The Set-Cookie header is special, it is not correct to combine multiple Set-Cookie headers into a single value separated by commas because it can change the semantics.

Reference:

• whatwg fetch, "Headers class": https://fetch.spec.whatwg.org/#headers-class (see the note directly below the structure definition of Headers)
• RFC6265 section 3: https://tools.ietf.org/html/rfc6265#section-3

I think the only correct way to parse this header with node-fetch is by using raw(). This is not at all obvious and should probably be documented.

Some (untested!) example code:

        if (cookieJar && res.headers.has('Set-Cookie')) {
            for (const [k, vv] of Object.entries(res.headers.raw())) {
                if (k.toLowerCase() === 'set-cookie') {
                    for (const v of vv) {
                        await promisify(cookieJar.setCookie)(v, url);
                    }
                }
            }
        }