搞了一个晚上了，求指点　Uncaught TypeError: Cannot read property 'offsetHeight' of null

<body>
    <div id="topcon" ref="bod" >
        <div class="top" id="top">
            <span class="sp-1">12112221212</span>
            <span class="sp-2">2323232323</span>
        </div>
        <div class="shop" ><a href="#">点击前往--></a></div>
        <div class="main">
            <div class="tipbox" >
                <span class="tip-1">内容</span>
                <span class="tip-2">优势</span>
            </div>
            <div class="title">

            </div>
        </div>
    </div>
<script>
    var div1 = document.getElementById("#top").offsetHeight;
    alert(div1+'px');
</script>   
</body>

这个高度一直拿不到

var div1 = document.getElementById("top").offsetHeight;

用getElementById方法不要加#号

搞了一个晚上了，求指点 Uncaught TypeError: Cannot read property 'offsetHeight' of null

搞了一个晚上了，求指点　Uncaught TypeError: Cannot read property 'offsetHeight' of null