如何获取JavaScript设置的标签的值

Question

此脚本在页面加载时已被触发，并且label1设置为“ Hello World”

<script type="text/javascript">            
        function hello() {
           document.getElementById('<%= Label1.ClientID %>').innerHTML = "Hello World";
        }
</script>

单击按钮时始终返回null

    protected void Button1_Click(object sender, EventArgs e){
     label2.text = label1.text; // always return null 
    }