An explanatory script to illustrate the timings of intervals and the event loop

nodeticks.js

#!/usr/bin/env node

const foreverTick = (fn) => {
  // Note: Using process.nextTick instead of setImmediate will result in an endless loop!
  const ticker = () => {
    // stopForever is a nice trick for a stoppable forever function
    const stopForever = fn()
    if (!stopForever) {
      setImmediate(ticker)
    }
  }
  setImmediate(ticker)
  return ticker
}

var calledThisTick = {}
var ticks = 0

foreverTick(() => {
  ticks += 1
  calledThisTick = {}
})

setInterval(function () { // foo!
  console.log('Foo called! bar=' + calledThisTick.bar, "baz=" + calledThisTick.baz, "ticks-since-last-interval=" + ticks)
  ticks = 0
  calledThisTick.foo = true
}, 250)

foreverTick(() => { // bar!
  if (calledThisTick.baz) {
    console.log('This can never occur, because baz will always be added after bar')
    process.exit(1)
  }
  calledThisTick.bar = true
})

foreverTick(() => { // bar!
  if (calledThisTick.bar != true) {
    console.log('This can never occur, because bar will always be executed before baz')
    process.exit(1)
  }
  if (calledThisTick.foo) {
    console.log('This can never occur, because all ticks are always executed in order, and the cleanup always executes before this tick')
    process.exit(1)
  }
  calledThisTick.baz = true
})