* {
 padding: 0;
 margin: 0;
}
#box1 {
 width: 500px;
 height: 500px;
 background: #999;
 position: relative;
 left: 100px;
 top: 100px;
}
#box {
 width: 100px;
 height: 100px;
 background: #334;
 position: absolute;
 cursor: move;
}

<div id="box1">
<div id="box"></div>
</div>

(function () {
 var dragging = false
 var boxX, boxY, mouseX, mouseY, offsetX, offsetY
 var box = document.getElementById('box')
 var box1 = document.getElementById('box1')

 // 鼠標按下的動作
 box.onmousedown = down
 // 鼠標的移動動作
 document.onmousemove = move
 // 釋放鼠標的動作
 document.onmouseup = up

 // 鼠標按下后的函數,e為事件對象
 function down(e) {
 dragging = true

 // 獲取元素所在的坐標
 boxX = box.offsetLeft
 boxY = box.offsetTop

 // 獲取鼠標所在的坐標
 mouseX = parseInt(getMouseXY(e).x)
 mouseY = parseInt(getMouseXY(e).y)

 // 鼠標相對元素左和上邊緣的坐標
 offsetX = mouseX - boxX
 offsetY = mouseY - boxY
 }

 // 鼠標移動調用的函數
 function move(e){
 if (dragging) {
  // 獲取移動后的元素的坐標
  var x = getMouseXY(e).x - offsetX
  var y = getMouseXY(e).y - offsetY

  // 計算可移動位置的大小， 保證元素不會超過可移動范圍
  // 此處就是父元素的寬度減去子元素寬度
  var width = box1.clientWidth - box.offsetWidth
  var height = box1.clientHeight - box.offsetHeight

  // min方法保證不會超過右邊界，max保證不會超過左邊界
  x = Math.min(Math.max(0, x), width)
  y = Math.min(Math.max(0, y), height)

  // 磁性吸附部分
  if (x < RANGE) {x = 0}
  if (width - x < RANGE) {x = width}
  if (y < RANGE) {y = 0}
  if (height - y < RANGE) {y = height}

  // 給元素及時定位
  box.style.left = x + 'px'
  box.style.top = y + 'px'
 }
 }

 // 釋放鼠標的函數
 function up(e){
 dragging = false
 }

 // 函數用于獲取鼠標的位置
 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
 }
 }
})()

// 磁性吸附部分
if (x < RANGE) {x = 0}
if (width - x < RANGE) {x = width}
if (y < RANGE) {y = 0}
if (height - y < RANGE) {y = height}