原生js实现弹出层登录拖拽功能

window.onload = function() { var btn = document.getElementsByClassName('login')[0] var close = document.getElementById('close'); var login = document.getElementById('login'); var top = (document.documentElement.clientHeight - 250) / 2; //top值等于(获取页面可视区域的宽度 - 登录框的高度) / 2 var left = (document.documentElement.clientWidth - 350) / 2; var open = document.getElementById('screen'); btn.onclick = function() {  login.style.display = 'block';  login.style.left = left + 'px';  login.style.top = top + 'px';  open.style.display = 'block';  open.style.width = getInner().width + 'px'; //弹出层的宽度等于可视窗口的宽度  open.style.height = getInner().height + 'px'; } close.onclick = function() {  login.style.display = 'none';  open.style.display = 'none'; } window.onresize = function() {   var top = (getInner().height - 250) / 2;   var left = (getInner().width - 350) / 2;   login.style.left = left + 'px';   login.style.top = top + 'px';  }  //跨浏览器获取可视窗口 function getInner() {  if (window.innerWidth != 'undefined') { //IE 不支持返回undefind   return {    width: window.innerWidth,    height: window.innerHeight   }  } else {   return {    width: document.documentElement.clientWidth,    height: document.documentElement.clientHeight   }  } } //实现拖拽功能 ，三个事件 mousemove , mouseup ,mousedown //clientX ,clientY 时鼠标指针相对于整个屏幕的坐标距离 //offsetLeft, offsetTop 获取当前元素相对于父元素的位置,在这里，父元素是document login.onmousedown = function(e) {   stop(e); //阻止事件默认行为   var e = e || window.event;   var oLeft = e.clientX - login.offsetLeft; //login.offsetLeft 获取盒子边框到浏览器左边框的距离   var otop = e.clientY - login.offsetTop;   document.onmousemove = function(e) { //移动的是整体的doucment    var e = e || window.event;    //不能移出可视区域    var left = e.clientX - oLeft;    var top = e.clientY - otop;    //左右    if (left < 0) { //如果盒子距左边的距离小于零，即超出     left = 0;    } else if (left > getInner().width - login.offsetWidth) { //可视区域的长度，减去盒子的长度 offsetWidth     left = getInner().width - login.offsetWidth;    }    //上下    if (top < 0) {     top = 0;    } else if (top > getInner().height - login.offsetHeight) {     top = getInner().height - login.offsetHeight;    }    login.style.left = left + 'px';    login.style.top = top + 'px';   }   document.onmouseup = function() {    document.onmousemove = null;    document.onmouseup = null;   }  }  //取消默认行为 function stop(e) {  var e = e || window.event;  if (typeof e.preventDefault != 'undefined') {   e.preventDefault(); //W3C  } else {   e.returnValue = false; //IE阻止事件默认行为  } }}

<div id="header"> <div class="logo"><img src="images/logo.gif" alt="" /></div> <div class="member">个人中心  <ul class="list">   <li><a href="###">设置</a></li>   <li><a href="###">换肤</a></li>   <li><a href="###">帮助</a></li>   <li><a href="###">退出</a></li>  </ul> </div> <div class="login">登录</div></div><div id="login"> <h2><img src="images/close.png" alt="" class="close" id="close" />登录</h2> <div class="user">用户名<input type="text" name="user" class="text" /></div> <div class="pass">密  码   <input type="password" name="pass" class="text" /></div> <div class="button">  <input type="button" class="submit" value="" /></div> <div class="other">注册新用户 | 忘记密码</div></div><div id="screen"></div><script type="text/javascript" src="demo.js"></script>