How to use the `PerformanceNavigationTiming` API to get page load time?

I am trying to to use the PerformanceNavigationTiming API to generate a page load metric.

The MDN API document linked above says that the PerformanceEntry.duration should give me what I need because it:

[r]eturns a timestamp that is the difference between the PerformanceNavigationTiming.loadEventEnd and PerformanceEntry.startTime properties.

However, when I check this property, I get simply 0. I’m accessing this API from within a React hook that runs a useEffect function that wait for the window load event and then checks the api like so:

export const useReportPageLoadTime = () => {
  useEffect(() => {
    const reportTime = () => {
      let navPerformance: PerformanceEntry
      navPerformance = window.performance.getEntriesByType('navigation')[0]

      console.log({
        duration: navPerformance.duration,
        blob: navPerformance.toJSON()
      })
    }

    if (document.readyState === 'complete') {
      reportTime()
      return null
    } else {
      window.addEventListener('load', reportTime)
      return () => window.removeEventListener('load', reportTime)
    }
  }, [])
}

As you can see there, I also call toJSON on the performance entry and indeed it shows that the values upon which duration (startTime and loadEventEnd) are both 0 as well:

Does anyone know why I am getting this value?

Answer

I was finally able to get this to work using a different method than the event listener. It certainly is logical that the data should be ready when the load event fires, but the only way I was able to get the data was to use another feature of the Performance API: the PerformanceObserver, which fires a callback when a new piece of data has become available.

Here is the code that worked for me:

export const useReportPageLoadMetrics = () => {
  useEffect(() => {
    const perfObserver = new PerformanceObserver((observedEntries) => {
      const entry: PerformanceEntry =
        observedEntries.getEntriesByType('navigation')[0]
      console.log('pageload time: ', entry.duration)
    })

    perfObserver.observe({
      type: 'navigation',
      buffered: true
    })
  }, [])
}