PerformanceResourceTiming: responseStart property

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since September 2017.

Note: This feature is available in Web Workers.

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

Specification
Resource Timing
# dom-performanceresourcetiming-responsestart

Browser compatibility

BCD tables only load in the browser

See also