PerformanceResourceTiming: responseStatus property

The responseStatus property can have the following values:

• A number indicating the HTTP response status code returned when fetching the resource.
• 0 if the CORS check fails.
• 0 for cross-origin <iframe> objects.

The responseStatus property can be used to check for cached resources with a 304 Not Modified response status code.

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) => {
    if (entry.responseStatus === 304) {
      console.log(`${entry.name} was loaded from cache`);
    }
  });
});

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) => {
  if (entry.responseStatus === 304) {
    console.log(`${entry.name} was loaded from cache`);
  }
});

Alternatively, if responseStatus is not available, you can check if the transferSize property returned 0.

If the value of the responseStatus property is 0, the resource might be a cross-origin request. To allow seeing cross-origin response status codes, the CORS Access-Control-Allow-Origin HTTP response header needs to be set.

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

Access-Control-Allow-Origin: https://developer.mozilla.org

• HTTP response status code
• Response.status
• CORS