JavaScript Canvas 实现自由落体与反弹动画

<!DOCTYPE html>
<html lang="en">
<head>
 <meta charset="UTF-8">
 <title>Title</title>
 <style>
  body {
   padding: 0;
   margin: 0;
   background-color: rgba(0, 0, 0, 1);
  }
  #canvas{
   display: block;
   margin: 10px auto;
  }
 </style>
</head>
<body>
<canvas id="canvas" width="600" height="600">your browser is not support canvas</canvas>
<script type="text/javascript">
 //自由落体 H=(gt^2)/2 动能 Ek=(mv^2)/2 重力势能:Ep = mgh
 let x=300,y=60,     //圆心坐标
  minHeight = 60,    //最小下落位移
  maxHeight = 446,   //最大下落位移
  dir = true;     //dir true下落，false为弹起
 (function(){
  let canvas= document.getElementById('canvas');
  let ctx = canvas.getContext('2d');
  draw(ctx);
 })();
 function draw(ctx){
  let currentTime = new Date().getTime();  //开始毫秒数,折返记录一次currentTime
  let arr_y = [];        //设置下落位移的数组
  window.requestAnimationFrame(function init(){
   if(dir){
    if(y >= maxHeight){
     dir = false;
     currentTime = new Date().getTime();
    }else{
     y = y + Math.pow((new Date().getTime() - currentTime)/1000,2)*10/2;
     arr_y.push(y);
    }
   }else{
    if(y <= minHeight){
     dir = true;
     currentTime = new Date().getTime();
    }else{
     y = arr_y.splice(-1,1)[0] || 60;
    }
   }
   drawArc(ctx,x,y);
   requestAnimationFrame(init);
  });
 }
 //绘制圆球和地面
 function drawArc(ctx,x,y){
  ctx.clearRect(0, 0, 600, 600);
  ctx.beginPath();
  ctx.arc(x,y,50,0,Math.PI*2);
  ctx.fillStyle='#98adf0';
  ctx.fill();
  ctx.save();
  ctx.beginPath();
  ctx.strokeStyle = '#ffffff';
  ctx.moveTo(0,500);
  ctx.lineTo(600,500);
  ctx.lineWidth = 4;
  ctx.stroke();
  ctx.closePath();
  ctx.save();
 }
</script>
</body>
</html>