let y = [];

let trans = [];

​

function setup() {

  createCanvas(400, 400);

  noStroke();

}

​

function draw() {

  background(0);

  for (let i = 0; i < x.length; i++) {

    fill(255, 255, 255, trans[i]);

    ellipse(x[i], y[i], 20, 20);

    trans[i]--;

  }

}

​

function mousePressed() {

  x.push(mouseX);

  y.push(mouseY);

  trans.push(255);

}