JSFiddle - React, Tailwind, and code Playground

JavaScript

var x;
if (x === undefined) alert('novalue for variable x');
else alert(x);

var y;
y='this variable has value';
if (y === undefined) alert('novalue for variable y');
else alert(y);

var z;
//z='this variable has value';
if (z === void 0) alert('novalue for variable z');
else alert(z);