createCanvas(400, 400);

  rectMode(CORNER);

}

​

function draw() {

  background(220);

  if (mouseX >= 150 && mouseX <= 150+100 && mouseY >= 150 && mouseY <= 150+100) {

  //if we've rolled over do something

    fill(0);

   } else {

        fill(255);

   }

    rect(150,150,100,100);

}