PerformanceResourceTiming: workerStart property

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.

Value

The workerStart property can have the following values:

Examples

Measuring ServiceWorker processing time

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

js
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.

js
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:

js
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 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:

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

Specifications

Specification
Resource Timing
# dom-performanceresourcetiming-workerstart

Browser compatibility

BCD tables only load in the browser

See also