Stay awake with the Screen Wake Lock API

# What is the Screen Wake Lock API?

To avoid draining the battery, most devices quickly go to sleep when left idle. While this is fine most of the time, some applications need to keep the screen awake to complete their work. Examples include cooking apps that show the steps of a recipe or a game like Ball Puzzle, which uses the device motion APIs for input.

The Screen Wake Lock API provides a way to prevent the device from dimming and locking the screen. This capability enables new experiences that, until now, required a platform-specific app.

The Screen Wake Lock API reduces the need for hacky and potentially power-hungry workarounds. It addresses the shortcomings of an older API that was limited to simply keeping the screen on and had a number of security and privacy issues.

# Suggested use cases for the Screen Wake Lock API

RioRun, a web app developed by The Guardian, was a perfect use case (though it's no longer available). The app takes you on a virtual audio tour of Rio, following the route of the 2016 Olympic marathon. Without wake locks, users' screens would turn off frequently while the tour played, making it hard to use.

Of course, there are plenty of other use cases:

• A recipe app that keeps the screen on while you bake a cake or cook dinner
• A boarding pass or ticket app that keeps the screen on until the barcode has been scanned
• A kiosk-style app that keeps the screen on continuously
• A web-based presentation app that keeps the screen on during a presentation

# Current status

# Using the Screen Wake Lock API

# Wake lock types

The Screen Wake Lock API currently provides just one type of wake lock: screen.

# screen wake lock

A screen wake lock prevents the device's screen from turning off so that the user can see the information that's displayed on screen.

# Getting a screen wake lock

To request a screen wake lock, you need to call the navigator.wakeLock.request() method that returns a WakeLockSentinel object. You pass this method the desired wake lock type as a parameter, which currently is limited to just 'screen' and therefore is optional. The browser can refuse the request for various reasons (for example, because the battery charge level is too low), so it's a good practice to wrap the call in a try…catch statement. The exception's message will contain more details in case of failure.

# Releasing a screen wake lock

You also need a way to release the screen wake lock, which is achieved by calling the release() method of the WakeLockSentinel object. If you don't store a reference to the WakeLockSentinel, there's no way to release the lock manually, but it will be released once the current tab is invisible.

If you want to automatically release the screen wake lock after a certain period of time has passed, you can use window.setTimeout() to call release(), as shown in the example below.

// The wake lock sentinel.
let wakeLock = null;

// Function that attempts to request a screen wake lock.
const requestWakeLock = async () => {
  try {
    wakeLock = await navigator.wakeLock.request();
    wakeLock.addEventListener('release', () => {
      console.log('Screen Wake Lock released:', wakeLock.released);
    });
    console.log('Screen Wake Lock released:', wakeLock.released);
  } catch (err) {
    console.error(`${err.name}, ${err.message}`);
  }
};

// Request a screen wake lock…
await requestWakeLock();
// …and release it again after 5s.
window.setTimeout(() => {
  wakeLock.release();
  wakeLock = null;
}, 5000);

The WakeLockSentinel object has a property called released that indicates whether a sentinel has already been released. Its value is initially false, and changes to true once a "release" event is dispatched. This property helps web developers know when a lock has been released so that they do not need to keep track of this manually. It is available as of Chrome 87.

# The screen wake lock lifecycle

When you play with the screen wake lock demo, you'll notice that screen wake locks are sensitive to page visibility. This means that the screen wake lock will automatically be released when you minimize a tab or window, or switch away from a tab or window where a screen wake lock is active.

To reacquire the screen wake lock, listen for the visibilitychange event and request a new screen wake lock when they occur:

const handleVisibilityChange = async () => {
  if (wakeLock !== null && document.visibilityState === 'visible') {
    await requestWakeLock();
  }
};

document.addEventListener('visibilitychange', handleVisibilityChange);

# Minimize your impact on system resources

Should you use a screen wake lock in your app? The approach you take depends on the needs of your app. Regardless, you should use the most lightweight approach possible for your app to minimize its impact on system resources.

Before adding a screen wake lock to your app, consider whether your use cases could be solved with one of the following alternative solutions:

• If your app is performing long-running downloads, consider using background fetch.
• If your app is synchronizing data from an external server, consider using background sync.

# Demo

Check out the Screen Wake Lock demo and demo source. Notice how the screen wake lock is automatically released when you switch tabs or apps.

# Screen Wake Locks in the OS task manager

You can use your operating system's task manager to see if an application is preventing your computer from sleeping. The video below shows the macOS Activity Monitor indicating that Chrome has an active screen wake lock that keeps the system awake.

# Feedback

The Web Platform Incubator Community Group (WICG) and the Chrome team want to hear about your thoughts and experiences with the Screen Wake Lock API.

# Tell us about the API design

Is there something about the API that doesn't work as expected? Or are there missing methods or properties that you need to implement your idea?

• File a spec issue on the Screen Wake Lock API GitHub repo or add your thoughts to an existing issue.

# Report a problem with the implementation

Did you find a bug with Chrome's implementation? Or is the implementation different from the spec?

• File a bug at https://new.crbug.com. Be sure to include as much detail as you can, provide simple instructions for reproducing the bug, and set Components to Blink>WakeLock. Glitch works great for sharing quick and easy repros.

# Show support for the API

Are you planning to use the Screen Wake Lock API? Your public support helps the Chrome team prioritize features and shows other browser vendors how critical it is to support them.

• Share how you plan to use the API on the WICG Discourse thread.
• Send a tweet to @ChromiumDev using the hashtag #WakeLock and let us know where and how you're using it.

# Helpful links

• Specification Candidate Recommendation | Editor's Draft
• Screen Wake Lock Demo | Screen Wake Lock Demo source
• Tracking bug
• ChromeStatus.com entry
• Experimenting with the Wake Lock API
• Blink Component: Blink>WakeLock

# Acknowledgements

Hero image by Kate Stone Matheson on Unsplash. Task manager video courtesy of Henry Lim.