js如何获取radio单选框选中的值

<code class="language-html"><form id="userlist" method="post" action="option.php">
    <input type="radio" name="userid" value="1">1
    <input type="radio" name="userid" value="2">2
    <input type="radio" name="userid" value="3">3
</form>
<script language="javascript">
    function usubmit(action){
        var radionum = document.getElementById("userlist").userid;
        for(var i=0;i<radionum.length;i++){
        if(radionum[i].checked){
            userid = radionum[i].value
        }
    }
    window.location.href='option.php?action='+action+'&userid='+userid;
}
</script></code>

<code class="language-javascript">document.getElementById("userlist").userid;</code>