JSFiddle - React, Tailwind, and code Playground

HTML

<p id="result">
  0
</p>
<button>toggle count</button>

JavaScript

var count = 0,
  counting = false;

function add() {
  if (counting) {
    count = count + 1;
    document.getElementById('result').innerHTML = count;
    setTimeout(add, 1000);
  }
}

document.getElementsByTagName('button')[0].addEventListener('click', function() {
  counting = !counting;
  add();
})