main.js

var start = document.querySelector('.start');
var pause = document.querySelector('.pause');
var reset = document.querySelector('.reset');

var minRef = document.querySelector('.min');
var secRef = document.querySelector('.sec');

var state = {
    min: '00',
    sec: '00'
};
var id;
setTime(state);
function computeNextTime(state) {
    var min = state.min;
    var sec = state.sec;

    if (sec === 59) {
        sec = '00';
        min++;
    }
    else {
        sec++;
    }

    return {
        min: min,
        sec: sec
    };
}

function setTime(state) {
    var min = state.min;
    var sec = state.sec;
    minRef.textContent = min;
    secRef.textContent = sec;
}

function handleStart() {

    if(id==undefined)
    id=setInterval(function () {
        var newState = computeNextTime(state);
        state = newState;
        setTime(state);
    }, 1000);
}

function handlePause() {
    if(id===undefined)
    {
        return;
    }
    clearInterval(id);
    id=undefined;
}


function handleReset() {
    clearInterval(id);
    state={
        min: '00',
        sec: '00'
    };
    setTime(state);
    id=undefined;
}

start.addEventListener('click', handleStart);
pause.addEventListener('click', handlePause);
reset.addEventListener('click', handleReset);