GeezerVison

One additional step you could take is this (pseudo-ish code):

if (isFd(fd)) validateInteger(fd, 'options.fd', 0, kMaxInt32);

While passing ex. -1 or -42 to libuv is Mostly Harmless (should fail with UV_EBADF) it's probably best to turn what is currently a runtime error into a validation error. That might make it semver-major though.

(And I vote to rename kMaxInt to kMaxUint32 because I find the former highly misleading.)