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

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.

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`);
  }
});

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

• Timing-Allow-Origin