canvas画小球

<canvas id="can" width="600px" height="300px" style="border: 1px solid black"></canvas>

var can = document.getElementById("can")
if(can.getContext){
    var ctx = can.getContext("2d");
    var res;
    var mov = false;
    var ball = {
        x : 40,
        y : 40,
        ra : 20,
        vx : 2,
        vy : 2
    }

    //画小球
    function draw(){
        ctx.fillStyle = "blue";
        ctx.beginPath();
        // ctx.moveTo(ball.x + ball.ra,ball.y + ball.ra);
        ctx.arc(ball.x, ball.y, ball.ra,0,Math.PI*2);
        ctx.fill();
    }
    draw();

    //小球移动
    function move(){
        //ctx.clearRect(0,0,can.width,can.height);
        //画长尾
        ctx.fillStyle = "rgba(255,255,255,0.4)";
        ctx.fillRect(0,0,can.width,can.height);
        //限制小球出边界
        if(ball.x+ball.ra+ball.vx>can.width||ball.x-ball.ra<0){
            ball.vx = -ball.vx;
        }
        if(ball.y+ball.ra+ball.vy>can.height||ball.y-ball.ra<0){
            ball.vy = -ball.vy;
        }

        ball.x += ball.vx;
        ball.y += ball.vy;
        draw();
        res = window.requestAnimationFrame(move);
    }

    /*//鼠标移入开始动
    can.addEventListener("mouseover", function(){
        res = window.requestAnimationFrame(move);
    });

    //鼠标移出停止
    can.addEventListener("mouseout", function(){
        window.cancelAnimationFrame(res); //进行传参
    });*/

    //如果小球没有移动,那么会跟着鼠标移动
    can.addEventListener("mousemove", function(e){
        if(!mov){
            ctx.clearRect(0,0,can.width,can.height);
            ball.x = e.clientX;
            ball.y = e.clientY;
            draw();
        }
    });

    //点击小球,小球进行移动
    can.addEventListener("click", function(e){
        mov = true;
        if(mov){
            res = window.requestAnimationFrame(move);
        }
    });

    //鼠标离开画面,停止移动
    can.addEventListener("mouseout", function(){
        window.cancelAnimationFrame(res);
        mov = false;
    });

}else{
    alert("not support");
}