setInterval(
function(){//函数表示每一帧干什么
render();
update();
},50//时间每隔多长时间 毫秒
);
<!DOCTYPE html>
<html>
<head>
<title></title>
</head>
<body>
<canvas id="canvas" style="display: block;margin: 50px auto;">
</canvas>
</body>
</html>
<script type="text/javascript">
var ball = {x:512,y:100,r:20,g:2,vx:-4,vy:0,color:'#005588'};
window.onload=function(){
var canvas = document.getElementById('canvas');
var context = canvas.getContext('2d');
canvas.width = 1024;
canvas.height = 768;
setInterval(
function(){//函数表示每一帧干什么
render(context);
update();
},50//时间每隔多长时间
);
}
function update(){
ball.x += ball.vx;
ball.y += ball.vy;
ball.vy += ball.g;
if(ball.y>=768-ball.r){
ball.y = 768-ball.r;
ball.vy = -ball.vy*0.8;
}
}
function render(cxt){
cxt.clearRect(0,0,cxt.canvas.width,cxt.canvas.height);
cxt.fillStyle = ball.color;
cxt.beginPath();
cxt.arc (ball.x,ball.y,ball.r,0,2*Math.PI);
cxt.closePath();
cxt.fill();
}
</script>