JavaScript 实现简单的碰撞检测效果

<head>
 <meta charset="UTF-8">
 <title></title>
 <style type="text/css">
 #all{
  width: 500px;
  height: 500px;
  border: 2px solid sandybrown;
  position: relative;
  margin: 0 auto;
 }
 #div1{
  width: 50px;
  height: 50px;
  background-color: red;
  position: absolute;
 }
 #center{
  width: 150px;
  height: 150px;
  background-color: black;
  position: absolute;
  margin: 175px;
 }
 </style>
</head>
<body>
 <div id="all">
 <div id="div1"></div>
 <div id="center"></div>
 </div>
 <script type="text/javascript">
 var oAll = document.getElementById("all");
 var oDiv1 = document.getElementById("div1");
 var oCenter = document.getElementById("center");
 
 var maxL = oAll.clientWidth - oDiv1.clientWidth;
 var maxT = oAll.clientHeight - oDiv1.clientHeight;
 
 oDiv1.onmousedown = function(){
  var ev = ev || window.event;
  var lessX = ev.clientX - oDiv1.offsetLeft;
  var lessY = ev.clientY - oDiv1.offsetTop;
  document.onmousemove = function(){
  var ev = ev || window.event;
  var posL = ev.clientX - lessX;
  var posT = ev.clientY - lessY;
  
  if(oCenter.offsetLeft-oDiv1.offsetWidth<posL && posL<325 && oCenter.offsetLeft-oDiv1.offsetWidth<posT && posT<325 ){
   oCenter.style.backgroundColor = "red"
  }else{
   oCenter.style.backgroundColor = "black"
  }
  if(posL<0){
   posL = 0;
  }
  if(posT<0){
   posT = 0;
  }
  if(posL>maxL){
   posL = maxL;
  }
  if(posT>maxT){
   posT = maxT;
  }
  oDiv1.style.left = posL + "px";
  oDiv1.style.top = posT + "px";
  }
 }
 document.onmouseup = function(){
  document.onmousemove = null;
  oDiv1.onmousemove = null;
 }
 </script>
</body>