Node is similar in design to and influenced by systems like Ruby's Event Machine or Python's Twisted. Node takes the event model a bit further: it presents the event loop as a language construct instead of as a library. In other systems, there is always a blocking call to start the event loop. Typically, one defines behavior through callbacks at the beginning of a script, and at the end starts a server through a blocking call like EventMachine::run(). In Node, there is no such start-the-event-loop call. Node simply enters the event loop after executing the input script. Node exits the event loop when there are no more callbacks to perform. This behavior is like browser Javascript: the event loop is hidden from the user.
