网页动态背景，弹球

<!DOCTYPE html>
<html lang="zh">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>弹球</title>
  <style>
html, body {
  margin: 0;
}

html {
  font-family: sans-serif;
  height: 100%;
}

body {
  overflow: hidden;
  height: inherit;
}


  </style>
</head>
<body>
    <canvas></canvas>

  <script>

// 设定画布
const canvas = document.querySelector("canvas");
const ctx = canvas.getContext("2d");

// 设定画布长宽
const width = (canvas.width = window.innerWidth);
const height = (canvas.height = window.innerHeight);

// 生成随机数的函数
function random(min, max) {
  return Math.floor(Math.random() * (max - min)) + min;
}

// 生成随机颜色的函数
function randomColor() {
  return (
    "rgb(" +
    random(0, 255) +
    ", " +
    random(0, 255) +
    ", " +
    random(0, 255) +
    ")"
  );
}
// 求
function Shape(x, y, velX, velY, exists) {
  this.x = x;
  this.y = y;
  this.velX = velX;
  this.velY = velY;
  this.exists = exists;
}
function Ball(x, y, velX, velY, color, size, exists) {
  Shape.call(this, x, y, velX, velY, exists);
  this.color = color;
  this.size = size;
}
Ball.prototype = Object.create(Shape.prototype);
Ball.prototype.constructor = Ball;

Ball.prototype.draw = function() {
  ctx.beginPath();
  ctx.fillStyle = this.color;
  ctx.arc(this.x, this.y, this.size, 0, 2 * Math.PI);
  ctx.fill();
};

Ball.prototype.update = function() {
  if (this.x + this.size >= width) {
    this.velX = -this.velX;
  }

  if (this.x - this.size <= 0) {
    this.velX = -this.velX;
  }

  if (this.y + this.size >= height) {
    this.velY = -this.velY;
  }

  if (this.y - this.size <= 0) {
    this.velY = -this.velY;
  }

  this.x += this.velX;
  this.y += this.velY;
};

Ball.prototype.collisionDetect = function() {
  for (var j = 0; j < balls.length; j++) {
    if (!(this === balls[j])) {
      var dx = this.x - balls[j].x;
      var dy = this.y - balls[j].y;
      var distance = Math.sqrt(dx * dx + dy * dy);

      if (distance < this.size + balls[j].size) {
        balls[j].color = this.color = randomColor();
      }
    }
  }
};
// 恶魔圈
function EvilCircle(x, y, velX, velY, exists) {
  Shape.call(this, x, y, velX, velY, exists);
  this.color = "red";
  this.size = 10;
}
EvilCircle.prototype = Object.create(Shape.prototype);
EvilCircle.prototype.constructor = EvilCircle;

EvilCircle.prototype.draw = function() {
  console.log(this.color, this.x, this.y, this.size);
  ctx.beginPath();
  ctx.strokeStyle = this.color;
  ctx.arc(this.x, this.y, this.size, 0, 2 * Math.PI);
  ctx.stroke()();
};
EvilCircle.prototype.checkBounds = function() {
  if (this.x + this.size >= width) {
    this.velX = -this.velX;
  }

  if (this.x - this.size <= 0) {
    this.velX = -this.velX;
  }

  if (this.y + this.size >= height) {
    this.velY = -this.velY;
  }

  if (this.y - this.size <= 0) {
    this.velY = -this.velY;
  }
};

var evilCircle = new EvilCircle(
  random(0, width),
  random(0, height),
  random(-7, 7),
  random(-7, 7)
);

var balls = [];

function loop() {
  // ctx.fillStyle = "rgba(0, 0, 0, 0.25)";

  ctx.fillRect(0, 0, width, height);

  while (balls.length < 20) {
    var ball = new Ball(
      random(0, width),
      random(0, height),
      random(-7, 7),
      random(-7, 7),
      randomColor(),
      random(10, 20)
    );
    balls.push(ball);
  }
  for (var i = 0; i < balls.length; i++) {
    balls[i].draw();
    balls[i].update();
    balls[i].collisionDetect();
  }

  requestAnimationFrame(loop);
}
loop();


  </script>
</body>
</html>