Baseline Widely available
Note: This feature is available in Web Workers.
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:
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
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