jQuery字段选择replacesselection不't为我工作

嗨，这是一个简单的jquery字段选择插件。

   /*
 * jQuery plugin: fieldSelection - v0.1.1 - last change: 2006-12-16
 * (c) 2006 Alex Brem <alex@0xab.cd> - http://blog.0xab.cd
 */
(function() {
    var fieldSelection = {
        getSelection: function() {
            var e = (this.jquery) ? this[0] : this;
            return (
                /* mozilla / dom 3.0 */
                ('selectionStart' in e && function() {
                    var l = e.selectionEnd - e.selectionStart;
                    return { start: e.selectionStart, end: e.selectionEnd, length: l, text: e.value.substr(e.selectionStart, l) };
                }) ||
                /* exploder */
                (document.selection && function() {
                    e.focus();
                    var r = document.selection.createRange();
                    if (r === null) {
                        return { start: 0, end: e.value.length, length: 0 }
                    }
                    var re = e.createTextRange();
                    var rc = re.duplicate();
                    re.moveToBookmark(r.getBookmark());
                    rc.setEndPoint('EndToStart', re);
                    return { start: rc.text.length, end: rc.text.length + r.text.length, length: r.text.length, text: r.text };
                }) ||
                /* browser not supported */
                function() { return null; }
            )();
        },
        replaceSelection: function() {
            var e = (typeof this.id == 'function') ? this.get(0) : this;
            var text = arguments[0] || '';
            return (
                /* mozilla / dom 3.0 */
                ('selectionStart' in e && function() {
                    e.value = e.value.substr(0, e.selectionStart) + text + e.value.substr(e.selectionEnd, e.value.length);
                    return this;
                }) ||
                /* exploder */
                (document.selection && function() {
                    e.focus();
                    document.selection.createRange().text = text;
                    return this;
                }) ||
                /* browser not supported */
                function() {
                    e.value += text;
                    return jQuery(e);
                }
            )();
        }
    };
    jQuery.each(fieldSelection, function(i) { jQuery.fn[i] = this; });
})();

我不明白为什么这样做，textarea元素中的选定文本不被替换。

$('.wysiwyg-editor-italic').live('click',function(){
   var _selection = $('.wysiwyg textarea').getSelection().text;
   var wcode = "";
   if(_selection){
       $('.wysiwyg textarea').replaceSelection("{italic}"+_selection+"{/italic}");
       console.log(_selection);
    }
    else{
        wcode = "{italic} italic {/italic}";
    }
    var wysiwyg_val = $('.wysiwyg textarea').val();
    $('.wysiwyg textarea').val("").val(wysiwyg_val+wcode);
     $('.wysiwyg-preview').html(wysiwyg_val);
    $('.wysiwyg textarea').focus();

    refresh_preview()
});

在控制台中我可以看到_selection，这是可以的，但之后没有被替换…也许我错过了什么?