let vy=1.5;

let ballx=300;

let bally = 300; 

​

function setup() {

  createCanvas(600, 600);

}

​

function draw() {

  background(220, 220, 220, 30);

​

  noStroke();

  fill(0);

  ellipse(ballx, bally, 30, 30);

​

  ballx += vx;

  bally += vy;

​

  if ((ballx-10) < 0 || (ballx+10) > width){

    vx *= -1;

  } if ((bally-10) < 0 || (bally+10) > height){

    vy *= -1;

  }

}