JSFiddle - React, Tailwind, and code Playground

JavaScript

'use strict';

// Create a global variable
var john = "Jo";
alert(john);        // "Jo"
alert(window.john); // "Jo", works as a property too
                    /* BUT I GET UNDEFINED HERE */
// Create a property of the global object
window.jane = "JJ";
alert(jane);        // "JJ", works as a variable too
alert(window.jane); // "JJ"

// Delete them
alert(delete window.john); // false
                           /* BUT I GET true HERE */
alert(delete window.jane); // true

alert(john); // "Jo"
alert(jane); // undefined
             /* BUT PROGRAM CRASHES HERE */
alert(this === window); // true