javascript 依次输入自动定焦框

<html>
<head>
<script type="text/javascript">
    function moveNext(object,index){
        if(object.value.length==4){
            document.forms[0].elements[index+1].focus();
        }
    } 

    function showResult(){
        var f=document.forms[0];
        var result="";
        for(var i=0;i<4;i++){
            result+=f.elements[i].value;
        }
        alert(result);
    }

</script>

</head>
<body onload="document.forms[0].elements[0].focus();">
    <form>
        <input type="text" size="3" maxlength="4" onkeyup="moveNext(this,0);">
        <input type="text" size="3" maxlength="4" onkeyup="moveNext(this,1);">
        <input type="text" size="3" maxlength="4" onkeyup="moveNext(this,2);">
        <input type="text" size="3" maxlength="4" onkeyup="moveNext(this,3);"> 

        <input type="button" value="显示" onclick="showResult();">
    </form>
</body>
</html>