原生JavaScript实现简单的DOM元素碰撞检测与反弹

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Title</title>
  <style>
    .box1,.box2{
      width: 100px;
      height: 100px;
      background-color: #f00;
      position:absolute;
    }
    .box2{
      background-color: #00f;
      left: 200px;
      top: 200px;
    }
    
  </style>
</head>
<body>
  <div class="box1"></div>
  <div class="box2"></div>
</body>
<script>
  var box1=document.querySelector(".box1");
  var box2=document.querySelector(".box2");
  box1.addEventListener("mousedown",mouseHandler);
  function mouseHandler(e){
    if(e.type==="mousedown"){
      e.preventDefault();
      document.elem=this;
      document.pointX= e.offsetX;
      document.pointY= e.offsetY;
      document.addEventListener("mousemove",mouseHandler);
      this.addEventListener("mouseup",mouseHandler);
    }else if(e.type==="mousemove"){
      this.elem.style.left= e.x-this.pointX+"px";
      this.elem.style.top= e.y-this.pointY+"px";
      hitText(this.elem,box2);
    }else if(e.type==="mouseup"){
      document.removeEventListener("mousemove",mouseHandler);
      this.removeEventListener("mouseup",mouseHandler);
    }
  }
  function hitText(elem1,elem2){
    var rect1=elem1.getBoundingClientRect();
    var rect2=elem2.getBoundingClientRect();
    var ponit1={x:rect1.x,y:rect1.y};
    var ponit4={x:rect1.x+rect1.width,y:rect1.y+rect1.height};
    if(
      ponit4.x>rect2.x
        &&ponit1.x<(rect2.x+rect2.width)
        &&ponit4.y>rect2.y
        &&ponit1.y<(rect2.y+rect2.height)){
      elem2.style.backgroundColor=randomColor();
      elem2.style.left=Math.round(Math.random()*document.documentElement.clientWidth)+"px";
      elem2.style.top=Math.round(Math.random()*document.documentElement.clientHeight)+"px";
    }
  }
  function randomColor(){
    var a=Math.round(Math.random()*255);
    var b=Math.round(Math.random()*255);
    var c=Math.round(Math.random()*255);
    var color="rgb("+ a+","+b+","+c+")";
    return color;
  }
</script>
</html>