function setup() {

  createCanvas(canW, canH);

}

​

let x=40,y=200, radius = 25;

let incX = 4, incY = 5;

function changeDir() {

  if(x >= canW - 8 || x <= 0 + 8) {// we added 8 because ball was rebounding after going some pixels deep inside the boundary walls

    incX *= (-1);

  } else if(y >= canH - 8 || y <= 0 + 8) {

    incY *= (-1);

  }

}

​

function draw() {

  background(0);

  strokeWeight(0);

  fill(200,100,200);

  x += incX;

  y += incY;

  changeDir();

  circle(x,y,radius);

}