PerformanceResourceTiming: connectStart property

The connectStart read-only property returns the timestamp immediately before the user agent starts establishing the connection to the server to retrieve the resource.

Value

The connectStart property can have the following values:

• A DOMHighResTimeStamp immediately before the browser starts to establish the connection to the server to retrieve 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 TCP handshake time

The connectStart and connectEnd properties can be used to measure how long it takes for the TCP handshake to happen.

const tcp = entry.connectEnd - entry.connectStart;

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 tcp = entry.connectEnd - entry.connectStart;
    if (tcp > 0) {
      console.log(`${entry.name}: TCP handshake duration: ${tcp}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 tcp = entry.connectEnd - entry.connectStart;
  if (tcp > 0) {
    console.log(`${entry.name}: TCP handshake duration: ${tcp}ms`);
  }
});

Cross-origin timing information

If the value of the connectStart 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