​

function setup() {

  createCanvas(400, 400);

}

​

function draw() {

  background(255);

​

  rect(150, 150, 100, 100);

​

  if (on) {

    fill(0); //on is false

  } else {

    fill(255); //on is true

  }

  print(on);

}

​

function mousePressed() {

​

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

on = !on

  }

​

}

​

// try replacing 

​

//     if (on) {

//       on = false;

//     } else {

//       on = true;

//     }

​

// with 

// on = !on;