Node's Event Loop implementation in pseudo code.

node-event-loop.js

// node's event loop behind the scenes
// node is event driven and single threaded, with background workers
// imagine we ran node index.js

const pendingTimers = []
const pendingOSTasks = []
const pendingOperations = []

// node executes the file and triggers timers, OSTasks and operations, filling in the above arrays.
index.runContents()

function shouldContinue () {
  // check one: Any pending setTimeout, setInterval or setImmediate?
  // check two: Any pending OS tasks? (Like server listening to port)
  // check three: Any pending operations? (Like fs module)
}

// every iteration of the loop is one 'tick'
while (shouldContinue()) {
  // 1: have any timers expired? If so, put their callbacks in the event loop and execute them.
  // 2: are there any resolved OS tasks? Like, if a response is recieved in a port, put the callback in the event loop and execute the callback
  // 3: the event loop is now empty, pause execution. 
  // 3b: continue execution when node is notified 
    // - timer has expired
    // - pending OS task is done
    // - operation completed
  // look at pending timers and call any setImmediate
  // handle any close/ cleanup events
}