PerformanceResourceTiming: firstInterimResponseStart property

Available in workers

The firstInterimResponseStart read-only property returns a timestamp immediately after the browser receives the first byte of the interim 1xx response (for example, 100 Continue or 103 Early Hints) from the server.

There is no end property for firstInterimResponseStart.

Value

The firstInterimResponseStart property can have the following values:

• A DOMHighResTimeStamp immediately after the browser receives the first interim bytes of the response from the server.
• 0 if the resource sent no interim response.
• 0 if the resource is a cross-origin request and no Timing-Allow-Origin HTTP response header is used.

[!NOTE] As Early Hints are typically only supported on the main navigation request, which is by definition same-origin, a 0 typically indicates Early Hints were not used.

When the firstInterimResponseStart is non-zero, that indicates it should be the same value as requestStart for supporting browsers.

Examples

Measuring request time

The firstInterimResponseStart and requestStart properties can be used to measure how long it takes to the browser to receive an interim response after the sending the request.

const request = entry.firstInterimResponseStart - entry.requestStart;

The following example uses a PerformanceObserver to notify of new resource performance entries as they are recorded in the browser’s performance timeline. The buffered option is used for accessing entries from before the observer creation.

const observer = new PerformanceObserver((list) => {
  list.getEntries().forEach((entry) => {
    const request = entry.firstInterimResponseStart - entry.requestStart;
    if (request > 0) {
      console.log(`${entry.name}: Interim response time: ${request}ms`);
    }
  });
});

observer.observe({ type: "resource", buffered: true });

The following example uses getEntriesByType(), which only shows resource performance entries present in the browser’s performance timeline at the time you call the method.

const resources = performance.getEntriesByType("resource");
resources.forEach((entry) => {
  const request = entry.firstInterimResponseStart - entry.requestStart;
  if (request > 0) {
    console.log(`${entry.name}: Interim response time: ${request}ms`);
  }
});

Cross-origin timing information

If the value of the firstInterimResponseStart 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

Specifications

Browser compatibility

See also

• Timing-Allow-Origin
• finalResponseHeadersStart