SOURCE

let x=50;
let y=50;
let a=3;
let b=5;
let d=50;
function setup(){
    createCanvas(500,500);
    noStroke(0);
}
function draw(){
    background(120);
    x+=a;
    fill(random(255));
    if((x>550)||(x<0)){
        a=-a;
    }
    y+=b;
    if((y>550)||(y<0)){
        b=-b;
    }
    ellipse(x,y,50,50);
}

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