textarea自动换行

<textarea id="info_text" name="info_text" class="t_area"  onpropertychange="this.style.height = this.scrollHeight + 'px';"
                                    oninput="this.style.height = '';this.style.height = this.scrollHeight + 'px';"
                                    runat="server">
 </textarea>

.t_area
{
    width: 171px;
    background: #F4FBFB;
    border: 0px;
    overflow-y: hidden;
}

 var textarea = document.getElementById("gg_text");
 textarea.style.posHeight = textarea.scrollHeight;