let hover = true; //mouse to hover over rectangle

​

function setup() {

  createCanvas(400, 400);

}

​

function draw() {

  background(220);

  //condition for mouse to be over rectangle

  if(mouseX>width/2){

    if(hover){

      hover = false;

      isOn = !isOn;

    } 

  }  else {

      hover = true;

    }

  if (isOn){

    fill ('red')

  } else {

    fill ('black')

  }

  rect(width/2,0,width/2,height)

}