fix: Narrow down from string | undefined to string

packages/next/src/build/webpack/plugins/define-env-plugin.ts

@@ -63,7 +63,7 @@ function getNextPublicEnvironmentVariables(): DefineEnv {
   for (const key in process.env) {
     if (key.startsWith('NEXT_PUBLIC_')) {
       const value = process.env[key]
-      if (value) {
+      if (value != null) {
         defineEnv[`process.env.${key}`] = value
       }
     }
@@ -80,7 +80,7 @@ function getNextConfigEnv(config: NextConfigComplete): DefineEnv {
   const env = config.env
   for (const key in env) {
     const value = env[key]
-    if (value) {
+    if (value != null) {
       errorIfEnvConflicted(config, key)
       defineEnv[`process.env.${key}`] = value
     }

test/integration/env-config/app/.env

@@ -15,4 +15,5 @@ ENV_FILE_EXPANDED_ESCAPED=\$ENV_FILE_KEY
 ENV_FILE_KEY_EXCLAMATION="hello!"
 ENV_FILE_PROCESS_ENV="env-file"
 ENV_KEY_IN_NEXT_CONFIG="hello from next.config.js"
-NEXT_PUBLIC_ENV_KEY_IN_NEXT_CONFIG="hello again from next.config.js"
+NEXT_PUBLIC_ENV_KEY_IN_NEXT_CONFIG="hello again from next.config.js"
+NEXT_PUBLIC_EMPTY_ENV_VAR=

test/integration/env-config/app/pages/index.js

@@ -25,13 +25,14 @@ const variables = [
   'NEW_ENV_LOCAL_KEY',
   'NEW_ENV_DEV_KEY',
   'NEXT_PUBLIC_HELLO_WORLD',
+  'NEXT_PUBLIC_EMPTY_ENV_VAR',
 ]
 
 export async function getStaticProps() {
   const items = {}
 
   variables.forEach((variable) => {
-    if (process.env[variable]) {
+    if (typeof process.env[variable] !== 'undefined') {
       items[variable] = process.env[variable]
     }
   })
@@ -53,6 +54,9 @@ export default function Page({ env }) {
       <div id="nextConfigNewPublicEnv">
         {process.env.NEXT_PUBLIC_NEW_NEXT_CONFIG_VALUE}
       </div>
+      <div id="nextPublicEmptyEnvVar">
+        {`${process.env.NEXT_PUBLIC_EMPTY_ENV_VAR}`}
+      </div>
     </>
   )
 }

test/integration/env-config/test/index.test.js

@@ -128,6 +128,13 @@ const runTests = (mode = 'dev', didReload = false) => {
     expect(data.ENV_FILE_PRODUCTION_LOCAL_OVERRIDEOVERRIDE_TEST).toEqual(
       isDev ? 'env' : 'localproduction'
     )
+    expect(data.NEXT_PUBLIC_EMPTY_ENV_VAR).toEqual('')
+
+    const browser = await webdriver(appPort, '/')
+    // Verify that after hydration, the empty env var is not replaced by undefined
+    expect(
+      await browser.waitForElementByCss('#nextPublicEmptyEnvVar').text()
+    ).toBe('')
   })
 }