---
id: making-a-progressive-web-app
title: Making a Progressive Web App
---

The production build has all the tools necessary to generate a first-class
[Progressive Web App](https://developers.google.com/web/progressive-web-apps/),
but **the offline/cache-first behavior is opt-in only**. By default,
the build process will generate a service worker file, but it will not be
registered, so it will not take control of your production web app.

In order to opt-in to the offline-first behavior, developers should look for the
following in their [`src/index.js`](https://github.com/facebook/create-react-app/blob/master/packages/react-scripts/template/src/index.js) file:

```js
// If you want your app to work offline and load faster, you can change
// unregister() to register() below. Note this comes with some pitfalls.
// Learn more about service workers: http://bit.ly/CRA-PWA
serviceWorker.unregister();
```

As the comment states, switching `serviceWorker.unregister()` to
`serviceWorker.register()` will opt you in to using the service worker.

## Why Opt-in?

Offline-first Progressive Web Apps are faster and more reliable than traditional web pages, and provide an engaging mobile experience:

- All static site assets are cached so that your page loads fast on subsequent visits, regardless of network connectivity (such as 2G or 3G). Updates are downloaded in the background.
- Your app will work regardless of network state, even if offline. This means your users will be able to use your app at 10,000 feet and on the subway.
- On mobile devices, your app can be added directly to the user's home screen, app icon and all. This eliminates the need for the app store.

However, they [can make debugging deployments more challenging](https://github.com/facebook/create-react-app/issues/2398) so, starting with Create React App 2, service workers are opt-in.

The [`workbox-webpack-plugin`](https://developers.google.com/web/tools/workbox/modules/workbox-webpack-plugin)
is integrated into production configuration,
and it will take care of generating a service worker file that will automatically
precache all of your local assets and keep them up to date as you deploy updates.
The service worker will use a [cache-first strategy](https://developers.google.com/web/fundamentals/instant-and-offline/offline-cookbook/#cache-falling-back-to-network)
for handling all requests for local assets, including
[navigation requests](https://developers.google.com/web/fundamentals/primers/service-workers/high-performance-loading#first_what_are_navigation_requests)
for your HTML, ensuring that your web app is consistently fast, even on a slow
or unreliable network.

## Offline-First Considerations

If you do decide to opt-in to service worker registration, please take the
following into account:

1. After the initial caching is done, the [service worker lifecycle](https://developers.google.com/web/fundamentals/primers/service-workers/lifecycle)
   controls when updated content ends up being shown to users. In order to guard against
   [race conditions with lazy-loaded content](https://github.com/facebook/create-react-app/issues/3613#issuecomment-353467430),
   the default behavior is to conservatively keep the updated service worker in the "[waiting](https://developers.google.com/web/fundamentals/primers/service-workers/lifecycle#waiting)"
   state. This means that users will end up seeing older content until they close (reloading is not
   enough) their existing, open tabs. See [this blog post](https://jeffy.info/2018/10/10/sw-in-c-r-a.html)
   for more details about this behavior.

1. Users aren't always familiar with offline-first web apps. It can be useful to
   [let the user know](https://developers.google.com/web/fundamentals/instant-and-offline/offline-ux#inform_the_user_when_the_app_is_ready_for_offline_consumption)
   when the service worker has finished populating your caches (showing a "This web
   app works offline!" message) and also let them know when the service worker has
   fetched the latest updates that will be available the next time they load the
   page (showing a "New content is available once existing tabs are closed." message). Showing
   these messages is currently left as an exercise to the developer, but as a
   starting point, you can make use of the logic included in [`src/serviceWorker.js`](https://github.com/facebook/create-react-app/blob/master/packages/react-scripts/template/src/serviceWorker.js), which
   demonstrates which service worker lifecycle events to listen for to detect each
   scenario, and which as a default, just logs appropriate messages to the
   JavaScript console.

1. Service workers [require HTTPS](https://developers.google.com/web/fundamentals/getting-started/primers/service-workers#you_need_https),
   although to facilitate local testing, that policy
   [does not apply to `localhost`](http://stackoverflow.com/questions/34160509/options-for-testing-service-workers-via-http/34161385#34161385).
   If your production web server does not support HTTPS, then the service worker
   registration will fail, but the rest of your web app will remain functional.

1. The service worker is only enabled in the [production environment](deployment.md),
   e.g. the output of `npm run build`. It's recommended that you do not enable an
   offline-first service worker in a development environment, as it can lead to
   frustration when previously cached assets are used and do not include the latest
   changes you've made locally.

1. If you _need_ to test your offline-first service worker locally, build
   the application (using `npm run build`) and run a simple http server from your
   build directory. After running the build script, `create-react-app` will give
   instructions for one way to test your production build locally and the [deployment instructions](deployment.md) have
   instructions for using other methods. _Be sure to always use an
   incognito window to avoid complications with your browser cache._

1. By default, the generated service worker file will not intercept or cache any
   cross-origin traffic, like HTTP [API requests](integrating-with-an-api-backend.md),
   images, or embeds loaded from a different domain.

## Progressive Web App Metadata

The default configuration includes a web app manifest located at
[`public/manifest.json`](https://github.com/facebook/create-react-app/blob/master/packages/react-scripts/template/public/manifest.json), that you can customize with
details specific to your web application.

When a user adds a web app to their homescreen using Chrome or Firefox on
Android, the metadata in [`manifest.json`](https://github.com/facebook/create-react-app/blob/master/packages/react-scripts/template/public/manifest.json) determines what
icons, names, and branding colors to use when the web app is displayed.
[The Web App Manifest guide](https://developers.google.com/web/fundamentals/engage-and-retain/web-app-manifest/)
provides more context about what each field means, and how your customizations
will affect your users' experience.

Progressive web apps that have been added to the homescreen will load faster and
work offline when there's an active service worker. That being said, the
metadata from the web app manifest will still be used regardless of whether or
not you opt-in to service worker registration.
