Adding value from th tag and set total in the final column javascript

I have values from my database and in my view I have added one column added for the total or average value.In every rows, it has 3 columns with values.In database there is no column for average yet instead of putting in database, I only put in my blade.php. How could i possibly do it in javascript?It seems like i don't know what to start since I'm not a master in java.

<tbody>
  @foreach($card['AllGrade'] as $subject)
    <tr>
      <th colspan="4">{!! $subject['subject'] !!}</th>
      <th colspan="2" >{!! $subject['term_1'] !!}</th>
      <th colspan="2" >{!! $subject['term_2'] !!}</th>
      <th colspan="2" >{!! $subject['term_3'] !!}</th>
      <th colspan="2" name ="ave" id ="ave" value=""> total value here</th>
    </tr>
  @endforeach

via Chee