​

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

    if (this.x > width || this.x < 0) {

      this.vx *= -1

    }

  }

}

​

function setup() {

  createCanvas(400, 400);

}

​

function draw() {

  background(220)

  if (mouseIsPressed) {

    particle = new Particle(mouseX, mouseY)

    listOfParticles.push(particle)

  }

  for (let particle of listOfParticles) {

    particle.draw()

    particle.update()

  }

}