How to Build a PWA from scratch with HTML, CSS, and JavaScript

How to Build a PWA from scratch with HTML, CSS, and JavaScript

In this article, we are going to build a PWA from scratch with HTML, CSS, and JavaScript. What is a Progressive Web App (PWA)? Progressive web apps are a way to bring that native app feeling to a traditional web app. With PWAs we can enhance our website with mobile app features which increase usability and offer a great user experience.

Progressive web apps are a way to bring that native app feeling to a traditional web app. With PWAs we can enhance our website with mobile app features which increase usability and offer a great user experience.

In this article, we are going to build a PWA from scratch with HTML, CSS, and JavaScript. Here are the topics we'll cover:

  • What is a Progressive Web App (PWA)?
  • Markup
  • Styling
  • Show data with JavaScript
  • Web App Manifest
  • What is a Service Worker?
  • Cache the assets
  • Fetch the assets
  • Register the Service Worker
  • Final thoughts

So, let's get started with an important question: What the heck is a PWA?

What is a Progressive Web App (PWA)?

A Progressive Web App is a web app that delivers an app-like experience to users by using modern web capabilities. In the end, it's just your regular website that runs in a browser with some enhancements. It gives you the ability:

  • To install it on a mobile home screen
  • To access it when offline
  • To access the camera
  • To get push notifications
  • To do background synchronization

And so much more.

However, to be able to transform our traditional web app to a PWA, we have to adjust it a little bit by adding a web app manifest file and a service worker.

Don't worry about these new terms – we'll cover them below.

First, we have to build our traditional web app. So let's start with the markup.

Markup

The HTML file is relatively simple. We wrap everything in the main tag.

  • In index.html
<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <meta http-equiv="X-UA-Compatible" content="ie=edge" />
    <link rel="stylesheet" href="css/style.css" />
    <title>Dev'Coffee PWA</title>
  </head>
  <body>
    <main>
      <nav>
        <h1>Dev'Coffee</h1>
        <ul>
          <li>Home</li>
          <li>About</li>
          <li>Blog</li>
        </ul>
      </nav>
      <div class="container"></div>
    </main>
    <script src="js/app.js"></script>
  </body>
</html>

And create a navigation bar with the nav tag. Then, the div with the class .container will hold our cards that we add later with JavaScript.

Now that we've gotten that out of the way, let's style it with CSS.

Styling

Here, as usual, we start by importing the fonts we need. Then we'll do some resets to prevent the default behavior.

  • In css/style.css
@import url("https://fonts.googleapis.com/css?family=Nunito:400,700&display=swap");
* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}
body {
  background: #fdfdfd;
  font-family: "Nunito", sans-serif;
  font-size: 1rem;
}
main {
  max-width: 900px;
  margin: auto;
  padding: 0.5rem;
  text-align: center;
}
nav {
  display: flex;
  justify-content: space-between;
  align-items: center;
}
ul {
  list-style: none;
  display: flex;
}

li {
  margin-right: 1rem;
}
h1 {
  color: #e74c3c;
  margin-bottom: 0.5rem;
}

Then, we limit the main element's maximum width to 900px to make it look good on a large screen.

For the navbar, I want the logo to be at the left and the links at the right. So for the nav tag, after making it a flex container, we use justify-content: space-between; to align them.

  • In css/style.css
.container {
  display: grid;
  grid-template-columns: repeat(auto-fit, minmax(15rem, 1fr));
  grid-gap: 1rem;
  justify-content: center;
  align-items: center;
  margin: auto;
  padding: 1rem 0;
}
.card {
  display: flex;
  align-items: center;
  flex-direction: column;
  width: 15rem auto;
  height: 15rem;
  background: #fff;
  box-shadow: 0 10px 20px rgba(0, 0, 0, 0.19), 0 6px 6px rgba(0, 0, 0, 0.23);
  border-radius: 10px;
  margin: auto;
  overflow: hidden;
}
.card--avatar {
  width: 100%;
  height: 10rem;
  object-fit: cover;
}
.card--title {
  color: #222;
  font-weight: 700;
  text-transform: capitalize;
  font-size: 1.1rem;
  margin-top: 0.5rem;
}
.card--link {
  text-decoration: none;
  background: #db4938;
  color: #fff;
  padding: 0.3rem 1rem;
  border-radius: 20px;
}

We'll have several cards, so for the container element it will be displayed as a grid. And, with grid-template-columns: repeat(auto-fit, minmax(15rem, 1fr)), we can now make our cards responsive so that they use at least 15rem width if there is enough space (and 1fr if not).

And to make them look nice we double the shadow effect on the .card class and use object-fit: cover on .card--avatar to prevent the image from stretching.

Now it looks much better – but we still don't have data to show.

Let's fix it in the next section

Show data with JavaScript

Notice that I used large images that take some time to load. This will show you in the best way the power of service workers.

As I said earlier, the .container class will hold our cards. Therefore, we need to select it.

  • In js/app.js
const container = document.querySelector(".container")
const coffees = [
  { name: "Perspiciatis", image: "images/coffee1.jpg" },
  { name: "Voluptatem", image: "images/coffee2.jpg" },
  { name: "Explicabo", image: "images/coffee3.jpg" },
  { name: "Rchitecto", image: "images/coffee4.jpg" },
  { name: " Beatae", image: "images/coffee5.jpg" },
  { name: " Vitae", image: "images/coffee6.jpg" },
  { name: "Inventore", image: "images/coffee7.jpg" },
  { name: "Veritatis", image: "images/coffee8.jpg" },
  { name: "Accusantium", image: "images/coffee9.jpg" },
]

Then, we create an array of cards with names and images.

  • In js/app.js
const showCoffees = () => {
  let output = ""
  coffees.forEach(
    ({ name, image }) =>
      (output += `
              <div class="card">
                <img class="card--avatar" src=${image} />
                <h1 class="card--title">${name}</h1>
                <a class="card--link" href="#">Taste</a>
              </div>
              `)
  )
  container.innerHTML = output
}

document.addEventListener("DOMContentLoaded", showCoffees)

With this code above, we can now loop through the array and show them on the HTML file. And to make everything work, we wait until the DOM (Document Object Model) content finishes loading to run the showCoffees method.

We've done a lot, but for now, we just have a traditional web app. So, let's change that in the next section by introducing some PWA features.

Web App Manifest

The web app manifest is a simple JSON file that informs the browser about your web app. It tells how it should behave when installed on the user's mobile device or desktop. And to show the Add to Home Screen prompt, the web app manifest is required.

Now that we know what a web manifest is, let's create a new file named manifest.jon (you have to name it like that) in the root directory. Then add this code block below.

  • In manifest.json
{
  "name": "Dev'Coffee",
  "short_name": "DevCoffee",
  "start_url": "index.html",
  "display": "standalone",
  "background_color": "#fdfdfd",
  "theme_color": "#db4938",
  "orientation": "portrait-primary",
  "icons": [
    {
      "src": "/images/icons/icon-72x72.png",
      "type": "image/png", "sizes": "72x72"
    },
    {
      "src": "/images/icons/icon-96x96.png",
      "type": "image/png", "sizes": "96x96"
    },
    {
      "src": "/images/icons/icon-128x128.png",
      "type": "image/png","sizes": "128x128"
    },
    {
      "src": "/images/icons/icon-144x144.png",
      "type": "image/png", "sizes": "144x144"
    },
    {
      "src": "/images/icons/icon-152x152.png",
      "type": "image/png", "sizes": "152x152"
    },
    {
      "src": "/images/icons/icon-192x192.png",
      "type": "image/png", "sizes": "192x192"
    },
    {
      "src": "/images/icons/icon-384x384.png",
      "type": "image/png", "sizes": "384x384"
    },
    {
      "src": "/images/icons/icon-512x512.png",
      "type": "image/png", "sizes": "512x512"
    }
  ]
}

In the end, it's just a JSON file with some mandatory and optional properties.

name: When the browser launches the splash screen, it will be the name displayed on the screen.

short_name: It will be the name displayed underneath your app shortcut on the home screen.

start_url: It will be the page shown to the user when your app is open.

display: It tells the browser how to display the app. There are several modes like minimal-ui, fullscreen, browser etc. Here, we use the standalone mode to hide everything related to the browser.

background_color: When the browser launches the splash screen, it will be the background of the screen.

theme_color: It will be the background color of the status bar when we open the app.

orientation: It tells the browser the orientation to have when displaying the app.

icons: When the browser launches the splash screen, it will be the icon displayed on the screen. Here, I used all sizes to fit any device's preferred icon. But you can just use one or two. It's up to you.

Now that we have a web app manifest, let's add it to the HTML file.

  • In index.html (head tag)
<link rel="manifest" href="manifest.json" />
<!-- ios support -->
<link rel="apple-touch-icon" href="images/icons/icon-72x72.png" />
<link rel="apple-touch-icon" href="images/icons/icon-96x96.png" />
<link rel="apple-touch-icon" href="images/icons/icon-128x128.png" />
<link rel="apple-touch-icon" href="images/icons/icon-144x144.png" />
<link rel="apple-touch-icon" href="images/icons/icon-152x152.png" />
<link rel="apple-touch-icon" href="images/icons/icon-192x192.png" />
<link rel="apple-touch-icon" href="images/icons/icon-384x384.png" />
<link rel="apple-touch-icon" href="images/icons/icon-512x512.png" />
<meta name="apple-mobile-web-app-status-bar" content="#db4938" />
<meta name="theme-color" content="#db4938" />

As you can see, we linked our manifest.json file to the head tag. And add some other links which handle the iOS support to show the icons and colorize the status bar with our theme color.

With that, we can now dive into the final part and introduce the service worker.

What is a Service Worker?

Notice that PWAs run only on https because the service worker can access the request and handle it. Therefore security is required.

A service worker is a script that your browser runs in the background in a separate thread. That means it runs in a different place and is completely separate from your web page. That's the reason why it can't manipulate your DOM element.

However, it's super powerful. The service worker can intercept and handle network requests, manage the cache to enable offline support or send push notifications to your users.

S0 let's create our very first service worker in the root folder and name it serviceWorker.js (the name is up to you). But you have to put it in the root so that you don't limit its scope to one folder.

Cache the assets

  • In serviceWorker.js
const staticDevCoffee = "dev-coffee-site-v1"
const assets = [
  "/",
  "/index.html",
  "/css/style.css",
  "/js/app.js",
  "/images/coffee1.jpg",
  "/images/coffee2.jpg",
  "/images/coffee3.jpg",
  "/images/coffee4.jpg",
  "/images/coffee5.jpg",
  "/images/coffee6.jpg",
  "/images/coffee7.jpg",
  "/images/coffee8.jpg",
  "/images/coffee9.jpg",
]

self.addEventListener("install", installEvent => {
  installEvent.waitUntil(
    caches.open(staticDevCoffee).then(cache => {
      cache.addAll(assets)
    })
  )
})

This code looks intimidating first but it just JavaScript (so don't worry).

We declare the name of our cache staticDevCoffee and the assets to store in the cache. And to perform that action, we need to attach a listener to self.

self is the service worker itself. It enables us to listen to life cycle events and do something in return.

The service worker has several life cycles, and one of them is the install event. It runs when a service worker is installed. It's triggered as soon as the worker executes, and it's only called once per service worker.

When the install event is fired, we run the callback which gives us access to the event object.

Caching something on the browser can take some time to finish because it's asynchronous.

So to handle it, we need to use waitUntil() which, as you might guess, waits for the action to finish.

Once the cache API is ready, we can run the open() method and create our cache by passing its name as an argument to caches.open(staticDevCoffee).

Then it returns a promise, which helps us store our assets in the cache with cache.addAll(assets).

Hopefully, you're still with me.

Now, we've successfully cached our assets in the browser. And the next time we load the page, the service worker will handle the request and fetch the cache if we are offline.

So, let's fetch our cache.

Fetch the assets

  • In serviceWorker.js
self.addEventListener("fetch", fetchEvent => {
  fetchEvent.respondWith(
    caches.match(fetchEvent.request).then(res => {
      return res || fetch(fetchEvent.request)
    })
  )
})

Here, we use the fetch event to, well, get back our data. The callback gives us access to fetchEvent. Then we attach respondWith() to prevent the browser's default response. Instead it returns a promise because the fetch action can take time to finish.

And once the cache ready, we apply the caches.match(fetchEvent.request). It will check if something in the cache matches fetchEvent.request. By the way, fetchEvent.request is just our array of assets.

Then, it returns a promise. And finally, we can return the result if it exists or the initial fetch if not.

Now, our assets can be cached and fetched by the service worker which increases the load time of our images quite a bit.

And most important, it makes our app available in offline mode.

But a service worker alone can't do the job. We need to register it in our project.

Register the Service Worker
  • In js/app.js
if ("serviceWorker" in navigator) {
  window.addEventListener("load", function() {
    navigator.serviceWorker
      .register("/serviceWorker.js")
      .then(res => console.log("service worker registered"))
      .catch(err => console.log("service worker not registered", err))
  })
}

Here, we start by checking if the serviceWorker is supported by the current browser (as it's still not supported by all browsers).

Then, we listen to the page load event to register our service worker by passing the name of our file serviceWorker.js to navigator.serviceWorker.register() as a parameter to register our worker.

With this update, we have now transformed our regular web app to a PWA.

Final thoughts

Throughout this article, we have seen how amazing PWAs can be. By adding a web app manifest file and a service worker, it really improves the user experience of our traditional web app. This is because PWAs are fast, secure, reliable, and – most importantly – they support offline mode.

Many frameworks out there now come with a service worker file already set-up for us. But knowing how to implement it with Vanilla JavaScript can help you understand PWAs.

And you can go even further with service workers by caching assets dynamically or limiting the size of your cache and so on.

Thanks for reading this article.

You can check it out live here and the source code is here.

Read more of my articles on this blog

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