 Cecilia Cong [LinkedIn](https://www.linkedin.com/in/cecilia-cong-697368237)
## The modern way
Periodic background sync lets you show fresh content when a progressive web app or service
worker-backed page is launched. It does this by downloading data in the background when the app or
page is not being used.
### Using the Periodic Background Sync API
After the service worker is installed, use the
[Permissions API](https://developer.mozilla.org/docs/Web/API/Permissions_API) to query for
`periodic-background-sync`. You can do this from either a window or a service worker context.
const status = await navigator.permissions.query({
name: 'periodic-background-sync',
});
if (status.state === 'granted') {
// Periodic background sync can be used.
} else {
// Periodic background sync cannot be used.
}
Registering a periodic sync requires both a tag and a minimum synchronization interval
(`minInterval`). The tag identifies the registered sync so that multiple syncs can be registered. In
the example below, the tag name is `'content-sync'` and the `minInterval` is one day.
navigator.serviceWorker.ready.then(async registration => {
try {
await registration.periodicSync.register('get-cats', { minInterval: 24 * 60 * 60 * 1000 });
console.log('Periodic background sync registered.');
} catch (err) {
console.error(err.name, err.message);
}
});
Call `periodicSync.getTags()` to retrieve an array of registration tags. The example below uses tag
names to confirm that cache updating is active to avoid updating again.
const registration = await navigator.serviceWorker.ready;
if ('periodicSync' in registration) {
const tags = await registration.periodicSync.getTags();
// Only update content if sync isn't set up.
if (!tags.includes('content-sync')) {
updateContentOnPageLoad();
}
} else {
// If periodic background sync isn't supported, always update.
updateContentOnPageLoad();
}
To respond to a periodic background sync event add a `periodicsync` event handler to your service
worker. The event object passed to it will contain a tag parameter matching the value used during
registration. For example, if a periodic background sync was registered with the name
`'content-sync'`, then `event.tag` will be `'content-sync'`.
self.addEventListener('periodicsync', (event) => {
if (event.tag === 'content-sync') {
event.waitUntil(syncContent());
}
});
### Browser compatibility
Browser Support
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[Source](https://developer.mozilla.org/docs/Web/API/PeriodicSyncManager)
## The classic way
Rather than updating data in the background so it's ready when the user loads the app, the classic
way simply consists of updating the data on load.
## Further reading
- [Richer offline experiences with the Periodic Background Sync API](https://web.dev/i18n/en/periodic-background-sync)
- [Web Periodic Background Synchronization API](https://developer.mozilla.org/docs/Web/API/Web_Periodic_Background_Synchronization_API)
## Demo
### HTML
How to periodically synchronize data in the background
How to periodically synchronize data in the background
Periodic background sync can be used. Install the app first.
Periodic background sync cannot be used.
Last updated
Never
Registered tags
### CSS
html {
box-sizing: border-box;
font-family: system-ui, sans-serif;
color-scheme: dark light;
}
*,
*:before,
*:after {
box-sizing: inherit;
}
body {
margin: 1rem;
}
### JS
const available = document.querySelector('.available');
const notAvailable = document.querySelector('.not-available');
const ul = document.querySelector('ul');
const lastUpdated = document.querySelector('.last-updated');
const updateContent = async () => {
const data = await fetch(
'https://worldtimeapi.org/api/timezone/Europe/London.json'
).then((response) => response.json());
return new Date(data.unixtime * 1000);
};
const registerPeriodicBackgroundSync = async (registration) => {
const status = await navigator.permissions.query({
name: 'periodic-background-sync',
});
if (status.state === 'granted' && 'periodicSync' in registration) {
try {
// Register the periodic background sync.
await registration.periodicSync.register('content-sync', {
// An interval of one day.
minInterval: 24 * 60 * 60 * 1000,
});
available.hidden = false;
notAvailable.hidden = true;
// List registered periodic background sync tags.
const tags = await registration.periodicSync.getTags();
if (tags.length) {
ul.innerHTML = '';
}
tags.forEach((tag) => {
const li = document.createElement('li');
li.textContent = tag;
ul.append(li);
});
// Update the user interface with the last periodic background sync data.
const backgroundSyncCache = await caches.open('periodic-background-sync');
if (backgroundSyncCache) {
const backgroundSyncResponse =
backgroundSyncCache.match('/last-updated');
if (backgroundSyncResponse) {
lastUpdated.textContent = `${await fetch('/last-updated').then(
(response) => response.text()
)} (periodic background-sync)`;
}
}
// Listen for incoming periodic background sync messages.
navigator.serviceWorker.addEventListener('message', async (event) => {
if (event.data.tag === 'content-sync') {
lastUpdated.textContent = `${await updateContent()} (periodic background sync)`;
}
});
} catch (err) {
console.error(err.name, err.message);
available.hidden = true;
notAvailable.hidden = false;
lastUpdated.textContent = 'Never';
}
} else {
available.hidden = true;
notAvailable.hidden = false;
lastUpdated.textContent = `${await updateContent()} (manual)`;
}
};
if ('serviceWorker' in navigator) {
window.addEventListener('load', async () => {
const registration = await navigator.serviceWorker.register('./sw.js');
console.log('Service worker registered for scope', registration.scope);
await registerPeriodicBackgroundSync(registration);
});
}