Document PerformanceResourceTiming.responseStatus (#22584)

files/en-us/web/api/performanceresourcetiming/index.md

@@ -61,6 +61,8 @@ The interface also supports the following properties which are listed in the ord
   - : A {{domxref("DOMHighResTimeStamp")}} immediately before the browser starts requesting the resource from the server.
 - {{domxref('PerformanceResourceTiming.responseStart')}} {{ReadOnlyInline}}
   - : A {{domxref("DOMHighResTimeStamp")}} immediately after the browser receives the first byte of the response from the server.
+- {{domxref('PerformanceResourceTiming.responseStatus')}} {{experimental_inline}} {{ReadOnlyInline}}
+  - : A number representing the HTTP response status code returned when fetching the resource.
 - {{domxref('PerformanceResourceTiming.responseEnd')}} {{ReadOnlyInline}}
   - : A {{domxref("DOMHighResTimeStamp")}} immediately after the browser receives the last byte of the resource or immediately before the transport connection is closed, whichever comes first.
 - {{domxref('PerformanceResourceTiming.transferSize')}} {{ReadOnlyInline}}

files/en-us/web/api/performanceresourcetiming/responsestatus/index.md

@@ -0,0 +1,83 @@
+---
+title: PerformanceResourceTiming.responseStatus
+slug: Web/API/PerformanceResourceTiming/responseStatus
+page-type: web-api-instance-property
+tags:
+  - API
+  - Property
+  - Reference
+  - Web Performance
+  - Experimental
+browser-compat: api.PerformanceResourceTiming.responseStatus
+---
+
+{{APIRef("Performance API")}} {{SeeCompatTable}}
+
+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](/en-US/docs/Web/API/Fetch_API).
+
+## Value
+
+The `responseStatus` property can have the following values:
+
+- A number indicating the [HTTP response status code](/en-US/docs/Web/HTTP/Status) returned when fetching the resource.
+- `0` if the [CORS](/en-US/docs/Web/HTTP/CORS) check fails.
+- `0` for cross-origin {{HTMLElement("iframe")}} objects.
+
+## Examples
+
+### Checking if a cache was hit
+
+The `responseStatus` property can be used to check for cached resources with a {{HTTPStatus("304")}} `Not Modified` response status code.
+
+Using a {{domxref("PerformanceObserver")}}:
+
+```js
+const observer = new PerformanceObserver((list) => {
+  list.getEntries().forEach((entry) => {
+    if (entry.responseStatus === 304) {
+      console.log(`${entry.name} was loaded from cache`);
+    }
+  });
+});
+
+observer.observe({ type: "resource", buffered: true });
+```
+
+Or using {{domxref("Performance.getEntriesByType()")}}, which only shows `resource` performance entries present in the browser's performance timeline at the time you call this method:
+
+```js
+const resources = performance.getEntriesByType("resource");
+resources.forEach((entry) => {
+  if (entry.responseStatus === 304) {
+    console.log(`${entry.name} was loaded from cache`);
+  }
+});
+```
+
+Alternatively, if `responseStatus` is not available, you can check if the {{domxref("PerformanceResourceTiming.transferSize", "transferSize")}} property returned `0`.
+
+### 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](/en-US/docs/Web/HTTP/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:
+
+```http
+Access-Control-Allow-Origin: https://developer.mozilla.org
+```
+
+## Specifications
+
+{{Specifications}}
+
+## Browser compatibility
+
+{{Compat}}
+
+## See also
+
+- [HTTP response status code](/en-US/docs/Web/HTTP/Status)
+- {{domxref("Response.status")}}
+- [CORS](/en-US/docs/Web/HTTP/CORS)

files/en-us/web/api/performanceresourcetiming/tojson/index.md

@@ -66,6 +66,7 @@ This would log a JSON object like so:
   "secureConnectionStart": 110.80000001192093,
   "requestStart": 117.30000001192093,
   "responseStart": 120.40000000596046,
+  "responseStatus": 200,
   "responseEnd": 122.40000000596046,
   "transferSize": 0,
   "encodedBodySize": 880,