小球运动代码与老师一样写，不知道为什么情况不一样

同学你好，代码修改如下：

代码：

​//小球类
function Ball(x, y) {
    this.x = x;
    this.y = y;
    this.r = 20;
    this.color = 'red';
    //小球x的增量和y的增量 
    //  this.dx = parseInt(Math.random() * 8 - 4);
    // this.dy = parseInt(Math.random() * 8 - 4);
    this.dx = parseInt(Math.random() * 8) - 4;
    this.dy = parseInt(Math.random() * 8) - 4;
    this.init();
    ballArr.push(this);

}
Ball.prototype.init = function() {
    this.dom = document.createElement('div');
    this.dom.className = 'ball';
    this.dom.style.width = this.r * 2 + 'px';
    this.dom.style.height = this.r * 2 + 'px';
    this.dom.style.left = this.x - this.r + 'px';
    this.dom.style.top = this.y - this.r + 'px';
    this.dom.style.backgroundColor = this.color;
    document.body.appendChild(this.dom);

};
Ball.prototype.update = function() {
    // this.x += 2;
    // this.y -= 2;
    // 把小球的增量赋给this.x和this.y;
    this.x += this.dx;
    this.y -= this.dy;
    this.dom.style.width = this.r * 2 + 'px';
    this.dom.style.height = this.r * 2 + 'px';
    this.dom.style.left = this.x - this.r + 'px';
    this.dom.style.top = this.y - this.r + 'px';
    // 更新的小球的颜色
    this.dom.style.backgroundColor = this.color;

};
var ballArr = [];
// new Ball(200, 200);
//视频中小球的坐标是200，600
new Ball(200, 600);
setInterval(function() {
    for (var i = 0; i < ballArr.length; i++) {
        ballArr[i].update();
    }
    // 计时器时间调快一点，例如：20
}, 20);
document.onmousemove = function(e) {
    var x = e.clientX;
    var y = e.clientY;
    new Ball(x, y);

};

同学自己试一试哦，祝学习愉快！