Environment Variables in Next.js

Variable Naming

Next.js has specific rules for environment variable exposure:

• Server-only: Any variable (no prefix required)
• Client-exposed: Must be prefixed with NEXT_PUBLIC_
• System variables: NODE_ENV, VERCEL_* are automatically available

File Structure

.env.local          # Local overrides (gitignored)
.env.development    # Development defaults
.env.production     # Production defaults
.env                # Defaults (committed)

Variable Types

Server-Only Variables

# .env.local
DATABASE_URL=postgresql://...
API_SECRET_KEY=secret123
STRIPE_SECRET_KEY=sk_live_...

Access in Server Components, API Routes, Server Actions:

// ✅ Server Component
export default async function Page() {
  const dbUrl = process.env.DATABASE_URL;
  // Only accessible on server
}

// ✅ API Route
export async function GET() {
  const apiKey = process.env.API_SECRET_KEY;
  return NextResponse.json({ data: 'secret' });
}

// ✅ Server Action
'use server';
export async function createOrder() {
  const stripeKey = process.env.STRIPE_SECRET_KEY;
  // Use stripeKey
}

Client-Exposed Variables

# .env.local
NEXT_PUBLIC_API_URL=https://api.example.com
NEXT_PUBLIC_APP_NAME=My App
NEXT_PUBLIC_ANALYTICS_ID=UA-123456

Access in Client Components:

'use client';

export default function ClientComponent() {
  const apiUrl = process.env.NEXT_PUBLIC_API_URL;
  // Available in browser (bundled at build time)
  
  return <div>API: {apiUrl}</div>;
}

Type Safety

Environment Variable Types

// env.d.ts or types/env.d.ts
namespace NodeJS {
  interface ProcessEnv {
    DATABASE_URL: string;
    API_SECRET_KEY: string;
    NEXT_PUBLIC_API_URL: string;
    NEXT_PUBLIC_APP_NAME: string;
  }
}

Validation at Startup

// lib/env.ts
function getEnvVar(name: string, required = true): string {
  const value = process.env[name];
  
  if (required && !value) {
    throw new Error(`Missing required environment variable: ${name}`);
  }
  
  return value || '';
}

export const env = {
  databaseUrl: getEnvVar('DATABASE_URL'),
  apiSecretKey: getEnvVar('API_SECRET_KEY'),
  publicApiUrl: getEnvVar('NEXT_PUBLIC_API_URL'),
  publicAppName: getEnvVar('NEXT_PUBLIC_APP_NAME'),
} as const;

Runtime vs Build-Time

Build-Time Variables

Client-exposed variables (NEXT_PUBLIC_*) are embedded at build time:

// ❌ This won't work - value is set at build time
const apiUrl = process.env.NEXT_PUBLIC_API_URL; // Set during `next build`

// ✅ Use different variables for different environments
// .env.development
NEXT_PUBLIC_API_URL=http://localhost:3001

// .env.production
NEXT_PUBLIC_API_URL=https://api.production.com

Runtime Variables (Server-Only)

Server variables are available at runtime:

// ✅ Server variables can change at runtime
export async function GET() {
  const dbUrl = process.env.DATABASE_URL; // Available at runtime
  return NextResponse.json({ url: dbUrl });
}

Environment-Specific Configuration

Using NODE_ENV

// lib/config.ts
const isDevelopment = process.env.NODE_ENV === 'development';
const isProduction = process.env.NODE_ENV === 'production';

export const config = {
  apiUrl: isDevelopment
    ? 'http://localhost:3001'
    : process.env.NEXT_PUBLIC_API_URL || 'https://api.example.com',
  enableDebug: isDevelopment,
  logLevel: isProduction ? 'error' : 'debug',
};

Custom Environment Variables

# .env.local
NEXT_PUBLIC_ENV=development

const env = process.env.NEXT_PUBLIC_ENV || 'development';

export const config = {
  apiUrl: env === 'production' 
    ? 'https://api.production.com'
    : 'http://localhost:3001',
};

Vercel Deployment

Vercel Environment Variables

Set in Vercel dashboard:

• Development: .env.development values
• Preview: Same as Production or custom
• Production: .env.production values

Automatic Variables

Vercel provides these automatically:

• VERCEL_URL - Deployment URL
• VERCEL_ENV - development, preview, or production
• NODE_ENV - production in production builds

export async function GET() {
  const baseUrl = process.env.VERCEL_URL 
    ? `https://${process.env.VERCEL_URL}`
    : 'http://localhost:3000';
    
  return NextResponse.json({ baseUrl });
}

Security Best Practices

1. Never commit secrets - Add .env.local to .gitignore
2. Use different keys per environment - Never reuse production keys
3. Validate required variables - Check at startup, fail fast
4. Limit client exposure - Only expose what's necessary via NEXT_PUBLIC_
5. Use secret management - Use Vercel Secrets, AWS Secrets Manager, etc.
6. Rotate keys regularly - Especially API keys and tokens

Common Patterns

API Configuration

// lib/api-config.ts
export const apiConfig = {
  baseUrl: process.env.NEXT_PUBLIC_API_URL || 'http://localhost:3001',
  timeout: parseInt(process.env.API_TIMEOUT || '5000', 10),
  retries: parseInt(process.env.API_RETRIES || '3', 10),
} as const;

Database Configuration

// lib/db-config.ts
import { Pool } from 'pg';

const pool = new Pool({
  connectionString: process.env.DATABASE_URL,
  ssl: process.env.NODE_ENV === 'production' ? { rejectUnauthorized: false } : false,
});

export default pool;

Feature Flags

// lib/features.ts
export const features = {
  enableAnalytics: process.env.NEXT_PUBLIC_ENABLE_ANALYTICS === 'true',
  enableBetaFeatures: process.env.NEXT_PUBLIC_ENABLE_BETA === 'true',
  maintenanceMode: process.env.MAINTENANCE_MODE === 'true',
} as const;

Troubleshooting

Variables Not Available

1. Restart dev server - Environment variables load at startup
2. Check prefix - Client variables need NEXT_PUBLIC_
3. Verify file location - .env.local should be in project root
4. Check .gitignore - Ensure .env.local is ignored

Type Errors

Add types to env.d.ts:

declare namespace NodeJS {
  interface ProcessEnv {
    YOUR_VAR: string;
  }
}