The workerStart read-only property of the PerformanceResourceTiming interface returns a DOMHighResTimeStamp immediately before dispatching the FetchEvent if a Service Worker thread is already running, or immediately before starting the Service Worker thread if it is not already running. If the resource is not intercepted by a Service Worker the property will always return 0.

The workerStart property can have the following values:

• A DOMHighResTimeStamp.
• 0 if no service worker is used.
• 0 if the resource is a cross-origin request and no Timing-Allow-Origin HTTP response header is used.

The workerStart and fetchStart properties can be used to measure the processing time of a ServiceWorker.

const workerProcessingTime = entry.fetchStart - entry.workerStart;

Example using a PerformanceObserver, which notifies of new resource performance entries as they are recorded in the browser's performance timeline. Use the buffered option to access entries from before the observer creation.

const observer = new PerformanceObserver((list) => {
  list.getEntries().forEach((entry) => {
    const workerProcessingTime = entry.fetchStart - entry.workerStart;
    if (workerProcessingTime > 0) {
      console.log(
        `${entry.name}: Worker processing time: ${workerProcessingTime}ms`,
      );
    }
  });
});

observer.observe({ type: "resource", buffered: true });

Example using Performance.getEntriesByType(), which only shows resource performance entries present in the browser's performance timeline at the time you call this method:

const resources = performance.getEntriesByType("resource");
resources.forEach((entry) => {
  const workerProcessingTime = entry.fetchStart - entry.workerStart;
  if (workerProcessingTime > 0) {
    console.log(
      `${entry.name}: Worker processing time: ${workerProcessingTime}ms`,
    );
  }
});

If the value of the workerStart property is 0, the resource might be a cross-origin request. To allow seeing cross-origin timing information, the Timing-Allow-Origin HTTP response header needs to be set.

For example, to allow https://developer.mozilla.org to see timing resources, the cross-origin resource should send:

Timing-Allow-Origin: https://developer.mozilla.org

• Timing-Allow-Origin