JSFiddle - React, Tailwind, and code Playground

HTML

<!DOCTYPE html>
<body>
<table>
<tr>
	<th>Name</th>
	<th>Gender</th>
	<th>Age</th>
</tr>
<tr>
	<td>John</td>
	<td>M</td>
	<td>18</td>
</tr>
<tr>
	<td>Alice</td>
	<td>W</td>
	<td>28</td>
</tr>
</table>
<script>
(function() {
	var jsonArr = [];
	var obj = {};
	var thNum = document.getElementsByTagName('th').length;
	var arrLength = document.getElementsByTagName('td').length;
	
	for(i = 0; i < arrLength; i++){
		if(i%3 === 0){
			obj = {};
		}
		var head = document.getElementsByTagName('th')[i%thNum].innerHTML;
		var content = document.getElementsByTagName('td')[i].innerHTML;
		obj[head] = content;
		if(head === 'Age'){
			jsonArr.push(obj);
		} 	
	}           
	document.write("<br>"+JSON.stringify(jsonArr));
})();
</script>
</body>
</html>