createCanvas(400, 400);

  background(50);

}

​

function draw() {

  // note that the mouse should move in Preview, not the text editing area.

  if (checkOutofCanvas(mouseX, mouseY) == true) {

    // if mouse position is out of the canvas,

    background(50);

  } else {

    // if mouse position is in the canvas,

    background(255, 255, 0);

  }

}

​

function checkOutofCanvas(x, y) {

  let value = false;

  if (x < 0 || x > width) {

    value = true;

  }

  if (y < 0 || y > height) {

    value = true;

  }

  return value;

}