What exactly is the event loop event loop

• First, the browser is running multi-threaded, it can be verified from the Task Manager and Activity Monitor.
  Open a new tab and add a plug-in will add a process, as shown below:
  

• Browser rendering process is multi-threaded, includes GUI rendering thread, js engine thread, the thread event-triggered, the timer thread, threading, asynchronous requests.
  Usually say js js engine is a single-threaded refers to the thread.