Allow passing in a custom logger

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

@@ -121,6 +121,10 @@ export type ServerConstructor = {
    * Hide error messages containing server information - @default false
    */
   quiet?: boolean
+  /**
+   * Custom logger for error - @default null
+   */
+  logger?: ((err: Error) => void) | null
   /**
    * Object what you would use in next.config.js - @default {}
    */
@@ -132,6 +136,7 @@ export type ServerConstructor = {
 export default class Server {
   protected dir: string
   protected quiet: boolean
+  protected logger: ((err: Error) => void) | null
   protected nextConfig: NextConfig
   protected distDir: string
   protected pagesDir?: string
@@ -173,13 +178,15 @@ export default class Server {
   public constructor({
     dir = '.',
     quiet = false,
+    logger = null,
     conf,
     dev = false,
     minimalMode = false,
     customServer = true,
   }: ServerConstructor & { conf: NextConfig; minimalMode?: boolean }) {
     this.dir = resolve(dir)
     this.quiet = quiet
+    this.logger = logger
     loadEnvConfig(this.dir, dev, Log)
 
     this.nextConfig = conf
@@ -282,6 +289,10 @@ export default class Server {
 
   public logError(err: Error): void {
     if (this.quiet) return
+    if (this.logger) {
+      this.logger(err)
+      return
+    }
     console.error(err)
   }
 

test/integration/custom-logger/pages/index.js

@@ -0,0 +1,12 @@
+export const getServerSideProps = ({ req }) => {
+  if (req.url === '/index?error=true') {
+    throw new Error('error from app')
+  }
+  return {
+    props: {},
+  }
+}
+
+const Page = () => 'hi'
+
+export default Page

test/integration/custom-logger/server.js

@@ -0,0 +1,33 @@
+const http = require('http')
+const next = require('next')
+
+const dev = process.env.NODE_ENV !== 'production'
+const dir = __dirname
+const port = process.env.PORT || 3000
+
+const app = next({
+  dev,
+  dir,
+  logger: (err) => {
+    console.error('custom logger:', err)
+  },
+})
+const handleNextRequests = app.getRequestHandler()
+
+app.prepare().then(() => {
+  const server = new http.Server(async (req, res) => {
+    if (/index/.test(req.url)) {
+      return app.render(req, res, '/index')
+    }
+
+    handleNextRequests(req, res)
+  })
+
+  server.listen(port, (err) => {
+    if (err) {
+      throw err
+    }
+
+    console.log(`> Ready on http://localhost:${port}`)
+  })
+})

test/integration/custom-logger/test/index.test.js

@@ -0,0 +1,78 @@
+/* eslint-env jest */
+
+import { join } from 'path'
+import getPort from 'get-port'
+import clone from 'clone'
+import {
+  initNextServerScript,
+  killApp,
+  renderViaHTTP,
+  nextBuild,
+} from 'next-test-utils'
+
+const appDir = join(__dirname, '../')
+
+let appPort
+let server
+jest.setTimeout(1000 * 60 * 2)
+
+const context = {}
+
+const startServer = async (optEnv = {}, opts) => {
+  const scriptPath = join(appDir, 'server.js')
+  context.appPort = appPort = await getPort()
+  const env = Object.assign(
+    {},
+    clone(process.env),
+    { PORT: `${appPort}` },
+    optEnv
+  )
+
+  server = await initNextServerScript(
+    scriptPath,
+    /ready on/i,
+    env,
+    /ReferenceError: options is not defined/,
+    opts
+  )
+}
+
+describe('Custom Logger', () => {
+  afterEach(() => killApp(server))
+
+  it('should not use custom logger in dev mode', async () => {
+    let stderr = ''
+    await startServer(
+      {},
+      {
+        onStderr(msg) {
+          stderr += msg || ''
+        },
+      }
+    )
+    const html = await renderViaHTTP(appPort, '/index?error=true')
+    expect(html).toContain('error from app')
+    expect(stderr).not.toContain('custom logger')
+    expect(stderr).toContain('error from app')
+  })
+
+  it('should use custom logger in production mode', async () => {
+    const { code } = await nextBuild(appDir)
+    expect(code).toBe(0)
+
+    let stderr = ''
+
+    await startServer(
+      { NODE_ENV: 'production' },
+      {
+        onStderr(msg) {
+          stderr += msg || ''
+        },
+      }
+    )
+
+    const html = await renderViaHTTP(appPort, '/index?error=true')
+    expect(html).toContain('Internal Server Error')
+    expect(stderr).toContain('custom logger: Error: error from app')
+  })
+})