02. Timer

function timer() {
   let timer;
   let seconds = 0;
   let minutes = 0;
   let hours = 0;
   let isRunning = false;
   $('#start-timer').on('click', function (){if (!isRunning){timer = setInterval(step, 1000); isRunning=true;}});
   $('#stop-timer').on('click', function (){if (isRunning){clearInterval(timer); isRunning=false;}});

   function step() {
       seconds++;
       if (seconds > 59)
       {
           seconds = 0;
           minutes++;
       }
       if (minutes > 59)
       {
           minutes = 0;
           hours++;
       }
       $('#seconds')["0"].textContent = padZero(seconds);
       $('#minutes')["0"].textContent = padZero(minutes);
       $('#hours')["0"].textContent = padZero(hours);
   }
   function padZero(n) {if (n < 10) {return ("0" + n);} else {return n;}}
}