let count = 0;

let timerP;

let interval = {

  isActive: false,

  id: -1,

  start(callback){

    this.id = setInterval(callback, 500);

    callback();

    this.isActive = true;

  },

  stop(){

    clearInterval(this.id);

    this.id = -1;

    this.isActive = false;

  }

};

​

function setup() {

  createCanvas(200, 200);

  timerP = select('.timer');

  timerP.parent(createDiv());

}

​

function mousePressed(){

  if(interval.isActive) {

    interval.stop();

  } else {

    interval.start(function(){

      timerP.html(count++);

    });

  }

}

​

function draw() {

  background(0);

  stroke(255);

  line(x, 0, x, height);

  x += 3;

  if (x > width) {

    x = 0;

  }

}