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 no Timing-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.

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.

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:

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:

Timing-Allow-Origin: https://developer.mozilla.org

Specifications

• Resource Timing

See also

• Timing-Allow-Origin