JavaScript Event Loop Explorer

Step through a sample program and watch the call stack, microtask queue (Promise callbacks) and macrotask queue (setTimeout callbacks) fill and drain. The order of the console output stops being mysterious once you can see the machine that produces it.

Program
Run
Speed
Step 1 / 1
Idle
Source 

    


    

      
      

        

          Call Stack
          LIFO · top runs
        

        

      


      

        
        

          

            Microtasks
            FIFO · promises
          

          

        

        
        

          

            Macrotasks
            FIFO · timers
          

          

        

      


      
⚡ When the stack empties, every microtask runs before one macrotask is taken — then microtasks drain again. That priority rule is the whole trick.


      
      

        

          Console output
          order of appearance
        

        

      

    

  


  
  

    

      Prompt — take this scenario back to Claude
      
    

    

  


  

    Keys: ← → step · Space play/pause · Home reset