js获取radio选中索引值

<form name="form1" onsubmit="return foo()">
<input type="radio" name="radiogroup1">
<input type="radio" name="radiogroup1">
<input type="radio" name="radiogroup1">
<input type="radio" name="radiogroup1">
<input type="radio" name="radiogroup1">
<input type="radio" name="radiogroup1">
<input type="submit">
</form>
<script>
function foo(){
    for(i=0;i<document.form1.radiogroup1.length;i++)
    {
       if (document.form1.radiogroup1[i].checked){
　　　　alert(i);
　　　　return i;
　　}
    }
 return false;
}
</script>