How to replace the currently selected text inside an html textarea?

Question

How do I edit the selected text of a textarea form element?

EDIT: as in edit it in-place, replacing the orignal text.

Answer 1

This works:

function replaceIt(txtarea, newtxt) {
  $(txtarea).val(
        $(txtarea).val().substring(0, txtarea.selectionStart)+
        newtxt+
        $(txtarea).val().substring(txtarea.selectionEnd)
   );  
}
    

$("button").on('click', function() {
  replaceIt($('textarea')[0], 'fun')
})

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<textarea>Hello world.</textarea>
<button>Replace with fun</button>

Answer 2

I found this:

function wrapText(elementID, openTag, closeTag) {
    var textArea = $('#' + elementID);
    var len = textArea.val().length;
    var start = textArea[0].selectionStart;
    var end = textArea[0].selectionEnd;
    var selectedText = textArea.val().substring(start, end);
    var replacement = openTag + selectedText + closeTag;
    textArea.val(textArea.val().substring(0, start) + replacement + textArea.val().substring(end, len));
}

$('button').on('click', function() {
  wrapText('test', '<b>', '</b>');
});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<textarea id="test">This is a test</textarea>
<button>Surround with &lt;b&gt; tag</button>

But personaly its not working with content-editable divs.