var increment = 20

​

function setup() {

  createCanvas(400, 400);

}

​

function draw() {

  squaremov = squaremov + increment

  background('grey');

  square(175, squaremov, 50);

  fill('pink')

  square(squaremov, 175, 50);

  fill('yellow')

  strokeWeight(0)

  if (squaremov > 350){

    increment = -2

  }

  else if (squaremov < 0){

    increment = 20

  }

}