SOURCE

const canvas = document.querySelector('canvas');
const ctx = canvas.getContext('2d');

const width = canvas.width = window.innerWidth;
const height = canvas.height = window.innerHeight;

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

// 生成随机颜色
function randomColor() {
    return `rgb(${random(0, 255)},${random(0, 255)},${random(0, 255)})`;
}

// 为程序中的小球建立模型
function Ball(x, y, velX, velY, color, size) {
    this.x = x;
    this.y = y;
    this.velX = velX;
    this.velY = velY;
    this.color = color;
    this.size = size;
}

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

// 更新小球数据
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;
}

// 碰撞检测
Ball.prototype.collisionDetect = function() {
    for (let j = 0; j < balls.length; j++) {
        if(balls[j] !== this) {
            const dx = this.x - balls[j].x;
            const dy = this.y - balls[j].y;
            const distance = Math.sqrt(dx * dx + dy * dy);
            if(distance < this.size + balls[j].size) {
                balls[j].color = this.color = randomColor();
            }
        }
    }
}

// 生成小球
let balls = [];
while(balls.length < 25) {
    let size = random(10, 20);
    let ball = new Ball(
        random(0 + size, width - size),
        random(0 + size, height - size),
        random(-10, 10),
        random(-10, 10),
        randomColor(),
        size
    )
    balls.push(ball);
}

// 让小球动起来
function loop() {
    ctx.fillStyle = 'rgba(0, 0, 0, 0.25)';
    ctx.fillRect(0, 0, width, height);
    for(let i = 0; i < balls.length; i++) {
        balls[i].draw();
        balls[i].update();  
        balls[i].collisionDetect();
    }
    requestAnimationFrame(loop);
}
loop();

<canvas></canvas>