function setup() {
  createCanvas(200, 200);
}

function draw() {
  background(0);
  noFill();
  stroke(255);
  strokeWeight(1);
  beginShape(LINES);
  for (let i = 0; i < 100; i = i + 10) {
    vertex(50, 50 + i);
    vertex(50 + i, 150);
    vertex(150, 150 - i);
    vertex(150 - i, 50);
  }
  endShape(CLOSE);
}