JSFiddle - React, Tailwind, and code Playground

HTML

<textarea id='text'></textarea>
<button id='button'>press me</button>

JavaScript

Ext.get('button').on('click', update);
function update(){
  var myTextArea = Ext.getDom('text')
     ,textInArea = myTextArea.value
     ,textToInsert = '???'
     ,caretPosition = myTextArea.selectionStart;

  myTextArea.value = textInArea.substring(0, caretPosition) 
      + textToInsert  + textInArea.substring(caretPosition); 
}