Get all text before SelectionStart and after SelectionEnd

I am creating a text editor from scratch.

code for getting bold

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

            var start = window.getSelection().toString();

            var bolded = '<b>' + start + '</b>';
            var cursorPositionBegin = $('#TextEditor').prop("selectionStart");
            var cursorPositionEnd = $('#TextEditor').prop("selectionEnd");
// to check 
                alert(bolded);
            })  

HTML CODE

<table width="50%">
    <tr>
        <td>
            <div>
                <div>
                    <span><input type=button value="B" id="bold"></span>
                </div>
                <div contenteditable="true">
                    <textarea cols="47" rows="23" placeholder="editor" id="TextEditor"></textarea>
                </div>
            </div>  
        </td>
    </tr>
</table>

When I click #bold for a selected set of characters I want to add characters in the #TextEditor. I thought maybe getting the beginning and ending position of cursor could help break up and join the begin and end together along with the characters to form what I require.

I also use jQuery

[update 1] Or is there an alternate method to do what I require? [update 2] added contenteditable="true" to div where #TextEditor id placed

Your help would be appreciated.

As @weBBer said you will not allowed to add tag inside textarea element use div with attribute contenteditable="true" instead

$('#bold').click(function(){
  var string = window.getSelection().toString();
  var bolded = '<b>' + string + '</b>';
  var selC, range;
      if (window.getSelection) {
          selC = window.getSelection();
          if (selC.rangeCount) {
              range = selC.getRangeAt(0);
              range.deleteContents();
              range.insertNode(document.createTextNode(bolded));
          }
      } else if (document.selection && document.selection.createRange) {
          range = document.selection.createRange();
          range.text = bolded;
      }
}); 

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<table width="50%">
    <tr>
        <td>
            <div>
                <div>
                    <span><input type=button value="B" id="bold"></span>
                </div>
                <div contenteditable="true" style="height: 300px;width: 300px;border: 1px solid black">
                    
                </div>
            </div>  
        </td>
    </tr>
</table>