check out http://browserquest.mozilla.org/ as a reference model. it uses node for the backend.
They use setTimeout on server side classes e.g character, mob, player. Messages are sent client-server client-server (move, attack etc) and server-client server-client (move,spawn,damage etc). There are client and server side versions of classes for major entities in the game world. Numeric constantsNumeric constants are used to keep messages light.