Hi shun,
if you are certain that there is only one document (I am actually not 100% sure if it works when there are multiple docs) then you can write:
await client.query(
q.Get(q.Match(q.Index('sessions_by_code'), 'ABCD'))
);
If there are multiple documents and you just want to get the first:
await client.query(
q.Select([0], q.Paginate(q.Match(q.Index('sessions_by_code'), 'ABCD')))
);