window before / on / after resize event

resize.js

var resizeTimer;
var timeoutInterval = 100; // Timeout interval for resize timer

window.onresize = function(event) {
  var status = document.getElementById("status");
  
  // ON RESIZING
  status.innerHTML = "Resizing...";
  
  clearTimeout(resizeTimer);
  resizeTimer = setTimeout(function() {

    // AFTER RESIZING
    status.innerHTML = "Resizing done.";
  }, timeoutInterval);
};

Working example: https://codepen.io/richardev/pen/QWWrvOp