JSFiddle - React, Tailwind, and code Playground

HTML

<input id="start" type="button" name="submit" value="Start" onclick="startPomodoro();">
<input id="shortbreak" type="button" name="submit" value="Short Break" style="display: none;" onclick="startShortBreak();">

<div id="tempo"></div>

JavaScript

var countPomodoro = new Number();
var minutesPomodoro = 2;
var countPomodoro = 60;
var countBreak = new Number();
var minutesBreak = 5;
var countBreak = 60;

var tempo = document.getElementById('tempo');
//Ao clicar no botão start ele começa uma contagem
function startPomodoro() {
  document.getElementById('start').style.display = "none";
  if ((countPomodoro - 1) >= 0 && (minutesPomodoro) >= 0) {
    countPomodoro = countPomodoro - 1;
    if (countPomodoro == 0) {
      minutesPomodoro = minutesPomodoro - 1;
      if (minutesPomodoro == -1) {
        countPomodoro = 0;
        minutesPomodoro = 0;
      }     else {
        countPomodoro = 59;
      }
    }
    if (countPomodoro < 10 && minutesPomodoro < 10) {
      tempo.innerText = "0" + minutesPomodoro + ":" + "0" + countPomodoro;
    } else if (countPomodoro < 10) {
      tempo.innerText = minutesPomodoro + ":" + "0" + countPomodoro;
    } else if (minutesPomodoro < 10) {
      tempo.innerText = "0" + minutesPomodoro + ":" + countPomodoro;
    } else {
      tempo.innerText = minutesPomodoro + ":" + countPomodoro;
    }
    setTimeout('startPomodoro();', 1);
		
    // Para mostrar, quando os segundos e o minutos estiverem zerados
    if (minutesPomodoro == 0 && countPomodoro == 0) {
    	document.getElementById('shortbreak').style.display = "block";
      countBreak = new Number();
      minutesBreak = 5;
      countBreak = 60;
    }

  }
}

//Ao clicar no botão Short Break começa uma contagem
function startShortBreak() {
  document.getElementById('shortbreak').style.display = "none";
  if ((countBreak - 1) >= 0 && (minutesBreak) >= 0) {
    countBreak = countBreak - 1;
    if (countBreak == 0) {
      minutesBreak = minutesBreak - 1;
      if (minutesBreak == -1) { // Para realizar a contagem, quando for no 00 minutos.
        countBreak = 0;
        minutesBreak = 0;
      } else {
       	countBreak = 59;
      }
    }
    if (countBreak < 10 && minutesBreak < 10) {
      tempo.innerText = "0" +...