JSFiddle - React, Tailwind, and code Playground

HTML

<span id="timeLeft"></span>

JavaScript

var timer = 60;

function startTimer(){
	timer = timer - 1;
  $("#timeLeft").html(timer);
}

var timerInterval;
$(window).focus(function() {
    if (!timerInterval)
        timerInterval = setInterval(startTimer, 1000);
});

$(window).blur(function() {
    clearInterval(timerInterval);
    timerInterval = 0;
});