如何使光标落在文本框的文本最后

<SCRIPT LANGUAGE="JavaScript">
function fun(obj)
{
var r = obj.createTextRange();
r.moveStart("character",obj.value.length);
r.select();
}
window.onload=function(){
document.getElementById('textbox1').focus()
}
</SCRIPT>
<TEXTAREA NAME="" ROWS="5" COLS="20" id="textbox1" onfocus="fun(this)">12312313131</TEXTAREA>