JSFiddle - React, Tailwind, and code Playground

HTML

<button id="bold">B</button>
<button id="add_txt">Add txtarea</button> 

<div id="vk_main">


</div>

CSS

textarea {
  font-weight: normal;
 }
  textarea:first-line {
 font-weight: bold;
 }

JavaScript

var c = 0;
$.activeEl = $();

$('#add_txt').on('click', function() {
    var $div = $('<textarea id="ta_' + (c) + '">Text here.</textarea>');
    $('#vk_main').append($div);
});

$("textarea").live("focus",function() {
   $.activeEl = $(this);
});

$("#bold").click(function() {
    $.activeEl.css("font-weight", "bold");

});