JSFiddle - React, Tailwind, and code Playground

CSS

body {
  background: #FFF;
}
code {
  display: block;
  padding: 1em;
  background: #F0F0F0;
  color: #555;
}

JavaScript

let myVar = 'Hello World';
let undefined = 'string';

document.body.innerHTML  = '<p>Value of myVar: <i>(' + (typeof myVar) + ') ' + myVar + '</i></p>';
document.body.innerHTML += '<hr><code>CHECKING (typeof myVar === undefined)<br></code>';

if (typeof myVar === undefined) {
    document.body.innerHTML += '<code>true</code>';
} else {
    document.body.innerHTML += '<code>false</code>';
}

document.body.innerHTML += '<hr><code>CHECKING (typeof myVar === \'undefined\')<br></code>';

if (typeof myVar === 'undefined') {
    document.body.innerHTML += '<code>true&nbsp;&nbsp;&nbsp;&nbsp;// This is the correct result</code>';
} else {
    document.body.innerHTML += '<code>false&nbsp;&nbsp;&nbsp;&nbsp;// This is the correct result</code>';
}