createCanvas(400, 400);

}

​

function draw() {

  background(220);

  let x = 0;

  while (x < width) {

    if (mouseX > x && mouseX < x + 20) {

      fill("red");

    } else {

      fill("white");

    }

    rect(x, 0, 40, height);

    x += 20; // t x to move to the next column

  }

}

​