SOURCE

let x=60;
let y=5;
let d=50;
let a=50;
let b=50;
function setup(){
    createCanvas(500,500);
    noStroke();
}
function draw(){
    background(120);
    x+=a;
    y+=b;
    fill(0);
    if(x>width+d/2||(x<d/2)){
        a=-a;
    }
    if(y>height-d/2||(y<d/2)){
        b=-b;
    }
    ellipse(x,y,d,d);
   
}

<script src='https://cdn.jsdelivr.net/npm/p5@1.1.9/lib/p5.min.js'></script>