ThedomainLookupStart read-only property returns thetimestamp immediately before the browser starts the domain name lookup for the resource.

Value

ThedomainLookupStart property can have the following values:

• ADOMHighResTimeStamp 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

ThedomainLookupStart anddomainLookupEnd properties can be used to measure how long it takes for the DNS lookup to happen.

const dns = entry.domainLookupEnd - entry.domainLookupStart;

Example using aPerformanceObserver, which notifies of newresource performance entries as they are recorded in the browser's performance timeline. Use thebuffered option to access entries from before the observer creation.

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 usingPerformance.getEntriesByType(), which only showsresource 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 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 thedomainLookupStart property is0, the resource might be a cross-origin request. To allow seeing cross-origin timing information, theTiming-Allow-Origin HTTP response header needs to be set.

For example, to allowhttps://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