let x=140;
let d=50;
let y=140;
let a=3;
let b=2;
function setup(){
colorMode(HSB,456,10,30);
createCanvas(500,500);
noStroke(0);
}
function draw(){
fill(random(305),120,110);
//background(120);
x+=a;
if((x>454)||(x<26)){
a=-a+random(-1,1)
}
y+=b;
if((y>474)||(y<26)){
b=-b+random(-1,1)
}
ellipse(x,y,d,d);
}
<script src="https://cdnjs.cloudflare.com/ajax/libs/p5.js/1.1.9/p5.min.js"></script>