SOURCE

let x =51
let sx =5
let y =20
let sy=3
function setup(){
    createCanvas(400,200);
}
function draw(){
    background(255,250,255);
    ellipse(x,y,30,30)
    x+=sx;
    y+=sy;

    if ((x>width)||(x<0)){
       sx*=-1;
    }
    if(y>height||y<0){
        sy*=-1;
    }
}

<script src=“https://cdnjs.cloudflare.com/ajax/libs/p5.js/1.1.9/p5.min.js”></script>