Cómo construir una PWA desde cero con HTML, CSS y JavaScript

Las aplicaciones web progresivas son una forma de llevar esa sensación de aplicación nativa a una aplicación web tradicional. Con PWA podemos mejorar nuestro sitio web con funciones de aplicaciones móviles que aumentan la usabilidad y ofrecen una excelente experiencia de usuario.

En este artículo, vamos a crear una PWA desde cero con HTML, CSS y JavaScript. Estos son los temas que cubriremos:

  • ¿Qué es una aplicación web progresiva?
  • Margen
  • Estilismo
  • Mostrar datos con JavaScript
  • Manifiesto de la aplicación web
  • ¿Qué es un trabajador de servicios?
  • Caché los activos
  • Recupera los activos
  • Registrar al trabajador del servicio
  • Pensamientos finales
  • Próximos pasos

Entonces, comencemos con una pregunta importante: ¿Qué diablos es una PWA?

¿Qué es una aplicación web progresiva?

Una aplicación web progresiva es una aplicación web que ofrece a los usuarios una experiencia similar a una aplicación mediante el uso de capacidades web modernas. Al final, es solo su sitio web habitual el que se ejecuta en un navegador con algunas mejoras. Te da la habilidad:

  • Para instalarlo en una pantalla de inicio móvil
  • Para acceder a él sin conexión
  • Para acceder a la cámara
  • Para recibir notificaciones push
  • Para hacer una sincronización en segundo plano

Y mucho más.

Sin embargo, para poder transformar nuestra aplicación web tradicional en una PWA, tenemos que ajustarla un poco agregando un archivo de manifiesto de aplicación web y un trabajador de servicio.

No se preocupe por estos nuevos términos, los cubriremos a continuación.

Primero, tenemos que construir nuestra aplicación web tradicional. Así que comencemos con el marcado.

Margen

El archivo HTML es relativamente simple. Envolvemos todo en la mainetiqueta.

  • En index.html
       Dev'Coffee PWA     

Dev'Coffee

  • Home
  • About
  • Blog

Y crea una barra de navegación con la navetiqueta. Luego, divcon la clase se .containerguardarán nuestras tarjetas que agreguemos más tarde con JavaScript.

Ahora que lo hemos aclarado, vamos a diseñarlo con CSS.

Estilismo

Aquí, como de costumbre, comenzamos importando las fuentes que necesitamos. Luego haremos algunos restablecimientos para evitar el comportamiento predeterminado.

  • En css/style.css
@import url("//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; } 

Luego, limitamos el mainancho máximo del elemento 900pxpara que se vea bien en una pantalla grande.

Para la barra de navegación, quiero que el logo esté a la izquierda y los enlaces a la derecha. Entonces, para la navetiqueta, después de convertirla en un contenedor flexible, usamos justify-content: space-between;para alinearlos.

  • En 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; } 

Tendremos varias tarjetas, por lo que para el elemento contenedor se mostrará como una cuadrícula. Y, con grid-template-columns: repeat(auto-fit, minmax(15rem, 1fr)), ahora podemos hacer que nuestras tarjetas respondan para que usen al menos el 15remancho si hay suficiente espacio (y 1frsi no).

Y para que se vean bonita se duplica el efecto de sombra en la .cardclase y el uso object-fit: coverde .card--avatarpara evitar que la imagen se estire.

Ahora se ve mucho mejor, pero todavía no tenemos datos para mostrar.

Arreglemoslo en la siguiente sección.

Mostrar datos con JavaScript

Observe que utilicé imágenes grandes que tardan algún tiempo en cargarse. Esto le mostrará de la mejor manera el poder de los trabajadores de servicios.

Como dije antes, la .containerclase tendrá nuestras cartas. Por lo tanto, debemos seleccionarlo.

  • En 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" }, ] 

Luego, creamos una matriz de tarjetas con nombres e imágenes.

  • En js/app.js
const showCoffees = () => { let output = "" coffees.forEach( ({ name, image }) => (output += ` 

${name}

Taste `) ) 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.

super-excited

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.json (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)

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.

wow

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).

image-cache

Hopefully, you're still with me.

desesperate

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 ) ) }) 

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.

let-s-do-it

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.

we-did-it

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 my blog

Next steps

Web Manifest Documentation

Service Worker Documentation

Web Manifest Generator

Browser Support