JavaScript结合Canvas绘画画运动小球

<canvas id="canvas" width="400" height="400"></canvas>
<script>
    let canvas = document.getElementById('canvas');
    let ctx = canvas.getContext('2d');
    let maxWidth = canvas.width,
        maxHeight = canvas.height;
        ctx.fillStyle = '#000';
        ctx.fillRect(0, 0, maxWidth, maxHeight);
</script>

function getRandomNum(minNum, maxNum) {
    switch (arguments.length) {
        case 1:
            return Math.round(Math.random() * minNum + minNum);
            break;
        case 2:
            return Math.round(
                Math.random() * (maxNum - minNum) + minNum);
            break;
        case 0:
            return 0;
            break;
    }
}
// 创建一个Ball的构造函数
    function Ball(maxWidth, maxHeight, ctx) {
        this.ctx = ctx;
        this.maxWidth = maxWidth;
        this.maxHeight = maxHeight;
        // 随机半径
        this.r = getRandomNum(5, 15);
        // 随机x,y坐标
        this.x = getRandomNum(this.r, this.maxWidth - this.r);
        this.y = getRandomNum(this.r, this.maxHeight - this.r);
        // 平移速度,正负区间是为了移动方向多样
        this.speedX = getRandomNum(-2, 2);
        this.speedY = getRandomNum(-2, 2);
        // 颜色随机
        this.color = `rgba(${getRandomNum(0, 255)},
        ${getRandomNum(0, 255)},
        ${getRandomNum(0, 255)},${Math.random()})`;
    }
    Ball.prototype = {
        draw: function () {
            ctx.beginPath();
            ctx.fillStyle = this.color;
            ctx.arc(this.x, this.y, this.r, 0, Math.PI * 2);
            ctx.fill();
            ctx.closePath();
        },
        move: function () {
            // 判断边界值,让圆球始终保证在画面内
            if (this.x > this.maxWidth - this.r || this.x < this.r) {
                this.speedX = -this.speedX;
            }
            if (this.y > this.maxHeight - this.r || this.y < this.r) {
                this.speedY = -this.speedY;
            }
            this.x += this.speedX;
            this.y += this.speedY;
        }
    };
    // 创建100个Ball实例
    let balls = [];
    for (let i = 0; i < 100; i++) {
        let newBall = new Ball(maxWidth, maxHeight, ctx);
        newBall.draw();
        balls.push(newBall);
    }

setInterval(() => {
    // 每次画之前都要清除画布
    ctx.clearRect(0, 0, maxWidth, maxHeight);
    ctx.fillStyle = '#000';
    ctx.fillRect(0, 0, maxWidth, maxHeight);
    for (let j = 0; j < 100; j++) {
        balls[j].draw(ctx);
        balls[j].move();
    }
}, 100);