JSFiddle - React, Tailwind, and code Playground

HTML

<div class="container">
  <textarea>asddas dasdasdas asdas</textarea>
  <div class="txt-underline"></div>
</div>

CSS

body{
  background-color: #ffffff; 
}
textarea{
  width: 100%;
  border: 0;
  outline: 0;
}
.txt-underline{
  background-color: rgba(0,0,0,0.12);
  height: 1px;
  width: 100%;
}
.txt-underline.active{
  background-color: #673ab7;
  height: 2px;
}

JavaScript

$(function(){
	$('body').on('focus', 'textarea', function(){
    $(this).next().addClass('active');
  });
  
  $('body').on('focusout', 'textarea', function(){
    $(this).next().removeClass('active');
  });
});