为什么 Button在布局的最下方，绑定的事件无法被触发，而在中间就可以触发Button所绑定的事件？

<form #sendmsg-window>
         <div class="maskinglayer_div">
            <div class="sendmsg">
                <div class="sendmsg_top">
                    <span class="sendmsg_top_1"><label>发送短消息</label></span>
                    <button id="sendmsg_min"></button>              
                    <button id="sendmsg_close"></button>
                </div>
                <div class="sendmsg_main">
                    <ul>
                        <li id="msgname"><label>接收人姓名</label><input id="sendmsg_phone" class="msgname_input"/></input></li>
                        <li class="msgname_label1"><label>请输入要发送的电话号码</label></li>
                        <li class="msgname_label1"><label>（多个手机号码间需要分号或空格隔开）</label></li>
                        <li><input class="msgname_input2"></input></li>
                        <li class="msgname_label1"><label>请输入短消息内容</label></li>
                         
                        <li class="msgname_label1"><label>（最多支持500个字符）</label></li>
                        <li><textarea class="sendmsg_content"id="sendmsg_content"></textarea></li>
                        <li class="msgname_label1"><label>请输入的字符个数：9</label></li>
                    </ul>
                </div>
                <div>
                <button id="sendmsg_sd1" class="msgname_btn1">发送</button><button class="msgname_btn1">关闭</button>
                </div>
            </div>
        </div>
    </form>

1楼的情况也可能是导致你的错误原因，你的脚本一定要放在你操作的DOM对象的后面，不然会undefined