let xSpeed = 4;

​

function setup() {

  createCanvas(400, 400);

}

​

function draw() {

  background(255);

  translate(xPos, 0);

  xPos = xPos + xSpeed;

  if(xPos >= 250 || xPos <= -100) {

   xSpeed = -xSpeed; 

  }

  //grid(10);  // This line will not work on your sketch!

  fill('yellow');

  rect(50, 150, 100, 160);

  fill('red');

  rect(100, 200, 50, 80);

  fill('blue');

  ellipse(125, 240, 50, 50);

}

​

function grid(inc) {

  for (let x = 0; x < width; x+=inc) {

    line(x, 0, x, height);

  }

  for (let y = 0; y < height; y+=inc) {

    line(0, y, width, y);

  }

}