let x=40;
let y=30;
let d=30;
let a=30;
let b=10;
function setup(){
createCanvas(500,500);
noStroke();
}
function draw(){
background(10,110);
x+=a;
y+=b;
if(x>width+d/2||(x<d/1)){
a=-a;
}
if(y>height-d/1||(y<d/3)){
b=-b;
}
ellipse(x,y,d,d);
}
<script src="https://cdnjs.cloudflare.com/ajax/libs/p5.js/1.1.9/p5.min.js"></script>