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PerformanceResourceTiming: responseStart property

The responseStart read-only property returns a timestamp immediately after the browser receives the first byte of the response from the server, cache, or local resource.

Value

The responseStart property can have the following values:

  • A DOMHighResTimeStamp 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 no Timing-Allow-Origin HTTP response header is used.

Examples

Measuring request time

The responseStart and requestStart properties can be used to measure how long the request takes.

js

const request = entry.responseStart - entry.requestStart;

Example using a PerformanceObserver, which notifies of new resource performance entries as they are recorded in the browser's performance timeline. Use the buffered option to access entries from before the observer creation.

js

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 using 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) => {
  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 the responseStart 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

Browser compatibility

Desktop Mobile
Chrome Edge Firefox Internet Explorer Opera Safari WebView Android Chrome Android Firefox for Android Opera Android Safari on IOS Samsung Internet
responseStart 43 12 31 10 30 11 43 43 31 30 11 4.0

See also

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Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/API/PerformanceResourceTiming/responseStart