JSFiddle - React, Tailwind, and code Playground

JavaScript

window.foo = true;

+function () {
    // Insert code mutating foo over time
}();

+function f() { 
   var a = window.foo;

   // Insert some long, blocking work here

   var b = window.foo; // again

   if (a != b) { totalMindExplosion() }
}();