The connectEnd read-only property returns the timestamp immediately after the browser finishes establishing the connection to the server to retrieve the resource. The timestamp value includes the time interval to establish the transport connection, as well as other time intervals such as TLS handshake and SOCKS authentication.

The connectEnd property can have the following values:

• A DOMHighResTimeStamp representing the time after a connection is established.
• 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 connectEnd and connectStart 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 connectEnd 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