JavaScript实现飘落星星动画特效

<!DOCTYPE html> 
<html lang="en"> 
<head> 
 <meta charset="UTF-8"> 
 <title>Title</title> 
 <style> 
 img{ 
  position: absolute; 
 } 
 body { 
 
  background-image: url(img/bg.jpg); 
  background-size: 100%; 
 
 } 
 </style> 
 
 <script> 
 
 function Star() { 
 
 
  this.speed=10; 
  this.img=new Image(); 
  this.img.src="img/star"+parseInt(Math.random()*4+1)+".png"; 
  this.img.style.width=50+'px'; 
  this.img.style.height=50+'px'; 
  this.img.style.top=Math.random()*window.innerHeight+1+'px'; 
  this.img.style.left=Math.random()*window.innerWidth+1+'px'; 
  document.body.appendChild(this.img); 
 } 
 
 Star.prototype.slip=function () { 
 
  var that=this; 
  function move() { 
  that.img.style.top=that.img.offsetTop+that.speed+'px'; 
  console.log(that.img.offsetTop+"star"); 
  console.log(window.innerHeight+"window"); 
  if(that.img.offsetTop>window.innerHeight){ 
   clearInterval(sh); 
   that.img.style.height=0; 
   that.img.style.width=0; 
  } 
  } 
  var sh=setInterval(move,100); 
 } 
 
  setInterval(function () { 
  for(var i=1;i<5;i++){ 
  new Star().slip(); 
  } 
  },1000) 
 
 </script> 
 
</head> 
 
<body> 
 
 
</body> 
</html>