New primitive

With unique IDs

Customized

Symbols were made for Metaprogramming

<!DOCTYPE html>
<html>
<head>
	<script src="https://cdnjs.cloudflare.com/ajax/libs/babel-core/5.8.29/browser.js"></script>
	<script type="text/babel">

		const id = Symbol();
		const courseInfo = {
			title: "ES6",
			topics: ["babel", "syntax", "functions", "classes"],
			id: "js-course"
		};

		courseInfo[id] = 41284;
		console.log(courseInfo);

	</script>
	<title>Symbols</title>
</head>
<body>
</body>	
</html>

Source: https://www.freecodecamp.org/news/how-did-i-miss-javascript-symbols-c1f1c0e1874a/