Baseline Widely available
Note: This feature is available in Web Workers.
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:
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
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