JSFiddle - React, Tailwind, and code Playground

HTML

<h1>WITH CALL BACK FUNCTION</h1>

JavaScript

var flag=false;
function ProgressBar(element, myimages, callback, flag) {
     //SOMESTUFF
    console.log('flag : '+flag);
    console.log('In Progress Bar called first');
      callback();
  }

ProgressBar(1, 2, function() { flag=true; console.log(' After flag : '+flag);}, flag);
 console.log('OUT flag : '+flag);