The key could be any object, but it must be an especific one when is called.

if it is a String, cant be Object or Other class

<!DOCTYPE html>
<html>
<head>
	<script src="https://cdnjs.cloudflare.com/ajax/libs/babel-core/5.8.29/browser.js"></script>
	<script type="text/babel">
		
		var course = new Map();
		course.set('react', {description: 'ui'});
		course.set('jest', {description: 'testing'});

		console.log(course);
		console.log(course.react);
		console.log(course.get('react'));

		var details = new Map([
			[new Date(), 'today'],
			['items', [1, 2]]
		]);

		console.log(details.size);

		details.forEach(function(item) {
			console.log(item);
		});

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

Output: