Vanilla JavaScript Timer

18 June, 2020 — 10 minute read
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Today we'll be looking into making a timer in JavaScript. A timer can be used in many ways for several purposes. In my case, it is a game timer. It will start once the game starts and keeps track of how long it takes someone to solve something.

Let's dive into it and make our first JavaScript timer.

Check out this countdown in JavaScript!

HTML Structure permalink

<div class="container">
  <div id="timer">00:00</div>
  <button onclick="startTimer()">Start</button>
</div>

We add a container for centering purposes inside we add our timer with the default of 00:00 so people can see what they are expecting. And then a button that will start the timer for now.

CSS Structure permalink

As for the CSS, it's a pure visual game in this project, nothing magic is going on.

.container {
  display: flex;
  justify-content: center;
  align-items: center;
  height: 100vh;
  flex-direction: column;
  background: #e9c46a;
}
#timer {
  margin-bottom: 25px;
  font-size: 3rem;
  font-weight: bold;
  color: #2a9d8f;
  &.odd {
    color: #264653;
  }
}
button {
  border-radius: 5px;
  display: inline-block;
  border: none;
  padding: 1rem 2rem;
  margin: 0;
  text-decoration: none;
  background: #e76f51;
  color: #ffffff;
  font-family: sans-serif;
  font-size: 1rem;
  cursor: pointer;
  text-align: center;
  transition: background 250ms ease-in-out;
  -webkit-appearance: none;
  -moz-appearance: none;
  &:hover {
    background: #f4a261;
  }
}

The central part here is the container, which uses the flex centering technique.

JavaScript Timer permalink

The fun part is the JavaScript to making the timer work.

const timer = document.getElementById('timer');
let timerInterval;

We start our JavaScript by defining the timer element and our interval for the timer.

Next, we will define the button's function we call startTimer.

For this, we are using the Arrow functions.

startTimer = () => {
  // Firs twe start by clearing the existing timer, in case of a restart
  clearInterval(timerInterval);
  // Then we clear the variables
  let second = 0,
    minute = 0,
    hour = 0;

  // Next we set a interval every 1000 ms
  timerInterval = setInterval(function () {
    // Toggle the odd class every interval
    timer.classList.toggle('odd');

    // We set the timer text to include a two digit representation
    timer.innerHTML =
      (hour ? hour + ':' : '') +
      (minute < 10 ? '0' + minute : minute) +
      ':' +
      (second < 10 ? '0' + second : second);

    // Next, we add a new second since one second is passed
    second++;

    // We check if the second equals 60 "one minute"
    if (second == 60) {
      // If so, we add a minute and reset our seconds to 0
      minute++;
      second = 0;
    }

    // If we hit 60 minutes "one hour" we reset the minutes and plus an hour
    if (minute == 60) {
      hour++;
      minute = 0;
    }
  }, 1000);
};

I've written comments between the code snippet to clarify what happens on each line.

The important part is we use setInterval to recall the function every 1000ms. Inside this, we manually add seconds and minutes and update the innerHTML of our timer div.

I've also included a fun part that will change our timer's color every second.

// Toggle the odd class every interval
timer.classList.toggle('odd');

You can find my article about using the classList in JavaScript here.

As usual, you can find the demo timer on Codepen.

See the Pen Vanilla JavaScript Timer by Chris Bongers (@rebelchris) on CodePen.

Shorthand Explanation permalink

(EDIT) I got a comment asking what the following part does:

// We set the timer text to include a two digit representation
timer.innerHTML =
  (hour ? hour + ':' : '') +
  (minute < 10 ? '0' + minute : minute) +
  ':' +
  (second < 10 ? '0' + second : second);

This is using the JavaScript shorthand properties and is basically the same as doing the following:

let string = '';
if (hour) {
  string += hour + ':';
}
if (minute < 10) {
  // Number is 0-9 so we want to prefix with a zero
  string += '0' + minute;
} else {
  // Number is 10 or more so no prefix needed
  string += minute;
}
if (second < 10) {
  // Number is 0-9 so we want to prefix with a zero
  string += ':0' + second;
} else {
  // Number is 10 or more so no prefix needed
  string += ':' + second;
}
timer.innerHTML = string;