​

class Particle {

  constructor(x, y) {

    this.x = x;

    this.y = y;

    this.vx = random(-1, 1);

    this.vy = random(-1, 1);

    this.startedAt = millis();

    this.lifetime = 3000;

  }

  draw() {

    let transparency = map(

      millis(),

      this.startedAt, this.startedAt + this.lifetime, 255, 0

    );

    fill(255, transparency);

    ellipse(this.x, this.y, 10);

  }

  update() {

    this.x = this.x + this.vx;

    this.y = this.y + this.vy;

  }

}

​

function setup() {

  createCanvas(400, 400);

  for (let i = 0; i < 10; i++) {

    particle = new Particle(random(width), random(height));

    listOfParticles.push(particle);

  }

}

​

function draw() {

  background(220);

  if (mouseIsPressed) {

    particle = new Particle(mouseX, mouseY);

    listOfParticles.push(particle);

  } 

  for (let particle of listOfParticles) {

    particle.draw();

    particle.update();

  }

}