JSFiddle - React, Tailwind, and code Playground

HTML

<form action="" method="get"> 
    <input type="hidden" id="myHiddenField" name="myHiddenField" value="" />
  <table width="766"  border="0" cellspacing="0" cellpadding="0"> 
   <input type="hidden" name="myHiddenField" value="**I want to store allRows.length+1 value here **" />
    <tr> 
      <td width="800"><input type="text" name="textfield_A" /></td> 

      <td width="191"><input type="text" name="textfield_B" /></td> 

      <td width="286"><input name="button" type="button" value="+" onclick="addRow(this.parentNode.parentNode)"></td> 
    </tr> 
  </table><br /><br /> 
  <input name="" type="submit" value="Submit" /> 
</form>

JavaScript

function addRow(r){ 
    var root = r.parentNode;//the root 
    var allRows = root.getElementsByTagName('tr');//the rows' collection 
    var cRow = allRows[0].cloneNode(true)//the clone of the 1st row 
    var cInp = cRow.getElementsByTagName('input');//the inputs' collection of the 1st row 
    for(var i=0;i<cInp.length;i++){//changes the inputs' names (indexes the names) 
    cInp[i].setAttribute('name',cInp[i].getAttribute('name')+'_'+(allRows.length+1)) 
    } 
    root.appendChild(cRow);//appends the cloned row as a new row 
    document.getElementById("myHiddenField").value = allRows.length;
}