SOURCE

let x=150;
let y=50;
let d=50;
let a=50;
let b=50;
function setup(){
    createCanvas(500,500);
    noStroke();
    colorMode(HSB,5,818,124);
}
function draw(){
    x+=a;
    y+=b;
    fill(random(2),random(85),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://cdn.jsdelivr.net/npm/p5@1.1.9/lib/p5.min.js'></script>