02. Timer

function timer() {
        let tick = 0;
        let clockOn = false;
        let myInterval = null;


        function step() {
            tick++;
            let seconds = ('0' + tick % 60).slice(-2);
            $('#seconds').text(seconds);
            let minutes = ('0' + Math.floor(tick/ 60)).slice(-2);
            if(minutes>=60) {
                minutes-=60;
                minutes=('0'+minutes).slice(-2);
            }
            $('#minutes').text(minutes);
            let hours = ('0' + Math.floor(tick / 60/60)).slice(-2);
            $('#hours').text(hours);
        }

        function myStopFunction() {
            clearInterval(myInterval);
            clockOn=false;
        }


            $('#start-timer').on('click', function () {
                if (!clockOn) {
                    myInterval = setInterval(step, 1000);
                }
                clockOn = true;
            });



        $('#stop-timer').on('click', myStopFunction);
    }