files/en-us/web/api/performanceresourcetiming/responsestatus/index.md
{{APIRef("Performance API")}}{{AvailableInWorkers}}
The responseStatus read-only property represents the HTTP response status code returned when fetching the resource.
This property maps to {{domxref("Response.status")}} from the Fetch API.
The responseStatus property can have the following values:
0 if the CORS check fails.0 for cross-origin {{HTMLElement("iframe")}} objects.The responseStatus property can be used to check for cached resources with a {{HTTPStatus("304")}} Not Modified response status code.
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) => {
if (entry.responseStatus === 304) {
console.log(`${entry.name} was loaded from cache`);
}
});
});
observer.observe({ type: "resource", buffered: true });
Example using {{domxref("Performance.getEntriesByType()")}}, which only shows resource performance entries present in the browser's performance timeline at the time you call this method:
const resources = performance.getEntriesByType("resource");
resources.forEach((entry) => {
if (entry.responseStatus === 304) {
console.log(`${entry.name} was loaded from cache`);
}
});
Alternatively, if responseStatus is not available, you can check if the {{domxref("PerformanceResourceTiming.transferSize", "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 {{HTTPHeader("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}}
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