Why would you need to know, except in some special cases? If you're using async/await, your next line is not executing anyway until the result comes, blocking or not.

Because otherwise you block the even loop or mismatch life cycle. Espacially reading an unfamiliar code base.

embrace higher-order functions. Use named functions and closures!

example code:

  var anchors = document.getElementsByTagName(“a”);
  for(var i=0,len=anchors.length; i<len; i++){
    alertClickAnchor(i); // Named function
  }

  function alertClickAnchor(i) { // Closure
    anchors[i].onclick = function() {
      alert(i);
    }
  }

This makes anything but basic life cycles very hard to read.

You can look at each function at a time and only have to understand what that function does. No more nesting.