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 for determining whether bfcache is not relevant to a particular navigation (as opposed to notRestoredReasons not being supported, in which case it would return undefined).

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