PerformanceNavigationTiming: notRestoredReasons property

Value

When the associated PerformanceNavigationTiming object represents a history navigation, notRestoredReasons returns a NotRestoredReasons object.

When the PerformanceNavigationTiming object does not represent a history navigation, notRestoredReasons will return null. This is useful to determine whether bfcache is not relevant to a particular navigation (as opposed to notRestoredReasons not being supported, in which case it would return undefined).

The notRestoredReasons property may return null even when the navigation type is reported as back/forward. The circumstances under which this happens include:

• Duplicating a back/forward navigation in a new tab. In such cases, some browsers copy the navigation type from the original tab, but as these are not actually back/forward navigations, notRestoredReasons returns null.
• Restoring a back/forward navigation tab after a browser restart. Since the bfcache stores fully loaded pages including DOM and JavaScript heap, developers cannot make bfcache survive a full restart. The property therefore returns null because there was no bfcache entry to block restoration in the first place.

Examples

PerformanceNavigationTiming data can be obtained from the performance timeline using Performance.getEntriesByType() or PerformanceObserver.

For example, you could invoke the following function to return all PerformanceNavigationTiming objects currently present in the performance timeline and log their notRestoredReasons:

function returnNRR() {
  const navEntries = performance.getEntriesByType("navigation");
  for (let i = 0; i < navEntries.length; i++) {
    console.log(`Navigation entry ${i}`);
    let navEntry = navEntries[i];
    console.log(navEntry.notRestoredReasons);
  }
}

The PerformanceNavigationTiming.notRestoredReasons property returns an object with the following structure, which provides reasons why the current document was blocked from using the bfcache. In this example the top-level frame has no embedded child <iframe>s:

{
  "children": [],
  "id": null,
  "name": null,
  "reasons": [{ "reason": "unload-listener" }],
  "src": "",
  "url": "example.com"
}

Specifications

Browser compatibility

See also

• Monitoring bfcache blocking reasons
• PerformanceResourceTiming