Environment variables in Next.js

// .env.local (git ignored)
DATABASE_URL=postgresql://localhost:5432/myapp
API_SECRET=secret_key_here

// Public variables (exposed to client)
NEXT_PUBLIC_API_URL=https://api.example.com
NEXT_PUBLIC_GA_ID=G-XXXXXXXXXX

// env.ts - Type-safe environment variables
import { z } from 'zod'

const envSchema = z.object({
  // Server-side variables
  DATABASE_URL: z.string().url(),
  API_SECRET: z.string().min(1),
  NODE_ENV: z.enum(['development', 'production', 'test']),
  
  // Client-side variables
  NEXT_PUBLIC_API_URL: z.string().url(),
  NEXT_PUBLIC_GA_ID: z.string().optional(),
})

// Validate at build time
const env = envSchema.parse({
  DATABASE_URL: process.env.DATABASE_URL,
  API_SECRET: process.env.API_SECRET,
  NODE_ENV: process.env.NODE_ENV,
  NEXT_PUBLIC_API_URL: process.env.NEXT_PUBLIC_API_URL,
  NEXT_PUBLIC_GA_ID: process.env.NEXT_PUBLIC_GA_ID,
})

export default env

// Usage in server components
import env from '@/lib/env'

async function getData() {
  const response = await fetch(env.NEXT_PUBLIC_API_URL + '/data', {
    headers: {
      'Authorization': `Bearer ${env.API_SECRET}`
    }
  })
  return response.json()
}