main.js

// setup canvas

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

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

// function to generate random number

function random(min, max) {
  const num = Math.floor(Math.random() * (max - min + 1)) + min
  return num
}

function Ball(x, y, velx, vely, size, color) {
  this.x = x
  this.y = y
  this.velx = velx
  this.vely = vely
  this.color = color
  this.size = size

}

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 (let j = 0; j < balls.length; j++) {
    if (!(this === balls[j])) {
      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 = 'rgb(' + random(0, 255) + ',' + random(0, 255) + ',' + random(0, 255) +')'
      }
    }
  }
}

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(-7,7),
    random(-7,7),
    size,
    'rgb(' + random(0,255) + ',' + random(0,255) + ',' + random(0,255) +')'
  )

  balls.push(ball);
}

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()