Next.js 16 Client-Server Boundary Module Resolution Errors

Problem

Next.js 16 build fails with errors like:

Module not found: Can't resolve 'dns'
Module not found: Can't resolve 'fs'
Module not found: Can't resolve 'net'
Module not found: Can't resolve 'tls'

The error trace shows:

Import trace:
  ./node_modules/.pnpm/pg@8.16.3/...
  ./packages/database/index.ts [Client Component Browser]
  ./apps/web/lib/utils/some-util.ts [Client Component Browser]
  ./apps/web/components/SomeClientComponent.tsx [Client Component Browser]

This happens when a client component ("use client") imports constants or utilities from a file that transitively imports Node.js-only modules (like Prisma, database clients, or server-side libraries).

Context / Trigger Conditions

When this occurs:

• Building Next.js 16 app with next build or turbo build
• Client component imports constants/types from a utility file
• That utility file also contains server-side functions that import database/Node.js modules
• The error mentions Node.js built-in modules: dns, fs, net, tls, child_process, etc.

Why it happens: Next.js 16 enforces strict client/server boundaries. When you import ANY export from a file, the entire file gets bundled. If that file imports server-only modules (even if you're only using constants), those server imports try to resolve in the browser bundle and fail.

Common scenarios:

1. Constants defined alongside database utility functions
2. Type definitions in files that also import ORM clients
3. Enums/config objects in server action files
4. Shared validators in files with server-side imports

Solution

Step 1: Identify the Import Chain

Look at the error's import trace to find where the client component is pulling in server code:

./apps/web/lib/utils/preferences.ts [Client Component Browser]
./apps/web/components/NotificationPreferencesClient.tsx [Client Component Browser]

This tells you preferences.ts is being bundled for the browser because a client component imports from it.

Step 2: Examine the Problematic File

Read the file that's causing issues (e.g., lib/utils/preferences.ts):

// ❌ PROBLEM: Server imports in same file as constants
import { db, PreferenceCategory } from "database";  // Server-only!

export const EMAIL_NOTIFICATION_KEYS = {
  BOOKING: "email.notification.booking",
  // ...
};

export async function getEmailPreference(db: any, userId: string) {
  // Server-only function using db
}

The client component only needs EMAIL_NOTIFICATION_KEYS, but importing it pulls in the entire file including the database import.

Step 3: Extract Constants to Separate File

Create a new constants-only file with NO server-side imports:

New file: lib/constants/email-preferences.ts

/**
 * Email Preference Constants
 *
 * Shared constants for email notification preferences.
 * This file has no server-side dependencies and can be safely imported in client components.
 */

export const EMAIL_NOTIFICATION_KEYS = {
  BOOKING: "email.notification.booking",
  PAYMENT: "email.notification.payment",
  MESSAGING: "email.notification.messaging",
  VERIFICATION: "email.notification.verification",
} as const;

export type EmailNotificationKey =
  (typeof EMAIL_NOTIFICATION_KEYS)[keyof typeof EMAIL_NOTIFICATION_KEYS];

export const MESSAGING_COOLDOWN_MS = 60 * 60 * 1000;

Step 4: Update Original Utility File

Import and re-export from the new constants file for backward compatibility:

Modified: lib/utils/preferences.ts

import { db, PreferenceCategory } from "database";  // Server imports OK here
import {
  EMAIL_NOTIFICATION_KEYS,
  MESSAGING_COOLDOWN_MS,
  type EmailNotificationKey,
} from "@/lib/constants/email-preferences";  // Import from constants file

// Re-export for backward compatibility
export { EMAIL_NOTIFICATION_KEYS, MESSAGING_COOLDOWN_MS };
export type { EmailNotificationKey };

// Server-only functions remain here
export async function getEmailPreference(db: any, userId: string) {
  // ...
}

Step 5: Update Client Component Imports

Change client component to import directly from constants file:

"use client";

// ❌ Before: Imports from file with server dependencies
import { EMAIL_NOTIFICATION_KEYS } from "@/lib/utils/preferences";

// ✅ After: Imports from constants-only file
import { EMAIL_NOTIFICATION_KEYS } from "@/lib/constants/email-preferences";

Step 6: Verify Fix

turbo build  # or next build

Build should succeed. The client component now imports from a file with no server dependencies.

Verification

1. Build succeeds: turbo build or next build completes without module resolution errors
2. Client bundle clean: No Node.js modules in browser bundle
3. Route renders: The page using the client component renders correctly
4. No runtime errors: No "Module not found" errors in browser console

Check the build output for your route:

Route (app)
├ ◐ /[locale]/settings/notifications  ✓ Should show without errors

Alternative Solutions

Option 1: Turbopack resolveAlias (Workaround, Not Recommended)

You can configure Turbopack to load empty modules for Node.js built-ins:

// next.config.ts
import type { NextConfig } from 'next'

const nextConfig: NextConfig = {
  turbopack: {
    resolveAlias: {
      fs: { browser: './empty.ts' },
      dns: { browser: './empty.ts' },
      net: { browser: './empty.ts' },
      tls: { browser: './empty.ts' },
    },
  },
}

export default nextConfig

Warning: This is a workaround that masks the problem. The recommended approach is to refactor your code to respect client/server boundaries.

Option 2: Duplicate Constants (Not Recommended)

Copying constants to both files works but creates maintenance burden:

// lib/utils/preferences.ts (server)
export const EMAIL_NOTIFICATION_KEYS = { /* ... */ };

// lib/constants/email-preferences.ts (client)
export const EMAIL_NOTIFICATION_KEYS = { /* ... */ };  // Duplicate!

Better: Extract once and import from the shared location.

Best Practices

1. Organize by Server/Client Capability

lib/
├── constants/          # Client-safe constants, types, enums (no server imports)
│   ├── email-preferences.ts
│   ├── routes.ts
│   └── validation-messages.ts
├── utils/              # Server-only utilities
│   ├── preferences.ts  # Can import database
│   └── auth.ts         # Can import server modules
└── shared/             # Truly universal utilities (no server or client-specific APIs)
    └── formatters.ts   # Pure functions only

2. Mark Constants Files Clearly

Add a comment banner to constants-only files:

/**
 * Client-Safe Constants
 *
 * This file has no server-side dependencies and can be safely imported
 * in both server components and client components.
 *
 * DO NOT add imports from: database, fs, path, crypto, or any Node.js modules.
 */

3. Use Separate Barrel Exports

If you have many constants, create a barrel export:

// lib/constants/index.ts
export * from './email-preferences';
export * from './routes';
export * from './validation';

// Client components import from barrel
import { EMAIL_NOTIFICATION_KEYS } from '@/lib/constants';

4. Validate Imports with ESLint

Consider using eslint-plugin-no-server-import-in-page or similar rules to catch these issues during development.

Notes

• This is specific to Next.js 16: Earlier versions had looser boundaries
• Applies to all client components: Any file with "use client" directive
• Transitive imports matter: Even if your file doesn't directly import Node.js modules, if it imports from a file that does, you'll hit this issue
• Server Components are safe: Server Components can import from files with Node.js modules without issues
• Type imports are usually safe: import type { ... } statements are typically fine because they're erased at runtime, but mixed imports (import { type X, Y }) will still pull in the runtime code

Common Mistakes

❌ Mistake 1: Adding "use client" to the utility file

"use client";  // ❌ Doesn't fix the problem, makes it worse
import { db } from "database";

❌ Mistake 2: Using dynamic imports for constants

// ❌ Overly complex, constants should be static imports
const keys = await import('@/lib/utils/preferences').then(m => m.EMAIL_NOTIFICATION_KEYS);

❌ Mistake 3: Ignoring the error with webpack fallbacks

// ❌ Masks the problem, doesn't fix architecture
webpack: { resolve: { fallback: { fs: false } } }

✅ Correct: Extract constants to separate file with no server imports

Example: Complete Refactor

Before (broken):

lib/utils/preferences.ts              <-- Has db imports
  - EMAIL_NOTIFICATION_KEYS           <-- Constants mixed with server code
  - getEmailPreference()              <-- Server function

components/NotificationClient.tsx     <-- "use client"
  - imports EMAIL_NOTIFICATION_KEYS   <-- ❌ Pulls in db imports

After (fixed):

lib/constants/email-preferences.ts    <-- NO server imports
  - EMAIL_NOTIFICATION_KEYS           <-- ✅ Pure constants

lib/utils/preferences.ts              <-- Has db imports
  - imports from constants file       <-- ✅ Uses constants, doesn't define them
  - re-exports for backward compat    <-- ✅ Existing code still works
  - getEmailPreference()              <-- Server function

components/NotificationClient.tsx     <-- "use client"
  - imports from constants file       <-- ✅ No server dependencies

References

• Next.js Module Not Found Error
• Next.js "use client" Directive
• Fixing Module not found: Can't resolve 'fs' in Next.js | Sentry
• How To Solve Module Not Found Can't Resolve 'fs' in Next.js
• Next.js Server Components vs. Client Components: Best Practices | Medium