SOURCE

const btnStart = document.querySelector('.btn-start');
const btnStop = document.querySelector('.btn-stop');
const btnReset = document.querySelector('.btn-reset');
const domText = document.querySelector('.text');

let count = 0;

let timer = null;

btnStart.addEventListener('click', function() {
    timer = setInterval(function() {
        count++;
        domText.textContent = count.toString();
    }, 1000)
})

btnStop.addEventListener('click', function() {
    clearInterval(timer);
})

btnReset.addEventListener('click', function() {
    clearInterval(timer);
    count = 0;
    domText.textContent = count.toString();
})

<button class="btn-start">start</button>
<button class="btn-stop">stop</button>
<button class="btn-reset">reset</button>
<p class="text"></p>