Document.selection.createRange () does not work in IE

I tried to create a simple ubb editor, but document.selection.createRange () does not work in IE and it just won’t get the selected text (try to warn the selected text, but you won’t get anything). I still can not find the cause and solve this problem.

Here you can see the script here

window.onload = function () {
    var ubb = document.getElementById('ubb_code');
    var ubba = ubb.getElementsByTagName('a');//ubb buttons
    var textarea = document.getElementsByTagName('textarea')[0];//editable textarea filed 

    //onclick to insert ubbcode
    ubba[0].onclick = function() {
        tag('[b]','[/b]');
    };

    //function used to insert ubbcode
    function tag(tag1,tag2){
        // code for IE
        if (document.selection){
            textarea.focus();
            var sel = textarea.document.selection.createRange();//It DOES NOT work!
            //alert(sel.text);
            sel.text = tag1 + sel.text + tag2;
        }else{  
        // code for none-IE webbrowsers
            var len = textarea.value.length;
            var start = textarea.selectionStart;
            var end = textarea.selectionEnd;


            var scrollTop = textarea.scrollTop;
            var scrollLeft = textarea.scrollLeft;


            var sel = textarea.value.substring(start, end);
            //alert(sel);
            var rep = tag1 + sel + tag2;
            textarea.value =  textarea.value.substring(0,start) + rep + textarea.value.substring(end,len);

            textarea.scrollTop = scrollTop;
            textarea.scrollLeft = scrollLeft;
        }
}