SOURCE

let x=51;
let y=50;
let A=5
let B=7;
let d=50;
function setup(){
    createCanvas(500,500);
}
function draw(){
    background(120,10);
    fill(random(255));
    x+=A;
    y+=B;
    if((x>500-d/2)||(x<d/2)){
        A=-A;
    }
    if((y>500-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><script src='https://cdn.jsdelivr.net/npm/p5@1.1.9/lib/p5.min.js'></script>