let x=5;
let y=5;
let d=50;
let a=10;
let b=10;
function setup(){
createCanvas(500,300);
noStroke();
}
function draw(){
background(120);
x+=a;
y+=b;
if(x>500||(x<d/5)){
a=-a;
}
if(y>300||(y<d/5)){
b=-b;
}
fill(random(255))
ellipse(x,y,d,d);
}
<script src="https://cdnjs.cloudflare.com/ajax/libs/p5.js/1.1.9/p5.min.js"></script>