The toJSON() method of the PerformanceServerTiming interface is a serializer; it returns a JSON representation of the PerformanceServerTiming object.

None.

A JSON object that is the serialization of the PerformanceServerTiming object.

Server timing metrics require the server to send the Server-Timing header. For example:

Server-Timing: cache;desc="Cache Read";dur=23.2

The serverTiming entries can live on navigation and resource entries.

Example using a PerformanceObserver, which notifies of new navigation and 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) => {
    entry.serverTiming.forEach((serverEntry) => {
      console.log(serverEntry.toJSON());
    });
  });
});

["navigation", "resource"].forEach((type) =>
  observer.observe({ type, buffered: true }),
);

This would log a JSON object like so:

{
  "name": "cache",
  "duration": 23.2,
  "description": "Cache Read"
}

To get a JSON string, you can use JSON.stringify(serverEntry) directly; it will call toJSON() automatically.