Returns the type/tag of a given value.

tagof.js

function tagof (value) {
	return value != value ? 'NaN' :
		Object.prototype.toString.call(value)
		.match(/\[object (\w+)\]/)[1]
		.toLowerCase();
}

test.js

[
	[], 
	false, 
	new Date(), 
	Error(), 
	function () {},
	1 / 'zero',
	null,
	0,
	{},
	RegExp(/a/),
	'a',
	Symbol(),
	undefined
]
.forEach(value => console.log(tagof(value)));

// That's all there is to it.