Workbox Routing

What is Workbox Routing?

A service worker can intercept network requests for a page. It may respond to the browser with cached content, content from the network or content generated in the service worker.

workbox-routing is a module which makes it easy to "route" these requests to different functions that provide responses.

How Routing is Performed

When a network request causes a service worker fetch event, workbox-routing will attempt to respond to the request using the supplied routes and handlers.

Workbox Routing Diagram

The main things to note from the above are:

The method of a request is important. By default, Routes are registered for GET requests. If you wish to intercept other types of requests, you’ll need to specify the method.
The order of the Route registration is important. If multiple Routes are registered that could handle a request, the Route that is registered first will be used to respond to the request.

There are a few ways to register a route: you can use callbacks, regular expressions or Route instances.

Matching and Handling in Routes

A "route" in workbox is nothing more than two functions: a "matching" function to determine if the route should match a request and a "handling" function, which should handle the request and respond with a response.

Workbox comes with some helpers that'll perform the matching and handling for you, but if you ever find yourself wanting different behavior, writing a custom match and handler function is the best option.

A match callback function is passed a ExtendableEvent, Request, and a URL object you can match by returning a truthy value. For a simple example, you could match against a specific URL like so:

const matchCb = ({url, request, event}) => {
  return (url.pathname === '/special/url');
};

Most use cases can be covered by examining / testing either the url or the request.

A handler callback function will be given the same ExtendableEvent, Request, and URL object along with a params value, which is the value returned by the "match" function.

const handlerCb = async ({url, request, event, params}) => {
  const response = await fetch(request);
  const responseBody = await response.text();
  return new Response(`${responseBody} <!-- Look Ma. Added Content. -->`); 
};

Your handler must return a promise that resolves to a Response. In this example, we're using async and await. Under the hood, the return Response value will be wrapped in a promise.

You can register these callbacks like so:

import {registerRoute} from 'workbox-routing';

registerRoute(matchCb, handlerCb);

The only limitation is that the "match" callback must synchronously return a truthy value, you can’t perform any asynchronous work. The reason for this is that the Router must synchronously respond to the fetch event or allow falling through to other fetch events.

Normally the "handler" callback would use one of the strategies provided by workbox-strategies like so:

import {registerRoute} from 'workbox-routing';
import {StaleWhileRevalidate} from 'workbox-strategies';

registerRoute(
  matchCb,
  new StaleWhileRevalidate()
);

In this page, we’ll focus on workbox-routing but you can learn more about these strategies on workbox-strategies.

How to Register a Regular Expression Route

A common practice is to use a regular expression instead of a "match" callback. Workbox makes this easy to implement like so:

import {registerRoute} from 'workbox-routing';

registerRoute(
  new RegExp('/styles/.*\\.css'),
  handlerCb
);

For requests from the same origin, this regular expression will match as long as the request’s URL matches the regular expression.

https://example.com/styles/main.css
https://example.com/styles/nested/file.css
https://example.com/nested/styles/directory.css

However, for cross-origin requests, regular expressions must match the beginning of the URL. The reason for this is that it’s unlikely that with a regular expression new RegExp('/styles/.*\\.css') you intended to match third-party CSS files.

https://cdn.third-party-site.com/styles/main.css
https://cdn.third-party-site.com/styles/nested/file.css
https://cdn.third-party-site.com/nested/styles/directory.css

If you did want this behaviour, you just need to ensure that the regular expression matches the beginning of the URL. If we wanted to match the requests for https://cdn.third-party-site.com we could use the regular expression new RegExp('https://cdn\\.third-party-site\\.com.*/styles/.*\\.css').

https://cdn.third-party-site.com/styles/main.css
https://cdn.third-party-site.com/styles/nested/file.css
https://cdn.third-party-site.com/nested/styles/directory.css

If you wanted to match both local and third parties you can use a wildcard at the start of your regular expression, but this should be done with caution to ensure it doesn’t cause unexpected behaviors in you web app.

How to Register a Navigation Route

If your site is a single page app, you can use a NavigationRoute to return a specific response for all navigation requests.

import {createHandlerBoundToURL} from 'workbox-precaching';
import {NavigationRoute, registerRoute} from 'workbox-routing';

// This assumes /app-shell.html has been precached.
const handler = createHandlerBoundToURL('/app-shell.html');
const navigationRoute = new NavigationRoute(handler);
registerRoute(navigationRoute);

Whenever a user goes to your site in the browser, the request for the page will be a navigation request and it will be served the cached page /app-shell.html. (Note: You should have the page cached via workbox-precaching or through your own installation step.)

By default, this will respond to all navigation requests. If you want to restrict it to respond to a subset of URLs, you can use the allowlist and denylist options to restrict which pages will match this route.

import {createHandlerBoundToURL} from 'workbox-precaching';
import {NavigationRoute, registerRoute} from 'workbox-routing';

// This assumes /app-shell.html has been precached.
const handler = createHandlerBoundToURL('/app-shell.html');
const navigationRoute = new NavigationRoute(handler, {
  allowlist: [
    new RegExp('/blog/'),
  ],
  denylist: [
    new RegExp('/blog/restricted/'),
  ],
});
registerRoute(navigationRoute);

The only thing to note is that the denylist will win if a URL is in both the allowlist and denylist.

Set a Default Handler

If you want to supply a "handler" for requests that don’t match a route, you can set a default handler.

import {setDefaultHandler} from 'workbox-routing';

setDefaultHandler(({url, event, params}) => {
  // ...
});

Set a Catch Handler

In the case of any of your routes throwing an error, you can capture and degrade gracefully by setting a catch handler.

import {setCatchHandler} from 'workbox-routing';

setCatchHandler(({url, event, params}) => {
  ...
});

Defining a Route for Non-GET Requests

All routes by default are assumed to be for GET requests.

If you would like to route other requests, like a POST request, you need to define the method when registering the route, like so:

import {registerRoute} from 'workbox-routing';

registerRoute(
  matchCb,
  handlerCb,
  'POST'
);
registerRoute(
  new RegExp('/api/.*\\.json'),
  handlerCb,
  'POST'
);

Router Logging

You should be able to determine the flow of a request using the logs from workbox-routing which will highlight which URLs are being processed through Workbox.

Routing Logs

If you need more verbose information, you can set the log level to debug to view logs on requests not handled by the Router. See our debugging guide for more info on setting the log level.

Debug and Log Routing Messages

Advanced Usage

If you want to have more control over when the Workbox Router is given requests, you can create your own Router instance and call it’s handleRequest() method whenever you want to use the router to respond to a request.

import {Router} from 'workbox-routing';

const router = new Router();
self.addEventListener('fetch', (event) => {
  const responsePromise = router.handleRequest(event);
  if (responsePromise) {
    // Router found a route to handle the request
    event.respondWith(responsePromise);
  } else {
    // No route found to handle the request
  }
});

When using the Router directly, you will also need to use the Route class, or any of the extending classes to register routes.

import {Route, RegExpRoute, NavigationRoute, Router} from 'workbox-routing';

const router = new Router();
router.registerRoute(new Route(matchCb, handlerCb));
router.registerRoute(new RegExpRoute(new RegExp(...), handlerCb));
router.registerRoute(new NavigationRoute(handlerCb));