js实现简易拖拽

<div id="box" style="background: #334;width: 100px;height: 100px;position: absolute;cursor: move;"></div>

(function () {
    var dragging = false
    var boxX, boxY, mouseX, mouseY, offsetX, offsetY
    var box = document.getElementById('box')

    // 鼠标按下的动作
    box.onmousedown = down
    // 鼠标的移动动作
    document.onmousemove = move
    // 释放鼠标的动作
    document.onmouseup = up

    // 鼠标按下后的函数,e为事件对象
    function down(e) {
        dragging = true

        // 获取元素所在的坐标
        boxX = box.offsetLeft
        boxY = box.offsetTop

        // 获取鼠标所在的坐标
        mouseX = parseInt(getMouseXY(e).x)
        mouseY = parseInt(getMouseXY(e).y)

        // 鼠标相对元素左和上边缘的坐标
        offsetX = mouseX - boxX
        offsetY = mouseY - boxY
    }

    // 鼠标移动调用的函数
    function move(e){
        if (dragging) {
            // 获取移动后的元素的坐标
            var x = getMouseXY(e).x - offsetX
            var y = getMouseXY(e).y - offsetY

            // 计算可移动位置的大小， 保证元素不会超过可移动范围
            var width = document.documentElement.clientWidth - box.offsetWidth
            var height = document.documentElement.clientHeight - box.offsetHeight

            // min方法保证不会超过右边界，max保证不会超过左边界
            x = Math.min(Math.max(0, x), width)
            y = Math.min(Math.max(0, y), height)

            // 给元素及时定位
            box.style.left = x + 'px'
            box.style.top = y + 'px'
        }
    }

    // 释放鼠标的函数
    function up(e){
        dragging = false
    }

    // 函数用于获取鼠标的位置
    function getMouseXY(e){
        var x = 0, y = 0
        e = e || window.event

        if (e.pageX) {
            x = e.pageX
            y = e.pageY
        } else {
            x = e.clientX + document.body.scrollLeft - document.body.clientLeft
            y = e.clientY + document.body.scrollTop - document.body.clientTop
        }

        return {
            x: x,
            y: y
        }
    }
})()

// 鼠标按下的动作
box.onmousedown = down
// 鼠标的移动动作
document.onmousemove = move
// 释放鼠标的动作
document.onmouseup = up

// 获取元素所在的坐标
boxX = box.offsetLeft
boxY = box.offsetTop

if (e.pageX) {
    x = e.pageX
    y = e.pageY
} else {
    x = e.clientX + document.body.scrollLeft - document.body.clientLeft
    y = e.clientY + document.body.scrollTop - document.body.clientTop
}