files/en-us/web/api/performanceresourcetiming/requeststart/index.md
{{APIRef("Performance API")}}{{AvailableInWorkers}}
The requestStart read-only property returns a {{domxref("DOMHighResTimeStamp","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 {{domxref("PerformanceResourceTiming.responseStart", "responseStart")}} - requestStart (see the example below).
The requestStart property can have the following values:
0 if the resource was instantaneously retrieved from a cache.0 if the resource is a cross-origin request and no {{HTTPHeader("Timing-Allow-Origin")}} HTTP response header is used.0 if the resource is a canceled request.When the {{domxref("PerformanceResourceTiming.firstInterimResponseStart", "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.
The requestStart and {{domxref("PerformanceResourceTiming.responseStart", "responseStart")}} properties can be used to measure how long the request takes.
const request = entry.responseStart - entry.requestStart;
Example using a {{domxref("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: ${request}ms`);
}
});
});
observer.observe({ type: "resource", buffered: true });
Example using {{domxref("Performance.getEntriesByType()")}}, which only shows resource performance entries present in the browser's performance timeline at the time you call this method:
const resources = performance.getEntriesByType("resource");
resources.forEach((entry) => {
const request = entry.responseStart - entry.requestStart;
if (request > 0) {
console.log(`${entry.name}: Request time: ${request}ms`);
}
});
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 {{HTTPHeader("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}}
{{Compat}}