ServiceWorkerGlobalScope: sync event

Secure context: This feature is available only in secure contexts (HTTPS), in some or all supporting browsers.

Note: This feature is only available in Service Workers.

The sync event of the ServiceWorkerGlobalScope interface is fired when the page (or worker) that registered the event with the SyncManager is running and as soon as network connectivity is available.

This event is not cancelable and does not bubble.


Use the event name in methods like addEventListener(), or set an event handler property.

addEventListener("sync", (event) => {});

onsync = (event) => {};

Event type

Event properties

Inherits properties from its ancestor, ExtendableEvent and Event.

SyncEvent.tag Read only

Returns the developer-defined identifier for this SyncEvent.

SyncEvent.lastChance Read only

Returns true if the user agent will not make further synchronization attempts after the current attempt.


The following example shows how to respond to a sync event in the service worker.

self.addEventListener("sync", (event) => {
  if (event.tag === "sync-messages") {

You can also set up the event handler using the onsync property:

self.onsync = (event) => {
  // ...


Web Background Synchronization
# dom-serviceworkerglobalscope-onsync

Browser compatibility

BCD tables only load in the browser

See also