// is remembered between calls to draw()

// can be used in setup() too if needed

let x = 0;

​

function setup() {

  createCanvas(400, 400);

  stroke(255);

  strokeWeight(5);

}

​

function draw() {

  background(0);

  line(x, 0, x, height);

  // move to the right for next time.

  x = x + 1;

  // wrap around to left side.

  if (x > width) {

    x = 0;

  }

}