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Troubleshoot and Debug

Working with service workers can be challenging, especially when starting out. This page will cover a few useful tips to help working with service workers and help with Workbox.

Get to Know Your Developer Tools

Chrome's developer tools have a number of tools to make it easier to work with service workers.

The most common features used are the following.

Update on Reload

The "update on reload" toggle will force Chrome to check for a new service worker every time you refresh the page. This means that any new changes will be found on each page load.

Highlighting where Update on Reload is in DevTools

Clear Storage

There will come a point where you'll want to start from a clean state. No service workers, not caching, nothing. You can clear everything with the "Clear site data" button.

Clear site data in DevTools

Bypass for Network

When you aren't working with service workers at all, you can make the browser go straight to the network (i.e. not use the service worker) with the "Bypass for Network" toggle.

Bybass for Network Checkbox in DevTools

You'll know that this is working because the network panel will stop showing "(from ServiceWorker)".

Network panel in DevTools showing a request through service workers

There are plenty more features, but the above list should help as you start working on your service worker files. Learn what other features exist in DevTools here.

Common Problems

There are a set of problems that are common for developers to hit when working with service workers.

Q: Why does my service worker load, but not get used?

A: This can occur if the "scope" of your service worker doesn't match your page.

You can check if this is the problem by following these steps:

  1. Open DevTools for your site and go to Application > Service Workers.
  2. Find your service worker and look for a Clients entry. If you don't see it, like the image below, then scoping is likely to be an issue.

    DevTools screenshot where clients is missing from a service worker

    If the scoping is correct, your web page will show up as a client, as shown below.

    DevTools screenshot where clients are displayed for a service worker

This normally occurs because the location of the service worker is not at the root of the site (i.e. instead of /sw.js it's under a directory like /scripts/sw.js).

The easiest solution is to move your service worker to the root of your domain. An alternative solution is to add a Service-Worker-Allowed header to the service worker response. You can then change the scope and register your service worker like so:

// OK when 'Service-Worker-Allowed' header is set to '/'
navigator.serviceWorker.register('/blog/sw.js', {scope: '/'});

You can learn more about service worker scope here.

Q: Why are changes to my service worker not shown?

A: The reason that you might find that your service worker isn't updating is because it's stuck in a pending state. This is normally caused by having multiple tabs or windows open that are using the service worker.

You can determine if this is the case by looking under your service worker in Application > Service Workers.

A service worker in pending state on service worker

You can fix this by clicking the "skipWaiting" link or by closing the extra tabs so you only have one tab using the website (this enables the service worker to update on a refresh).

Q: My service worker isn't pending, but it's still not loading new changes.

A: Browsers will actually use a cached version of a service worker if the service worker file is served with a cache expiration header. The cached version will only be used for up to 24 hours, but it can still be difficult to reason with. The best practice is to set the cache expiration header to 0 for your service worker file. This forces the browser to update the service worker whenever your page loads.

Debugging Workbox

If you are finding it difficult to debug what Workbox is doing, there are a few things you can do to get some extra information.

Use a Debug Build

The debug builds of Workbox perform extra checks on the input and output arguments, provide extra logging and come unminified. This should help you debug a range of issues.

If you are using workbox-sw, you just need to set the config to debug: true to force Workbox to use debug builds.

  debug: true

Enable 'debug' Logs

The debug builds of Workbox will, by default, log extensive messages to the JavaScript console. These messages are tagged with specific log levels, and your JavaScript console might not be configured to show all log levels by default.

We recommend checking your JavaScript console's log level filtering and adjusting it accordingly. Setting it to display the "Verbose" level will result in seeing the most detailed messages, and can help in your debugging.

Stack Overflow

If you are still struggling to figure out your problem, try posting a question to Stack Overflow with the workbox tag. This enables a wide audience of people to view, answer and learn from your question.