fill transparent in p5.js code example

Example: p5 transparancy

function draw() {
  clear();
  background(200);
  squareColor = color(100, 50, 100);
  squareColor.setAlpha(128 + 128 * sin(millis() / 1000));
  fill(squareColor);
  rect(13, 13, width - 26, height - 26);
}