Add perf data experiment

packages/next/build/index.ts

@@ -1159,6 +1159,15 @@ export default async function build(
     printCustomRoutes({ redirects, rewrites, headers })
   }
 
+  if (config.experimental.analyticsId) {
+    console.log(
+      chalk.bold.green('Next.js Analytics') +
+        ' is enabled for this production build. ' +
+        "You'll receive a Real Experience Score computed by all of your visitors."
+    )
+    console.log('')
+  }
+
   if (tracer) {
     const parsedResults = await tracer.profiler.stopProfiling()
     await new Promise((resolve) => {

packages/next/build/webpack-config.ts

@@ -1015,6 +1015,9 @@ export default async function getBaseWebpackConfig(
         'process.env.__NEXT_i18n_SUPPORT': JSON.stringify(
           !!config.experimental.i18n
         ),
+        'process.env.__NEXT_ANALYTICS_ID': JSON.stringify(
+          config.experimental.analyticsId
+        ),
         ...(isServer
           ? {
               // Fix bad-actors in the npm ecosystem (e.g. `node-formidable`)

packages/next/client/performance-relayer.ts

@@ -8,13 +8,53 @@ import {
   ReportHandler,
 } from 'web-vitals'
 
+const initialHref = location.href
 let isRegistered = false
 let userReportHandler: ReportHandler | undefined
 
 function onReport(metric: Metric) {
   if (userReportHandler) {
     userReportHandler(metric)
   }
+
+  // This code is not shipped, executed, or present in the client-side
+  // JavaScript bundle unless explicitly enabled in your application.
+  //
+  // When this feature is enabled, we'll make it very clear by printing a
+  // message during the build (`next build`).
+  if (
+    process.env.NODE_ENV === 'production' &&
+    // This field is empty unless you explicitly configure it:
+    process.env.__NEXT_ANALYTICS_ID
+  ) {
+    const body: Record<string, string> = {
+      dsn: process.env.__NEXT_ANALYTICS_ID,
+      id: metric.id,
+      page: window.__NEXT_DATA__.page,
+      href: initialHref,
+      event_name: metric.name,
+      value: metric.value.toString(),
+      speed:
+        'connection' in navigator &&
+        navigator['connection'] &&
+        'effectiveType' in navigator['connection']
+          ? (navigator['connection']['effectiveType'] as string)
+          : '',
+    }
+
+    const blob = new Blob([new URLSearchParams(body).toString()], {
+      // This content type is necessary for `sendBeacon`:
+      type: 'application/x-www-form-urlencoded',
+    })
+    const vitalsUrl = 'https://vitals.vercel-analytics.com/v1/vitals'
+    ;(navigator.sendBeacon && navigator.sendBeacon(vitalsUrl, blob)) ||
+      fetch(vitalsUrl, {
+        body: blob,
+        method: 'POST',
+        credentials: 'omit',
+        keepalive: true,
+      })
+  }
 }
 
 export default (onPerfEntry?: ReportHandler) => {

packages/next/next-server/server/config.ts

@@ -60,6 +60,7 @@ const defaultConfig: { [key: string]: any } = {
     optimizeImages: false,
     scrollRestoration: false,
     i18n: false,
+    analyticsId: process.env.VERCEL_ANALYTICS_ID || '',
   },
   future: {
     excludeDefaultMomentLocales: false,

test/integration/relay-analytics-disabled/pages/_app.js

@@ -0,0 +1,15 @@
+/* global localStorage */
+/* eslint-disable camelcase */
+import App from 'next/app'
+
+export default class MyApp extends App {}
+
+/*
+  Method is experimental and will eventually be handled in a Next.js plugin
+*/
+export function reportWebVitals(data) {
+  localStorage.setItem(
+    data.name || data.entryType,
+    data.value !== undefined ? data.value : data.startTime
+  )
+}

test/integration/relay-analytics-disabled/pages/index.js

@@ -0,0 +1,25 @@
+if (typeof navigator !== 'undefined') {
+  window.__BEACONS = window.__BEACONS || []
+
+  navigator.sendBeacon = async function () {
+    const args = await Promise.all(
+      [...arguments].map((v) => {
+        if (v instanceof Blob) {
+          return v.text()
+        }
+        return v
+      })
+    )
+
+    window.__BEACONS.push(args)
+  }
+}
+
+export default () => {
+  return (
+    <div>
+      <h1>Foo!</h1>
+      <h2>bar!</h2>
+    </div>
+  )
+}

test/integration/relay-analytics-disabled/test/index.test.js

@@ -0,0 +1,72 @@
+/* eslint-env jest */
+
+import fs from 'fs-extra'
+import { findPort, killApp, nextBuild, nextStart } from 'next-test-utils'
+import webdriver from 'next-webdriver'
+import path, { join } from 'path'
+
+const appDir = join(__dirname, '../')
+let appPort
+let server
+jest.setTimeout(1000 * 60 * 2)
+
+let buildManifest
+
+describe('Analytics relayer (disabled)', () => {
+  let stdout
+  beforeAll(async () => {
+    appPort = await findPort()
+    ;({ stdout } = await nextBuild(appDir, [], {
+      stdout: true,
+    }))
+    buildManifest = require(path.join(
+      appDir,
+      '.next/build-manifest.json'
+    ), 'utf8')
+    server = await nextStart(appDir, appPort)
+  })
+  afterAll(() => killApp(server))
+
+  it('Does not relay any data', async () => {
+    const browser = await webdriver(appPort, '/')
+    await browser.waitForElementByCss('h1')
+    const h1Text = await browser.elementByCss('h1').text()
+    const firstContentfulPaint = parseFloat(
+      await browser.eval('localStorage.getItem("FCP")')
+    )
+
+    expect(h1Text).toMatch(/Foo!/)
+
+    expect(firstContentfulPaint).not.toBeNaN()
+    expect(firstContentfulPaint).toBeGreaterThan(0)
+
+    const beacons = (await browser.eval('window.__BEACONS')).map(([, value]) =>
+      Object.fromEntries(new URLSearchParams(value))
+    )
+
+    expect(beacons.length).toBe(0)
+
+    expect(stdout).not.toMatch('Next.js Analytics')
+
+    await browser.close()
+  })
+
+  it('Does not include the code', async () => {
+    const pageFiles = [
+      ...new Set([
+        ...buildManifest.pages['/'].filter((file) => file.endsWith('.js')),
+        ...buildManifest.pages['/_app'].filter((file) => file.endsWith('.js')),
+      ]),
+    ]
+
+    expect(pageFiles.length).toBeGreaterThan(1)
+
+    for (const pageFile of pageFiles) {
+      const content = await fs.readFile(
+        path.join(appDir, '.next', pageFile),
+        'utf8'
+      )
+      expect(content).not.toMatch('vercel-analytics')
+    }
+  })
+})

test/integration/relay-analytics/pages/index.js

@@ -1,3 +1,20 @@
+if (typeof navigator !== 'undefined') {
+  window.__BEACONS = window.__BEACONS || []
+
+  navigator.sendBeacon = async function () {
+    const args = await Promise.all(
+      [...arguments].map((v) => {
+        if (v instanceof Blob) {
+          return v.text()
+        }
+        return v
+      })
+    )
+
+    window.__BEACONS.push(args)
+  }
+}
+
 export default () => {
   return (
     <div>

test/integration/relay-analytics/test/index.test.js

@@ -10,9 +10,13 @@ let server
 jest.setTimeout(1000 * 60 * 2)
 
 describe('Analytics relayer', () => {
+  let stdout
   beforeAll(async () => {
     appPort = await findPort()
-    await nextBuild(appDir)
+    ;({ stdout } = await nextBuild(appDir, [], {
+      env: { VERCEL_ANALYTICS_ID: 'test' },
+      stdout: true,
+    }))
     server = await nextStart(appDir, appPort)
   })
   afterAll(() => killApp(server))
@@ -54,6 +58,35 @@ describe('Analytics relayer', () => {
     expect(cls).not.toBeNull()
     expect(largestContentfulPaint).not.toBeNaN()
     expect(largestContentfulPaint).toBeGreaterThan(0)
+
+    const beacons = (await browser.eval('window.__BEACONS')).map(([, value]) =>
+      Object.fromEntries(new URLSearchParams(value))
+    )
+
+    beacons.sort((a, b) => a.event_name.localeCompare(b.event_name))
+
+    expect(beacons.length).toBe(2)
+    expect(beacons[0]).toMatchObject({
+      dsn: 'test',
+      event_name: 'FCP',
+      href: expect.stringMatching('http://'),
+      id: expect.stringContaining('-'),
+      page: '/',
+      speed: '4g',
+      value: expect.stringContaining('.'),
+    })
+    expect(beacons[1]).toMatchObject({
+      dsn: 'test',
+      event_name: 'TTFB',
+      href: expect.stringMatching('http://'),
+      id: expect.stringContaining('-'),
+      page: '/',
+      speed: '4g',
+      value: expect.stringContaining('.'),
+    })
+
+    expect(stdout).toMatch('Next.js Analytics')
+
     await browser.close()
   })
 })