let x, y;

// previous x, y

let px, py;

​

function setup() {

  createCanvas(windowWidth, windowHeight);

  background(255);

​

  // initialize variables

  x = 0;

  y = height / 2;

  px = x;

  py = y;

}

​

function draw() {

  // move x position by 10 pixels to the right every frame

  x += 10; // same as x = x + 10

​

  y = random(-height / 4, height / 4) + height / 2;

​

  line(px, py, x, y);

​

  // set previous to current

  px = x;

  py = y;

}