Pomodoro timer - counting

How to implement counting in JavaScript

Posted on Fri 17 February 2017

Welcome to today's blog post! This blog post will be about implementing countdown in JavaScript and also about some CSS work I have to do so my timer looks decent.

Core functionality of pomodoro timer

As the name suggests the core functionality of a timer is to count down time. In the case of this timer, I will be using 25 minutes as a timer that needs to be counted down. I decided that for the time being, I will have only two control buttons for the timer: start & restart.

Implementing timer in JavaScript

As I know what I want to accomplish the first thing is the look of my timer. I was wondering if it will be better to write some CSS from scratch and learn this language too but when I start doing that I realized that I can spend a whole week only on this task. Instead, I decided to use Material Design Lite. This is a collection of CSS and JavaScript that allows me to use Google Material Design. To get started all I need to do is include some code from google CDN:

  <link rel="stylesheet" href="https://fonts.googleapis.com/icon?family=Material+Icons">
  <link rel="stylesheet" href="https://code.getmdl.io/1.3.0/material.indigo-pink.min.css">
  <script defer src="https://code.getmdl.io/1.3.0/material.min.js"></script>

You may have noticed that script has defer attribute which means that this script will be executed after the document has been parsed. I also add my custom style.css:

.display__time-left {
  font-weight: 100;
  font-size: 20rem;
  margin: 0;
  color: black;
  flex: 1;
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;

.control_buttons {
  flex: 1;
  display: flex;
  justify-content: space-around;
  align-items: center;

Much of the code from style.css is based on Wes Bos code from here. In display__time-left I set up a few properties of the font that will be showing how many minutes and seconds are still in one pomodoro. I also made this element flex which fits element in its available space. .control_buttons are evenly spaced on the webpage with space between them by space-around. After loading a page it looks like this:

Basic layout

I am aware that this look needs a bit of work though. As I have my styles ready I add this HTML to the body:

<h1 class="display__time-left">25:00</h1>
<div class="control_buttons">
  <button class="mdl-button mdl-js-button mdl-button--raised mdl-js-ripple-effect mdl-button--accent" data-action="start">

  <button class="mdl-button mdl-js-button mdl-button--raised mdl-js-ripple-effect mdl-button--accent" data-action="stop">

<audio id="end_sound" src="sound.wav"></audio>

At the beginning, I show time left in pomodoro which by default is 25 minutes. Next, I have my control buttons with classes from Material Design Lite. At the end, there is an audio file which I will be playing at the end of each pomodoro.

How is the counting implemented? For this you need to look into script.js:

let countdown;
const timerDisplay = document.querySelector('.display__time-left');
const startTimeBtn = document.querySelector('[data-action="start"]');
const restartTimeBtn = document.querySelector('[data-action="stop"]');

Here I just select necessary elements from HTML. I'm using querySelector to take class and data attributes. As I have my startTimeBtn selected then I add an event listener to it:

startTimeBtn.addEventListener('click', () => {
  if (countdown) return;

I'm listening for click event and if this happens I set up my timer for 1500 seconds which is 25 minutes. But before running timer(1500) I check if countdown element is defined. Why? Before the user can click as many times as he/ she wanted and start the timer from the beginning. Then I run timer:

function timer(seconds) {
  const now = Date.now();
  const then = now + (seconds * 1000);


At the beginning, I define now which tells me what is current time right now. Then I foresee at which time my pomodoro timer will end. Then I call displayTimeLeft:

function displayTimeLeft(seconds) {
  const minutes = Math.floor(seconds / 60);
  const remainderSeconds = seconds % 60;
  const display = `${minutes}:${remainderSeconds < 10 ? '0' : ''}${remainderSeconds}`;
  timerDisplay.textContent = display;

Which is a simple function to display time in min:sec format. I compute minutes & remainderSeconds and then use es6 template string to neatly interpolate variables into the string. At the end, I set textContent of my timerDisplay which is h1 HTML element.

Let's go back to timer:

function timer(seconds) {
  // variables

  countdown = setInterval(() => {
    const secondsLeft = Math.round((then - Date.now()) / 1000);

    if (secondsLeft < 0) {

  }, 1000);

Here to countdown, I assign interval which will be executed every second. This is the place when this variable is defined and has an integer value. In the interval I calculate secondsLeft and if they are less than 0 it means it's time to stop interval by clearInterval, play sound and exit the function. At the end, I display changing time. playAudio is a simple function:

const endSound = document.querySelector('#end_sound');

function playAudio() {
  const sound = new Audio(endSound.src);

By the way most of these functions I take from JavaScript 30 day 29 by Wes Bos.

There is the last thing to do - restart my timer:

restartTimeBtn.addEventListener('click', () => {
  countdown = undefined;
  timerDisplay.textContent = '25:00';

I stop interval, set the countdown to undefined so I can start my timer again. I also redisplay remaining time.

What is next?

That's all for today! Thanks for reading but don't worry there is still a lot to do:

  • checking if pomodoro was good or bad
  • breakes
  • notifications
  • storing good & bad pomodoros

Please feel free to comment! If you have another way to do any of this don't hesitate and write to me.

Repo with this code is available on github.

tags: javascript,

Comments !