• I can't remove any default header.
• Setting "connection": "keep-alive" throws an error.
• I can't overwrite default "sec-fetch-mode" header correctly.

1. I can't remove the default headers

I can't remove the default headers: host, connection, accept, accept-encoding, accept-language, sec-fetch-mode, user-agent.

For example, curl allows to remove any header (even host header):
curl http://127.0.0.1:3000 -H 'User-Agent:' -H 'Accept:' -H 'Host:'
And yes, it works.

Using of "" will send "" as the header's value, using of null will send "null" as the header's value.

2. "connection": "keep-alive"

Also, if I manually set connection: "keep-alive" I get the error (code: 'UND_ERR_INVALID_ARG').

const resp = await fetch(url, {
    headers: {
        "connection": "keep-alive"
    }
});

It's important for code compatibility reason — since node-fetch uses connection: close by default.

3. "sec-fetch-mode"

It uses "sec-fetch-mode": "cors" by default.

As I said above, I can't remove this header (as well as any other default header).

More over, I can't change it correctly.

For example, using of:

await fetch(url, {
    headers: {
        "sec-fetch-mode": "navigate"
    }
});

Will produce a request with sec-fetch-mode: navigate, cors, while it should be sec-fetch-mode: navigate.

"undici": "4.16.0"