JSFiddle - React, Tailwind, and code Playground

HTML

<input type="button" id="button" value="Clicky" />
<textarea id="texty" style="width:400px;height:300px"> asd sad asdsa http://www.youtube.com/watch?v=u7TwqpWiY5s text</textarea>

JavaScript

$('#button').click(function() {
 
     

   
      el = document.getElementById('texty');
      if (el.setSelectionRange) {
          
         el.value = el.value.substring(0,el.selectionStart) + "[highlight]" + el.value.substring(el.selectionStart,el.selectionEnd) + "[/highlight]" + el.value.substring(el.selectionEnd,el.value.length);
          
        
      }
      else if(document.selection.createRange()) {
          document.selection.createRange().text = "[highlight]" + document.selection.createRange().text + "[/highlight]"; 
      }
    
  
        
        
});