[Ayoub-Mabrouk]

Test setting domain to a value (e.g., 'example.com')
Test default value (should be undefined)

[yxshee]

I ran this branch: 149 passing and eslint clean. The two new tests also pass on master, which is expected for a coverage-only PR.

I tried a quick mutation to see what they protect. I deleted domain: this.domain from the Cookie#data getter in session/cookie.js, so Domain is never sent in Set-Cookie. With that change, the full suite including these two tests still passes (175 on current master plus this file). new Cookie({ domain: 'example.com' }).serialize('sid', 'x') then returns sid=x; Path=/; HttpOnly with no Domain. Both tests read cookie.domain, which the generic for (var key in options) copy in the constructor sets for any key, so they'd pass for a made-up option too.

One extra assertion would cover the part that can actually break, for example:

assert.strictEqual(cookie.data.domain, 'example.com')
assert.ok(/; Domain=example\.com/.test(cookie.serialize('sid', 'x')))

Also, this and #1100 insert at the same spot after the priority block, so whichever lands second will conflict in test/cookie.js. I checked by merging both onto master. The branch is also 15 commits behind master.