Precaching with the Angular service worker

Use the Angular service worker to make your app faster and more reliable on networks with poor connectivity.

Minko Gechev

Dealing with limited connectivity

When users have limited network access—or none at all—web app functionality can significantly degrade and often fails. Using a service worker to provide precaching lets you intercept network requests and deliver responses directly from a local cache instead of retrieving them from the network. Once your app's assets have been cached, this approach can really speed up an app and make it work when the user is offline.

This post walks through how to set up precaching in an Angular app. It assumes you're already familiar with precaching and service workers in general. If you need a refresher, check out the Service workers and the Cache Storage API post.

Introducing the Angular service worker

The Angular team offers a service worker module with precaching functionality that's well integrated with the framework and the Angular command line interface (CLI).

To add the service worker, run this command in the CLI:

ng add @angular/pwa

@angular/service-worker and @angular/pwa should now be installed in the app and should appear in package.json. The ng-add schematic also adds a file called ngsw-config.json to the project, which you can use to configure the service worker. (The file includes a default configuration that you'll customize a little later.)

Now build the project for production:

ng build --prod

Inside the dist/service-worker-web-dev directory you'll find a file called ngsw.json. This file tells the Angular service worker how to cache the assets in the app. The file is generated during the build process based on the configuration (ngsw-config.json) and the assets produced at build time.

Now start an HTTP server in the directory containing your app's production assets, open the public URL, and check out its network requests in Chrome DevTools:

1. Press `Control+Shift+J` (or `Command+Option+J` on Mac) to open DevTools.
2. Click the Network tab.

Note that the network tab has a bunch of static assets directly downloaded in the background by the ngsw-worker.js script:

This is the Angular service worker precaching the static assets specified in the generated ngsw.json manifest file.

One important asset is missing though: nyan.png. To precache this image you need to add a pattern that includes it to ngsw-config.json, which lives in the root of the workspace:

{
  "$schema": "./node_modules/@angular/service-worker/config/schema.json",
  "index": "/index.html",
  "assetGroups": [
    {
      "name": "app",
      "installMode": "prefetch",
      "resources": {
      "files": [
        "/favicon.ico",
        "/index.html",
        "/*.css",
        "/*.js",
        "/assets/*.png"
        ]
      }
    },
    ...
}

This change adds all PNG images in the /assets folder to the app resource asset group. Since the installMode for this asset group is set to prefetch, the service worker will precache all the specified assets—which now include PNG images.

Specifying other assets to be precached is just as straightforward: update the patterns in the app resource asset group.

Conclusion

Using a service worker for precaching can improve the performance of your apps by saving assets to a local cache, which makes them more reliable on poor networks. To use precaching with Angular and the Angular CLI:

1. Add the @angular/pwa package to your project.
2. Control what the service worker caches by editing ngsw-config.json.