let speedX= 5;

​

function setup() {

  createCanvas(400, 400);

}

​

function draw() {

  background(220);

  circleX=circleX+=speedX;

  circle(circleX,height/2,20);

  if(circleX>400){

    speedX=-5;

    circle(circleX,height/2,20);

  }if(circleX<0){

   speedX=5;

  }

}