可移动的对话框

难点：

• 页面布局
• 先求出鼠标在盒子内的坐标，再利用鼠标在盒子内的坐标，求出盒子的offsetLeft和offsetTop
• mousedown、mousemove 和 mouseup的使用顺序和逻辑 以下是实现代码 css部分

<style>
        *{
            padding: 0;
            margin: 0;
        }
        a{
            text-decoration: none;
            color: #000;
        }
        .login-header{
            text-align: center;
            font-weight: 400;
            font-size: 26px;
        }
        .login{
            width: 400px;
            height: 220px;
            border-radius: 30px;
            background: #ffffff;
            box-shadow: 0px 0px 20px #ddd;
            z-index: 9999;
            transform: translate(0%, -50%);
            text-align: center;
            position: relative;
            margin: 300px auto;
            display: none;
        }
        .login-title{
            font-size: 18px;
            padding: 10px;
        }
        .login-title span{
            font-size: 12px;
            width: 40px;
            height: 40px;
            background-color: #fff;
            position: absolute;
            margin:-20px 140px;
            border-radius: 40px;
           line-height: 40px;
        }
        .login-input-content{
            width: 300px;
            height: 110px;
            margin: 0 auto;
            font-size: 14px;
        }
        .login-input-content .login-input{
            padding: 10px;
            position: relative;
            width: 280px;
            height: 30px;
        }
        .login-input-content .login-input input{
             width: 260px;
             height: 26px;
             position: absolute;
             left: 50px;
             border: 1px solid #ccc;
        }
        .login-input-content .login-input input:focus{
            outline:none;
        }
        .login-input-content .login-input label{
            text-align: right;
            width: 80px;
            height: 26px;
            position: absolute;
            right: 260px;
            line-height: 26px;
        }
        .login-button{
            width: 200px;
            height: 30px;
            line-height: 30px;
            border: 1px solid #ccc;
            border-radius: 5px;
            margin: 0 auto;
        }
        .login-bg{
            height: 100%;
            width: 100%;
            position: fixed;
            top: 0;
            left: 0;
            background-color: rgba(0, 0, 0, 0.3);
            display: none;
        }
    </style>

body部分

 <div class="login-header"><a href="javascript:;" id="link">点击弹出登录框</a></div>
    <div class="login" id="login">
        <div class="login-title" id="title">登录会员
            <span><a href="javascript:void(0)" id="closeBtn" class="close-login">关闭</a></span>
        </div>
        
        <div class="login-input-content">
            <div class="login-input">
                <label>用户名:</label>
                <input type="text" placeholder="请输入用户名" name="info[username]">
            </div>
            <div class="login-input">
                <label>登录密码:</label>
                <input type="text" placeholder="请输入登录密码" name="info[password]" id="password">
            </div>
        </div>

        <div class="login-button" id="loginBtn"><a href="javascript:;" id="login-button-submit">登录会员</a></div>
    </div>
    <!-- 遮罩层 -->
    <div id="bg" class ="login-bg"></div>

js部分

<script>
        // 1.获取元素
        var login = document.querySelector('#login');
        var mask = document.querySelector('#bg');
        var link = document.querySelector('#link');
        var closeBtn = document.querySelector('#closeBtn');
        // 2.点击弹出层这个链接link，让mask和login显示出来
        link.addEventListener('click',function(){
            login.style.display = 'block';
            mask.style.display = 'block';
        })
        // 3.点击closeBtn 就隐藏mask和login
        closeBtn.addEventListener('click',function(){
            login.style.display = 'none';
            mask.style.display = 'none';
        })
        // 4.开始拖拽
        // （1）当我们鼠标按下，就获得鼠标在盒子内的坐标
        var title = document.querySelector('#title');
        title.addEventListener('mousedown',function(e){
            // 求鼠标在盒子的坐标（此时按下了）
            var x = e.pageX - login.offsetLeft;
            var y = e.pageY - login.offsetTop;
            // (2)当鼠标移动的时候，把鼠标在页面中的坐标，减去鼠标在盒子内的坐标就是摸态框的left和top值
            document.addEventListener('mousemove',move);
            function move(e){
                var left = e.pageX - x + 'px';
                var top = e.pageY - y + 'px';
                login.style.left = left;
                login.style.top = top;
            }
            // （3）鼠标弹起，就让鼠标移动事件移除
            document.addEventListener('mouseup',function(){
                document.removeEventListener('mousemove',move);
            });
        });
    </script>