demo.js

var heads = ['NAME', 'AGE', 'GENER'];
var rows = [['TOM', 21, 'MALE'],['LUCY', 16, 'FEMALE'],['JERRY', 23, 'MALE']];

// '<table><thead><tr>'+
// 在这里循环 head
// for head in heads:
//   <th>head</th>
// '</tr></thead><tbody>'+
// 在这里循环 rows
// for row in rows:
//   <tr>
//     这里循环 row 里面的单元格
//     for cell in row:
//       <td>cell</td>
//   </tr>  
// '</tbody></table>';

var html = '<table><thead><tr>';

for (var i = 0; i < heads.length; i++)
  html += '<th>'+heads[i]+'</th>';
  
html += '</tr></thead><tbody>';

for (i = 0; i < rows.length; i++) {
  html += '<tr>';
  for (var j = 0; j < rows[i].length; j++)
    html += '<td>'+ rows[i][j] +'</td>';
  html += '</tr>';
}

html += '</tbody></table>';
console.log(html);