JSFiddle - React, Tailwind, and code Playground

HTML

<textarea id="mytextarea">
here is some text to select
</textarea>

JavaScript

function getSelectionInsideTextarea(textComponent)
{
  var selectedText;
 
  if (document.selection != undefined)
  {
    textComponent.focus();
    var sel = document.selection.createRange();
    selectedText = sel.text;
  }
  else if (textComponent.selectionStart != undefined)
  {
    var startPos = textComponent.selectionStart;
    var endPos = textComponent.selectionEnd;
    selectedText = textComponent.value.substring(startPos, endPos)
  }

  return selectedText;
}
var textArea = document.getElementById('mytextarea');
textArea.onselect = function(){           alert(getSelectionInsideTextarea(textArea));
};