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

// Create a property of the global object
window.jane = "JJ";
alert(jane);        // "JJ", works as a variable too
alert(window.jane); // "JJ"

// Delete them
try { alert(delete window.john); } catch (e) { alert(e); }
// false
/* if strict mode were enforced would actually cause
   the following fatal error:
   TypeError:
   property "john" is non-configurable and can't be deleted   
*/
try { alert(delete window.jane); } catch (e) { alert(e); }
// true

alert(john); // "Jo"
try { alert(jane); } catch (e) { alert(e); } // undefined in book
/* but actually gives a:
   ReferenceError: jane is not defined
   which is a fatal error causing the script to exit
   if not caught*/
alert(this === window); // true