TheresponseStart read-only property returns atimestamp immediately after the browser receives the first byte of the response from the server, cache, or local resource.

Value

TheresponseStart property can have the following values:

• ADOMHighResTimeStamp immediately after the browser receives the first byte 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 noTiming-Allow-Origin HTTP response header is used.
• 0 if the resource is a canceled request.

Examples

Measuring request time

TheresponseStart andrequestStart properties can be used to measure how long the request takes.

const request = entry.responseStart - entry.requestStart;

Example using aPerformanceObserver, which notifies of newresource performance entries as they are recorded in the browser's performance timeline. Use thebuffered option to access entries from before the observer creation.

const observer = new PerformanceObserver((list) => {  list.getEntries().forEach((entry) => {    const request = entry.responseStart - entry.requestStart;    if (request > 0) {      console.log(`${entry.name}: Request time: ${request}ms`);    }  });});observer.observe({ type: "resource", buffered: true });

Example usingPerformance.getEntriesByType(), which only showsresource performance entries present in the browser's performance timeline at the time you call this method:

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

Cross-origin timing information

If the value of theresponseStart property is0, the resource might be a cross-origin request. To allow seeing cross-origin timing information, theTiming-Allow-Origin HTTP response header needs to be set.

For example, to allowhttps://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