let isOn = false;

​

// Keep track of whether I've left the room

let hasLeft = true;

​

function setup() {

  createCanvas(400, 400);

}

​

function draw() {

  background(220);

  // Check to see if I'm in the room

  if(mouseX > width/2) {

    // Check to see if I've left the room

    if(hasLeft) {

      // I've now officially not left the room - This take away my ability to toggle the light in the next frame.

      hasLeft = false;

      // Toggle the light switch

      isOn = !isOn;

    }

  } else {

    // I've now officially left the room - This re-enables my ability to toggle the light in the next frame.

    hasLeft = true;

  } 

  // Check to see if the light is on

  if(isOn) {

    fill('red');

  }

  else {

   fill('black');

  }

  // Draw the rectangle

  rect(width/2, 0, width/2, height);

}