let y = 50;

 let speed = 1;

​

 function setup() {

   createCanvas(400, 400);

 }

​

 function draw() {

​

   background(255);

   ellipse(x, y, 100, 100);

​

   x = x + speed;

   y = y + speed;

​

   if (x > width - 50) {

     speed = -1;

   } else if (x == 50) {

     speed = 1;

   }

 }