fix(next/babel): read env from caller, not process

This PR fixes a bug where `next/babel` would accidentally enable development transforms for a production build (`next build`).

---

Fixes vercel#18929
Fixes vercel#19001
x-ref vercel#19046

packages/next/build/babel/preset.ts

@@ -1,9 +1,8 @@
 import { PluginItem } from 'next/dist/compiled/babel/core'
 import { dirname } from 'path'
-const env = process.env.NODE_ENV
-const isProduction = env === 'production'
-const isDevelopment = env === 'development'
-const isTest = env === 'test'
+
+const isLoadIntentTest = process.env.NODE_ENV === 'test'
+const isLoadIntentDevelopment = process.env.NODE_ENV === 'development'
 
 type StyledJsxPlugin = [string, any] | string
 type StyledJsxBabelOptions =
@@ -65,6 +64,18 @@ module.exports = (
 ): BabelPreset => {
   const supportsESM = api.caller(supportsStaticESM)
   const isServer = api.caller((caller: any) => !!caller && caller.isServer)
+  const isCallerDevelopment = api.caller((caller: any) => caller?.isDev)
+
+  // Look at external intent if used without a caller (e.g. via Jest):
+  const isTest = isCallerDevelopment == null && isLoadIntentTest
+
+  // Look at external intent if used without a caller (e.g. Storybook):
+  const isDevelopment =
+    isCallerDevelopment === true ||
+    (isCallerDevelopment == null && isLoadIntentDevelopment)
+
+  // Default to production mode if not `test` nor `development`:
+  const isProduction = !(isTest || isDevelopment)
 
   const useJsxRuntime =
     options['preset-react']?.runtime === 'automatic' ||

packages/next/build/webpack-config.ts

@@ -395,8 +395,6 @@ export default async function getBaseWebpackConfig(
       : [require('pnp-webpack-plugin')],
   }
 
-  const webpackMode = dev ? 'development' : 'production'
-
   const terserOptions: any = {
     parse: {
       ecma: 8,
@@ -948,7 +946,10 @@ export default async function getBaseWebpackConfig(
             [`process.env.${key}`]: JSON.stringify(config.env[key]),
           }
         }, {}),
-        'process.env.NODE_ENV': JSON.stringify(webpackMode),
+        // TODO: enforce `NODE_ENV` on `process.env`, and add a test:
+        'process.env.NODE_ENV': JSON.stringify(
+          dev ? 'development' : 'production'
+        ),
         'process.env.__NEXT_CROSS_ORIGIN': JSON.stringify(crossOrigin),
         'process.browser': JSON.stringify(!isServer),
         'process.env.__NEXT_TEST_MODE': JSON.stringify(
@@ -1014,7 +1015,7 @@ export default async function getBaseWebpackConfig(
           : undefined),
         // stub process.env with proxy to warn a missing value is
         // being accessed in development mode
-        ...(config.experimental.pageEnv && process.env.NODE_ENV !== 'production'
+        ...(config.experimental.pageEnv && dev
           ? {
               'process.env': `
             new Proxy(${isServer ? 'process.env' : '{}'}, {

test/unit/next-babel-loader.unit.test.js

@@ -1,11 +1,4 @@
 /* eslint-env jest */
-
-// avoid generating __source annotations in JSX during testing:
-const NODE_ENV = process.env.NODE_ENV
-process.env.NODE_ENV = 'production'
-require('next/dist/build/babel/preset')
-process.env.NODE_ENV = NODE_ENV
-
 function interopRequireDefault(mod) {
   return mod.default || mod
 }
@@ -370,8 +363,8 @@ describe('next-babel-loader', () => {
         { resourcePath: pageFile, isServer: false, development: true }
       )
 
-      expect(output).toMatchInlineSnapshot(
-        `"var __jsx=React.createElement;import React from\\"react\\";export var __N_SSG=true;export default function Home(_ref){var greeting=_ref.greeting;return __jsx(\\"h1\\",null,greeting);}_c=Home;var _c;$RefreshReg$(_c,\\"Home\\");"`
+      expect(output).toMatch(
+        /var _jsxFileName="[^"]+";var __jsx=React\.createElement;import React from"react";export var __N_SSG=true;export default function Home\(_ref\)\{var greeting=_ref\.greeting;return __jsx\("h1",\{__self:this,__source:\{fileName:_jsxFileName,lineNumber:8,columnNumber:20\}\},greeting\);\}_c=Home;var _c;\$RefreshReg\$\(_c,"Home"\);/
       )
     })
 

test/unit/next-babel.unit.test.js

@@ -3,23 +3,18 @@ import { transform } from '@babel/core'
 
 const trim = (s) => s.join('\n').trim().replace(/^\s+/gm, '')
 
-// avoid generating __source annotations in JSX during testing:
-const NODE_ENV = process.env.NODE_ENV
-process.env.NODE_ENV = 'production'
-const preset = require('next/dist/build/babel/preset')
-process.env.NODE_ENV = NODE_ENV
-
 const babel = (code, esm = false, presetOptions = {}, filename = 'noop.js') =>
   transform(code, {
     filename,
-    presets: [[preset, presetOptions]],
+    presets: [[require('next/dist/build/babel/preset'), presetOptions]],
     babelrc: false,
     configFile: false,
     sourceType: 'module',
     compact: true,
     caller: {
       name: 'tests',
       supportsStaticESM: esm,
+      isDev: false,
     },
   }).code