The unloadEventStart read-only property returns a DOMHighResTimeStamp representing the time immediately before the current document's unload event handler starts.

The unloadEventStart property can have the following values:

• A DOMHighResTimeStamp representing the time immediately before the current document's unload event handler starts.
• 0 if there is no previous document.
• 0 if the previous page was on another origin.

The unloadEventStart property can be used to measure how long it takes process theunload event handler.

This is useful to measure the time of long running unload event handlers.

window.addEventListener("unload", (event) => {
  // Some long running code
});

Example using a PerformanceObserver, which notifies of new navigation 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 unloadEventTime = entry.unloadEventEnd - entry.unloadEventStart;
    if (unloadEventTime > 0) {
      console.log(
        `${entry.name}: unload event handler time: ${unloadEventTime}ms`,
      );
    }
  });
});

observer.observe({ type: "navigation", buffered: true });

Example using Performance.getEntriesByType(), which only shows navigation performance entries present in the browser's performance timeline at the time you call this method:

const entries = performance.getEntriesByType("navigation");
entries.forEach((entry) => {
  const loadEventTime = entry.unloadEventEnd - entry.unloadEventStart;
  if (unloadEventTime > 0) {
    console.log(`${entry.name}:
      load event handler time: ${unloadEventTime}ms`);
  }
});

• unload event