SOURCE

let x=40;
let y=70;
let d=70;
let a=15;
let b=20;
function setup(){
    createCanvas(500,500);
    noStroke();
    colorMode(HSB,250,100,200);
}
function draw(){
    x+=a;
    y+=b;
    fill(random(2),random(50),random(294));
    if(x>width+50||(x<d/2)){
        a=-a+random(-5,5);
    }
    if(y>height+50||(y<d/2)){
        b=-b+random(-50,50);
    }
    rect(x,y,d,d);
   
}

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