JSFiddle - React, Tailwind, and code Playground

HTML

<script src="//wzrd.in/standalone/expect@latest"></script>
<script src="//wzrd.in/standalone/deep-freeze@latest"></script>
<body>
</body>

JavaScript

const concat = (list, element) => {
	// list.push(element);
  return list.concat(element);
};

const testConcat = () => {
   const listBefore = [];
   const listAfter = [0];
  deepFreeze(listBefore);
   expect(
     concat(listBefore, 0)
   ).toEqual(listAfter);
  
};

const logAndDisplay = (arg) => {
	document.querySelector('body').innerHTML = arg;
  console.log(arg)
};

testConcat();
logAndDisplay('All tests passed');