Record presence of reportWebVitals

packages/next/build/index.ts

@@ -69,6 +69,7 @@ import {
   PageInfo,
   printCustomRoutes,
   printTreeView,
+  getNamedExports,
 } from './utils'
 import getBaseWebpackConfig from './webpack-config'
 import { writeBuildId } from './write-build-id'
@@ -466,6 +467,7 @@ export default async function build(dir: string, conf = null): Promise<void> {
   )
 
   let customAppGetInitialProps: boolean | undefined
+  let namedExports: Array<string> | undefined
 
   process.env.NEXT_PHASE = PHASE_PRODUCTION_BUILD
 
@@ -538,6 +540,17 @@ export default async function build(dir: string, conf = null): Promise<void> {
           runtimeEnvConfig
         )
 
+        namedExports = getNamedExports(
+          isLikeServerless
+            ? serverBundle
+            : path.join(
+                distDir,
+                SERVER_DIRECTORY,
+                `/static/${buildId}/pages/_app.js`
+              ),
+          runtimeEnvConfig
+        )
+
         if (customAppGetInitialProps) {
           console.warn(
             chalk.bold.yellow(`Warning: `) +
@@ -921,6 +934,7 @@ export default async function build(dir: string, conf = null): Promise<void> {
         pagePaths.length -
         (staticPages.size + ssgPages.size + serverPropsPages.size),
       hasStatic404: useStatic404,
+      hasReportWebVitals: namedExports?.includes('reportWebVitals') ?? false,
     })
   )
 

packages/next/build/utils.ts

@@ -789,3 +789,11 @@ export function hasCustomGetInitialProps(
   }
   return mod.getInitialProps !== mod.origGetInitialProps
 }
+
+export function getNamedExports(
+  bundle: string,
+  runtimeEnvConfig: any
+): Array<string> {
+  require('../next-server/lib/runtime-config').setConfig(runtimeEnvConfig)
+  return Object.keys(require(bundle))
+}

packages/next/telemetry/events/build.ts

@@ -44,6 +44,7 @@ type EventBuildOptimized = {
   hasDunderPages: boolean
   hasTestPages: boolean
   hasStatic404: boolean
+  hasReportWebVitals: boolean
 }
 
 export function eventBuildOptimize(

test/integration/telemetry/pages/_app_withoutreportwebvitals.empty

@@ -0,0 +1,2 @@
+import App from 'next/app'
+export default App

test/integration/telemetry/pages/_app_withreportwebvitals.empty

@@ -0,0 +1,5 @@
+import App from 'next/app'
+export default App
+export function reportWebVitals(metric) {
+  console.log(metric)
+}

test/integration/telemetry/test/index.test.js

@@ -369,4 +369,55 @@ describe('Telemetry CLI', () => {
 
     expect(stderr).toMatch(/isSrcDir.*?true/)
   })
+
+  it('detect reportWebVitals correctly for `next build`', async () => {
+    // Case 1: When _app.js does not exist.
+    let build = await nextBuild(appDir, [], {
+      stderr: true,
+      env: { NEXT_TELEMETRY_DEBUG: 1 },
+    })
+
+    let event1 = /NEXT_BUILD_OPTIMIZED[\s\S]+?{([\s\S]+?)}/
+      .exec(build.stderr)
+      .pop()
+    expect(event1).toMatch(/hasReportWebVitals.*?false/)
+
+    // Case 2: When _app.js exist with reportWebVitals function.
+    await fs.rename(
+      path.join(appDir, 'pages', '_app_withreportwebvitals.empty'),
+      path.join(appDir, 'pages', '_app.js')
+    )
+
+    build = await nextBuild(appDir, [], {
+      stderr: true,
+      env: { NEXT_TELEMETRY_DEBUG: 1 },
+    })
+
+    await fs.rename(
+      path.join(appDir, 'pages', '_app.js'),
+      path.join(appDir, 'pages', '_app_withreportwebvitals.empty')
+    )
+
+    event1 = /NEXT_BUILD_OPTIMIZED[\s\S]+?{([\s\S]+?)}/.exec(build.stderr).pop()
+    expect(event1).toMatch(/hasReportWebVitals.*?true/)
+
+    // Case 3: When _app.js exist without reportWebVitals function.
+    await fs.rename(
+      path.join(appDir, 'pages', '_app_withoutreportwebvitals.empty'),
+      path.join(appDir, 'pages', '_app.js')
+    )
+
+    build = await nextBuild(appDir, [], {
+      stderr: true,
+      env: { NEXT_TELEMETRY_DEBUG: 1 },
+    })
+
+    await fs.rename(
+      path.join(appDir, 'pages', '_app.js'),
+      path.join(appDir, 'pages', '_app_withoutreportwebvitals.empty')
+    )
+
+    event1 = /NEXT_BUILD_OPTIMIZED[\s\S]+?{([\s\S]+?)}/.exec(build.stderr).pop()
+    expect(event1).toMatch(/hasReportWebVitals.*?false/)
+  })
 })