let xSpeed=5;

​

function setup() {

  createCanvas(400, 400);

  noStroke();

}

​

function draw() {

  background(255);

  fill(250,100,10);

  ellipse(xPosition,width/2,100,100);

  xPosition += xSpeed;

  if(xPosition > width || xPosition <0){

   xSpeed *= -1; 

​

  }

}