Next.js
Next.js 13.5
Using App Router
API Reference
Functions
useReportWebVitals

useReportWebVitals

The useReportWebVitals hook allows you to report Core Web Vitals ↗, and can be used in combination with your analytics service.

app/_components/web-vitals.js

'use client'
 
import { useReportWebVitals } from 'next/web-vitals'
 
export function WebVitals() {
  useReportWebVitals((metric) => {
    console.log(metric)
  })
}

app/layout.js

import { WebVitals } from './_components/web-vitals'
 
export default function Layout({ children }) {
  return (
    <html>
      <body>
        <WebVitals />
        {children}
      </body>
    </html>
  )
}

Since the useReportWebVitals hook requires the "use client" directive, the most performant approach is to create a separate component that the root layout imports. This confines the client boundary exclusively to the WebVitals component.

useReportWebVitals

The metric object passed as the hook’s argument consists of a number of properties:

  • id: Unique identifier for the metric in the context of the current page load
  • name: The name of the performance metric. Possible values include names of Web Vitals metrics (TTFB, FCP, LCP, FID, CLS) specific to a web application.
  • delta: The difference between the current value and the previous value of the metric. The value is typically in milliseconds and represents the change in the metric’s value over time.
  • entries: An array of Performance Entries ↗ associated with the metric. These entries provide detailed information about the performance events related to the metric.
  • navigationType: Indicates the type of navigation ↗ that triggered the metric collection. Possible values include "navigate", "reload", "back_forward", and "prerender".
  • rating: A qualitative rating of the metric value, providing an assessment of the performance. Possible values are "good", "needs-improvement", and "poor". The rating is typically determined by comparing the metric value against predefined thresholds that indicate acceptable or suboptimal performance.
  • value: The actual value or duration of the performance entry, typically in milliseconds. The value provides a quantitative measure of the performance aspect being tracked by the metric. The source of the value depends on the specific metric being measured and can come from various Performance API ↗s.

Web Vitals

Web Vitals ↗ are a set of useful metrics that aim to capture the user experience of a web page. The following web vitals are all included:

You can handle all the results of these metrics using the name property.

app/components/web-vitals.tsx

'use client'
 
import { useReportWebVitals } from 'next/web-vitals'
 
export function WebVitals() {
  useReportWebVitals((metric) => {
    switch (metric.name) {
      case 'FCP': {
        // handle FCP results
      }
      case 'LCP': {
        // handle LCP results
      }
      // ...
    }
  })
}

Usage on Vercel

Vercel Speed Insights ↗ are automatically configured on Vercel deployments, and don’t require the use of useReportWebVitals. This hook is useful in local development, or if you’re using a different analytics service.

Sending results to external systems

You can send results to any endpoint to measure and track real user performance on your site. For example:

useReportWebVitals((metric) => {
  const body = JSON.stringify(metric)
  const url = 'https://example.com/analytics'
 
  // Use `navigator.sendBeacon()` if available, falling back to `fetch()`.
  if (navigator.sendBeacon) {
    navigator.sendBeacon(url, body)
  } else {
    fetch(url, { body, method: 'POST', keepalive: true })
  }
})

Good to know: If you use Google Analytics ↗, using the id value can allow you to construct metric distributions manually (to calculate percentiles, etc.)

useReportWebVitals(metric => {
  // Use `window.gtag` if you initialized Google Analytics as this example:
  // https://github.com/vercel/next.js/blob/canary/examples/with-google-analytics/pages/_app.js
  window.gtag('event', metric.name, {
    value: Math.round(metric.name === 'CLS' ? metric.value * 1000 : metric.value), // values must be integers
    event_label: metric.id, // id unique to current page load
    non_interaction: true, // avoids affecting bounce rate.
  });
}

Read more about sending results to Google Analytics ↗.

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