var commentsTree = [
	{
		id: 1,
		text: 'Some lorem',
		comments: [
			{
				id: 2,
				text: 'Another lorem',
				comments: [
					{
						id: 3,
						text: 'Third lorem',
						comments: [
							{
								id: 11,
								text: 'Hello'
							},
							{
								id: 12,
								text: 'Bye'
							}
						]
					},
				]
			},
			{
				id: 9,
				text: 'True'
			}
		]
	},
	{
		id: 4,
		text: 'Functional programming',
		comments: [
			{
				id: 5,
				text: 'Yes'
			},
			{
				id: 6,
				text: 'No'
			}
		]
	}
];

var countCommentsES5 = function(comments) {
	return comments.reduce(function(count, comment) {
			if (comment.comments) {
				return count + 1 + countCommentsES5(comment.comments);
			} else {
				return count + 1;
			}
			}, 0);
};

var countCommentsES6 = comments => {
	return comments.reduce(
		(count, comment) => (
			comment.comments
			? count + 1 + countCommentsES6(comment.comments)
			: count + 1
		),
		0
	)
}

console.log(countCommentsES5(commentsTree));