let px, py;

​

function setup() {

  createCanvas(windowWidth, windowHeight);

​

  x = width / 2;

  y = height / 2;

  px = x;

  py = y;

​

  background(0);

}

​

function draw() {

​

  x += random(-50, 50);

  y += random(-50, 50);

​

  if (x > width) {

    px = x = 0;

  }

  if (x < 0) {

    px = x = width;

  }

  if (y > height) {

    py = y = 0;

  }

  if (y < 0) {

    py = y = height;

  }

​

  stroke(255);

  line(x, y, px, py);

​

  px = x;

  py = y;

}