JSFiddle - React, Tailwind, and code Playground

HTML

<table id="mytable">
  <tr id="tr1">
      <td>2</td>
      <td>3</td>
  </tr>
  <tr id="tr2">
    <td>4</td>
    <td>5</td>
    <td>6</td>
    <td>8</td>
  </tr>
  <tr id="tr3">
    <td>7</td>
    <td>8</td>
  </tr>
</table>

CSS

#mytable{
    /*border:1px solid red;*/
}

#mytable td{
    border:1px solid red;
    
}

JavaScript

$(document).ready(function(){
    
    addCol("tr3", 4);

});

function addCol(rowid, coldes){
    var tr = $("#" + rowid + " td");
    var trctr = tr.length;
    troffset = coldes - trctr;
    alert(troffset);
    for(var i = 1; i <= troffset; i++){
        tr.parent().append("<td></td>");
    }
}