js拖动元素

<div class="dv"></div>

.dv {
    width:100px;
    height:100px;
    background-color:green;
    border-radius:50%;
    position:absolute;
}

//获取元素
    var dv = document.getElementsByClassName('dv')[0];
    var x = 0;
    var y = 0;
    var l = 0;
    var t = 0;
    var isDown = false;
    //鼠标按下事件
    dv.onmousedown = function(e) {
        //获取x坐标和y坐标
        x = e.clientX;
        y = e.clientY;

        //获取左部和顶部的偏移量
        l = dv.offsetLeft;
        t = dv.offsetTop;
        //开关打开
        isDown = true;
        //设置样式  
        dv.style.cursor = 'move';
    }
    //鼠标移动
    window.onmousemove = function(e) {
        if (isDown == false) {
            return;
        }
        //获取x和y
        var nx = e.clientX;
        var ny = e.clientY;
        //计算移动后的左偏移量和顶部的偏移量
        var nl = nx - (x - l);
        var nt = ny - (y - t);

        dv.style.left = nl + 'px';
        dv.style.top = nt + 'px';
    }
    //鼠标抬起事件
    dv.onmouseup = function() {
        //开关关闭
        isDown = false;
        dv.style.cursor = 'default';
    }