PerformanceResourceTiming: responseStatus property

The responseStatus read-only property represents the HTTP response status code returned when fetching the resource.

This property maps to Response.status from the Fetch API.

Value

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.

Examples

Checking if a cache was hit

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.

Cross-origin response status codes

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

Specifications

Browser compatibility

See also

• HTTP response status code
• Response.status
• CORS