<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
}