PerformanceResourceTiming: workerStart property

{{APIRef("Performance API")}}{{AvailableInWorkers}}

The workerStart read-only property of the {{domxref("PerformanceResourceTiming")}} interface returns a {{domxref("DOMHighResTimeStamp")}} immediately before dispatching the {{domxref("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.

Value

The workerStart property can have the following values:

• A {{domxref("DOMHighResTimeStamp")}}.
• 0 if no service worker is used.
• 0 if the resource is a cross-origin request and no {{HTTPHeader("Timing-Allow-Origin")}} HTTP response header is used.

Examples

Measuring ServiceWorker processing time

The workerStart and {{domxref("PerformanceResourceTiming.fetchStart", "fetchStart")}} properties can be used to measure the processing time of a {{domxref("ServiceWorker")}}.

const workerProcessingTime = entry.fetchStart - entry.workerStart;

Example using a {{domxref("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 {{domxref("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`,
    );
  }
});

Cross-origin timing information

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 {{HTTPHeader("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

Specifications

{{Specifications}}

Browser compatibility

{{Compat}}

See also

• {{HTTPHeader("Timing-Allow-Origin")}}