Experimental: This is an experimental technology
Check the Browser compatibility table carefully before using this in production.

The finalResponseHeadersStart read-only property returns a timestamp immediately after the browser receives the first byte of the final document response (for example, 200 OK) from the server.

This differs from firstInterimResponseStart), as this starts from the first bytes of any response including interim responses (for example, 103 Early Hints) with the final response coming potentially much later.

When there are no interim responses, requestStart is the same as finalResponseHeadersStart and firstInterimResponseStart is 0.

There is no end property for finalResponseHeadersStart.

Value

The finalResponseHeadersStart property can have the following values:

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

Examples

Measuring request time

The finalResponseHeadersStart and requestStart properties can be used to measure how long it takes for the browser to start receive the final response after the sending the request.

const request = entry.finalResponseHeadersStart - 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.finalResponseHeadersStart - entry.requestStart;
    if (request > 0) {
      console.log(`${entry.name}: final 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.finalResponseHeadersStart - entry.requestStart;
  if (request > 0) {
    console.log(`${entry.name}: final response time: ${request}ms`);
  }
});

The following example shows how to measure the time between the first and final response headers.

const observer = new PerformanceObserver((list) => {
  list.getEntries().forEach((entry) => {
    const diff = entry.finalResponseHeadersStart - entry.responseStart;
    if ((entry.finalResponseHeadersStart > 0) & (diff > 0)) {
      console.log(
        `${entry.name}: time between first and final response start: ${diff}ms`,
      );
    }
  });
});

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

Cross-origin timing information

If the value of the finalResponseHeadersStart 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:

http

Timing-Allow-Origin: https://developer.mozilla.org

Specifications

Specification
Resource Timing # dom-performanceresourcetiming-finalresponseheadersstart