mongojs do not close nodejs app after call

[askucher]

iso = get-iso new Date!
query = iso : $lte: iso
err, items <-! db.rates.find(query).sort( iso : -1 ).limit 10
#db.close!
#app still running - actual
#app closed - expected

if I add db.close! everything works fine but odd. whats is wrong in my code?