input 焦点放在文本最后

<input id="a" name="test" value="aaaaaaaaaaaaaaaaaabbb2aaaaaaaaaaaaaaaaaabbb2aaaaaaaaaaaaaaaaaabbb2aaaaaaaaaaaaaaaaaabbb2"><input>
<script>
    var obj;
    var val;
    var len;
    var start = 0;
    (function m1(){
        obj=document.getElementById('a');
        val=obj.value;
        len=val.length-1;
        if(len<30) return;

        obj.value=""
       m2();
    })();
    function m2(){
        obj.value = obj.value + val.substr(start,1) ;
       
        start++;
        if(start<=len){
            setTimeout('m2()',1);
        } else {
         document.getElementById('a').focus();
        }
       
    }
</script>