JSFiddle - React, Tailwind, and code Playground

HTML

<div id="output"></div>

JavaScript

var foo = {
    a: 1,
    b: undefined
}

var o = document.getElementById('output');

o.innerHTML = ("a" in foo) + "<br>" + 
    ("b" in foo) + "<br>" + 
    ("c" in foo) + "<br>";