实现拖拽一个元素
拖拽的流程：
当鼠标在被拖拽元素上按下时，开始拖拽 onmousedown
当鼠标移动时被拖拽元素跟随鼠标移动 onmousemove
当鼠标松开时，被拖拽元素固定在当前位置 onmouseup
<style>
#box1{
width: 100px;
height: 100px;
background-color: red;
position: absolute;
}
</style>
<body>
<div id="box1"></div>
</body>
</html>
<script type="text/javascript">
window.onload = function(){
var box1 = document.getElementById("box1");
//为box1绑定一个鼠标按下事件,当鼠标在被拖拽元素上按下时，开始拖拽 onmousedown
box1.onmousedown = function(event){
event = event || window.event;
//div的偏移量 鼠标.clentX - 元素.offsetLeft
//div的偏移量 鼠标.clentY - 元素.offsetTop
var ol = event.clientX - box1.offsetLeft;
var ot = event.clientY - box1.offsetTop; //为document绑定一个onmousemove事件
document.onmousemove = function(event){
event = event || window.event;
//当鼠标移动时被拖拽元素跟随鼠标移动 onmousemove
//获取鼠标的坐标
var left = event.clientX - ol;
var top = event.clientY - ot; //修改box1的位置
box1.style.left = left+"px";
box1.style.top = top+"px";
}; //为document绑定一个鼠标松开事件
document.onmouseup = function(){
//当鼠标松开时，被拖拽元素固定在当前位置 onmouseup
//取消document的onmousemove事件
document.onmousemove = null;
//取消document的onmouseup事件
document.onmouseup = null;
};
};
};
</script>