let y = 0;

let incDec = true;

let mattincDec = true;

function setup() {

  createCanvas(400, 400);

}

function draw() {

  background(220);

​

  //

​

  if (incDec == true) {

    x+=2;

  }

  if (incDec == false) {

    x-=2;

  }

  if (x == 400) {

    incDec = false;

  }

  if (x == 0) {

    incDec = true;

  }

  rect(x, y, 50);

​

  //test if x is greater than width or less than 0

}

​