js中是不是可以省略getElementById，下面的代码有没有注释部分都照样成功运行啊

<head>
    <title>无标题页</title>
    <style type="text/css">
        .day{background-color:Blue;}
        .night{background-color:Black;}
    </style>
    <script type="text/javascript">
        function switchLight(){
                       //var btnSwitch=document.getElementById("btnSwitch");
            if(document.body.className=="night"){
                document.body.className="day";
                btnSwitch.value="关灯"
            }
            else{
                document.body.className="night";
                btnSwitch.value="开灯";
            }
        }
    </script>
</head>
<body class="day">
    <input type="button" id="btnSwitch" value="关灯" onclick="switchLight()" />
</body>
</html>