PerformanceResourceTiming: requestStart property

Available in workers

The requestStart read-only property returns a timestamp of the time immediately before the browser starts requesting the resource from the server, cache, or local resource. If the transport connection fails and the browser retries the request, the value returned will be the start of the retry request.

There is no end property for requestStart. To measure the request time, calculate responseStart - requestStart (see the example below).

Value

The requestStart property can have the following values:

• A DOMHighResTimeStamp representing the time immediately before the browser starts requesting the resource 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.
• 0 if the resource is a canceled request.

When the firstInterimResponseStart is non-zero, that indicates it should be the same value as requestStart for supporting browsers.

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

Examples

Measuring request time

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

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.

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 getEntriesByType(), which only shows resource 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 the requestStart 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

Specifications

Browser compatibility

See also

• Timing-Allow-Origin