js操作textarea方法集合封装(兼容IE,firefox)

完整测试代码：
代码如下:
<textarea id="testlujun" style="width: 300px; height: 50px;">abcdefghijklmnopqrstuvwxyz</textarea>
<br />
<input onclick="alert(TT.getCursorPosition(test))" type="button" value="光标位置" />
<input onclick="TT.setCursorPosition(test,3)" type="button" value="设置光标到3第位置" />
<input onclick="TT.add(test,'你好')" type="button" value="添加'你好'到光标后面" />
<input onclick="TT.del(test,2)" type="button" value="删除光标前2个字符" />
<input onclick="TT.del(test,-2)" type="button" value="删除光标后2个字符" />
<input onclick="TT.sel(test,0,2)" type="button" value="选中字符0-2的位置" />
<input onclick="TT.selString(test,'mno')" type="button" value="选中字符'mno'的位置" />
<script type="text/javascript">// <![CDATA[
var test = document.getElementById('testlujun');
var TT = {

 /*
  * 获取光标位置
  * @Method getCursorPosition
  * @param t element
  * @return number
  */
 getCursorPosition: function(t){
  if (document.selection) {
   t.focus();
   var ds = document.selection;
   var range = ds.createRange();
   var stored_range = range.duplicate();
   stored_range.moveToElementText(t);
   stored_range.setEndPoint("EndToEnd", range);
   t.selectionStart = stored_range.text.length - range.text.length;
   t.selectionEnd = t.selectionStart + range.text.length;
   return t.selectionStart;
  } else return t.selectionStart
 },

 
 /*
  * 设置光标位置
  * @Method setCursorPosition
  * @param t element
  * @param p number
  * @return
  */
 setCursorPosition:function(t, p){
  this.sel(t,p,p);
 },

 /*
  * 插入到光标后面
  * @Method add
  * @param t element
  * @param txt String
  * @return
  */
 add:function (t, txt){
  var val = t.value;
  if(document.selection){
   t.focus()
   document.selection.createRange().text = txt; 
  } else {
   var cp = t.selectionStart;
   var ubbLength = t.value.length;
   var s = t.scrollTop;
  // document.getElementById('aaa').innerHTML += s + '<br />';
   t.value = t.value.slice(0,t.selectionStart) + txt + t.value.slice(t.selectionStart, ubbLength);
   this.setCursorPosition(t, cp + txt.length);
  // document.getElementById('aaa').innerHTML += t.scrollTop + '<br />';