PerformanceResourceTiming: requestStart property

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.

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 Performance.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://big.rakal.top to see timing resources, the cross-origin resource should send:

Timing-Allow-Origin: https://big.rakal.top

Specifications

Browser compatibility

See also

• Timing-Allow-Origin