PerformanceResourceTiming: domainLookupStart property
The domainLookupStart
read-only property returns the timestamp
immediately before the browser starts the domain name lookup for the resource.
Value
The domainLookupStart
property can have the following values:
- A
DOMHighResTimeStamp
immediately before the browser starts the domain name lookup for the resource. 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 DNS lookup time
The domainLookupStart
and domainLookupEnd
properties can be used to measure how long it takes for the DNS lookup to happen.
js
const dns = entry.domainLookupEnd - entry.domainLookupStart;
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 dns = entry.domainLookupEnd - entry.domainLookupStart;
if (dns > 0) {
console.log(`${entry.name}: DNS lookup duration: ${dns}ms`);
}
});
});
observer.observe({ type: "resource", buffered: true });
Example using Performance.getEntriesByType()
, which only shows resource
performance entries present in the browser's performance timeline at the time you call this method:
js
const resources = performance.getEntriesByType("resource");
resources.forEach((entry) => {
const dns = entry.domainLookupEnd - entry.domainLookupStart;
if (dns > 0) {
console.log(`${entry.name}: DNS lookup duration: ${dns}ms`);
}
});
Cross-origin timing information
If the value of the domainLookupStart
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-domainlookupstart |
Browser compatibility
BCD tables only load in the browser