SOURCE

let x=50;
let y=50;
let d=60;
let a=25;
let b=25;
function setup(){
    createCanvas(550,500);
    noStroke();
}
function draw(){
    background(120,110);
    x+=a;
    y+=b;
    fill(random(10));
    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://cdnjs.cloudflare.com/ajax/libs/p5.js/1.1.9/p5.min.js"></script>