How to make a substring in RichTextBox to bold

/**
* Replace selected elments with the element wrapped in a <tag></tag>
* This function is supposed to be cross-platform
* This function copied from http://codingforums.com/showthread.php?t=134113
*
* @return void
*/
function formatText(el,tag) {
      var selectedText=document.selection?document.selection.createRange().text:el.value.substring(el.selectionStart,el.selectionEnd);// IE:Moz
      var newText='<'+tag+'>'+selectedText+'</'+tag+'>';
      // Do IE compatible replacements
      if(document.selection){
            document.selection.createRange().text=newText;
      }
      // Do mozilla compatible replacements
      else{
            el.value=el.value.substring(0,el.selectionStart)+newText+el.value.substring(el.selectionEnd,el.value.length);
      }
}


Then call this like so input type="button" onclick="formatText('element', '<b>')"

richtextbox1.select allows you select from some index to some length of text.
After selecting, just make the selection bold as shown by jaime.

Indeed I showed how to use the Select method too.
 

Looks like Jaime is the winner here :)