PerformanceResourceTiming: requestStart 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 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 retires 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 noTiming-Allow-Origin
HTTP response header is used.
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://developer.mozilla.org
to see timing resources, the cross-origin resource should send:
Timing-Allow-Origin: https://developer.mozilla.org
Specifications
Specification |
---|
Resource Timing # dom-performanceresourcetiming-requeststart |
Browser compatibility
BCD tables only load in the browser