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.

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 was instantaneously retrieved from a cache.
• 0 if the resource is a cross-origin request and no Timing-Allow-Origin HTTP response header is used.

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 Performance.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`);
  }
});

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

• Timing-Allow-Origin