]

Resolved via #1358