Understanding the Event Loop

eventloop.js

function callback(arg) {
  setTimeout(console.log, 0, arg, '   #  4 timeout  ');
  setImmediate(console.log, arg, '    # 5 immediate');
  process.nextTick(console.log, arg, ' #    2 nextTick ');
  (async () => { await undefined; console.log(arg, '  #   3 await    '); })();
  console.log(arg, '#     1 sync     ');
}

setTimeout(callback, 0, '   #  4 timeout  ');
setImmediate(callback, '    # 5 immediate');
process.nextTick(callback, ' #    2 nextTick ');
(async () => { await undefined; callback('  #   3 await    '); })();
callback('#     1 sync     ');

/* Result:
#     1 sync      #     1 sync     
 #    2 nextTick  #     1 sync     
#     1 sync       #    2 nextTick 
 #    2 nextTick   #    2 nextTick 
  #   3 await     #     1 sync     
#     1 sync        #   3 await    
 #    2 nextTick    #   3 await    
  #   3 await       #   3 await    
  #   3 await      #    2 nextTick 
   #  4 timeout   #     1 sync     
   #  4 timeout    #    2 nextTick 
   #  4 timeout     #   3 await    
#     1 sync         #  4 timeout  
 #    2 nextTick     #  4 timeout  
  #   3 await        #  4 timeout  
    # 5 immediate #     1 sync     
    # 5 immediate  #    2 nextTick 
    # 5 immediate   #   3 await    
#     1 sync          # 5 immediate
 #    2 nextTick      # 5 immediate
  #   3 await         # 5 immediate
   #  4 timeout       # 5 immediate
   #  4 timeout      #  4 timeout  
    # 5 immediate    #  4 timeout  
    # 5 immediate     # 5 immediate
*/

scheduling-methods.js

// https://nodejs.org/en/docs/guides/event-loop-timers-and-nexttick
// https://javascript.info/event-loop
setImmediate(console.log, 'immediate')
setTimeout(console.log, 0, 'timeout')
queueMicrotask(() => console.log('microtask'))
;(async () => { await undefined; console.log('await') })()
new Promise(resolve => { console.log('promise'); resolve('promised') }).then(console.log)
process.nextTick(console.log, 'nextTick')
console.log('sync')

/* Result:
promise
sync
nextTick
microtask
await
promised
timeout
immediate
*/