let direction = 1;

​

function setup() {

  createCanvas(600,600); 

}

​

function draw() {

  background(220); 

  ellipse(x, height / 2, 20);

  x = x + direction;

  // if(x > width) {

  //   direction = -1;   

  //  }

  // if(x < 0) {

  //   direction = 1;

  // }

  if(x > width || x < 0) {

    direction = -direction;

  }

}

​