JS (canvas) 两个小球碰撞

<style media="screen">
           * {
               margin: 0;
               padding: 0;
           }
           canvas {
               box-shadow: 0 0 40px black;
               margin: 50px;
           }
</style>


 <canvas id="canvas" width="500" height="500">您的浏览器不支持canvas</canvas>

<script>
        var canvas = document.querySelector('canvas');
        var ctx = canvas.getContext('2d');

        var canvasW = canvas.width;
        var canvasH = canvas.height;

        //随机数函数
        function random(min,max) {
            return parseInt(Math.random()*(max - min) + min);
        }

        //构造函数
        function Ball(x, y, r, speedX, speedY) {
            this.r = r || random(10, 30);
            this.x = x || random(this.r, canvasW - this.r);
            this.y = y || random(this.r, canvasH - this.r);
            this.speedX = speedX || random(2, 5);
            this.speedY = speedY || random(2, 5);
            //draw方法
            this.draw = function() {
                ctx.beginPath();
                ctx.arc(this.x, this.y, this.r, 0, Math.PI*2, true);
                ctx.fillStyle = 'red';
                ctx.fill();
            }
            //move方法
            this.move = function() {
                this.x += this.speedX;
                this.y += this.speedY;
                if(this.x < this.r || this.x > canvasW - this.r) {
                    this.speedX = -this.speedX;
                }
                if(this.y < this.r || this.y > canvasH - this.r) {
                    this.speedY = -this.speedY;
                }
                this.draw();//调用draw方法
            }
        }

        var ball1 = new Ball();
        var ball2 = new Ball();
        ball1.draw();
        ball2.draw();

        // 碰撞函数
        function isCrash(obj1, obj2) {
            var x = obj1.x - obj2.x;
            var y = obj1.y - obj2.y;
            var distance = Math.sqrt(x*x + y*y);//开方函数
            if(distance < obj1.r + obj2.r) {//判断碰撞
                obj1.speedX = -obj1.speedX;
                obj1.speedY = -obj1.speedY;
                obj2.speedX = -obj2.speedX;
                obj2.speedY = -obj2.speedY;
            }
        }

        var frameNum = 0;
        var prevDate = new Date();

        function gameloop() {
            frameNum++;
            ctx.clearRect(0, 0, canvasW, canvasH);
            ball1.move();
            ball2.move();
            isCrash(ball1,ball2);
            window.requestAnimationFrame(gameloop);//跟setTimeout相似，根据帧率执行
        }
        gameloop();
</script>