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Next.js Environment Variables: The Complete Guide to Avoiding Common Mistakes

#nextjs #security #typescript #webdev

Environment variables in Next.js are more nuanced than they appear. The client vs server distinction, the build-time vs runtime behavior, and the local vs production differences trip up almost everyone at least once.

Here's the complete guide.

The Core Rule

Next.js has two execution environments: server and browser. Environment variables work differently in each.

  • Server-only variables: Available in API routes, Server Components, middleware
  • Browser variables: Must be prefixed with NEXT_PUBLIC_ 
# Server-only (never sent to browser)
DATABASE_URL=postgresql://...
ANTHROPIC_API_KEY=sk-ant-...
STRIPE_SECRET_KEY=sk_live_...
NEXTAUTH_SECRET=...

# Browser-accessible (bundled into client JavaScript)
NEXT_PUBLIC_STRIPE_PUBLISHABLE_KEY=pk_live_...
NEXT_PUBLIC_ANALYTICS_ID=G-...
NEXT_PUBLIC_APP_URL=https://myapp.com
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If you access process.env.DATABASE_URL in a Client Component, Next.js will return undefined -- not the value. It doesn't throw an error, which makes this bug easy to miss.

File Priority Order

Next.js loads env files in this priority (later overrides earlier):

.env                    # Always loaded
.env.local              # Local overrides (git-ignored)
.env.development        # Loaded in development
.env.development.local  # Local dev overrides (git-ignored)
.env.production         # Loaded in production
.env.production.local   # Local prod overrides (git-ignored)
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For most projects:

  • .env -- defaults that can be committed
  • .env.local -- secrets and local overrides (NEVER commit)
  • Vercel/Railway/Render dashboard -- production values

Validate at Startup

The worst bug in production: your app starts, a user hits a page, and it crashes because process.env.STRIPE_SECRET_KEY is undefined.

Validate env vars at startup so missing values fail immediately:

// src/lib/env.ts
import { z } from "zod"

const envSchema = z.object({
  // Database
  DATABASE_URL: z.string().url("DATABASE_URL must be a valid URL"),

  // Auth
  NEXTAUTH_URL: z.string().url(),
  NEXTAUTH_SECRET: z.string().min(32, "NEXTAUTH_SECRET must be at least 32 chars"),
  GOOGLE_CLIENT_ID: z.string().optional(),
  GOOGLE_CLIENT_SECRET: z.string().optional(),

  // AI
  ANTHROPIC_API_KEY: z.string().startsWith("sk-ant-"),

  // Stripe
  STRIPE_SECRET_KEY: z.string().startsWith("sk_"),
  STRIPE_WEBHOOK_SECRET: z.string().startsWith("whsec_"),

  // Public (client-accessible)
  NEXT_PUBLIC_STRIPE_PUBLISHABLE_KEY: z.string().startsWith("pk_"),
  NEXT_PUBLIC_APP_URL: z.string().url(),

  // Optional
  NODE_ENV: z.enum(["development", "test", "production"]).default("development"),
})

// This throws at startup if anything is missing or wrong
export const env = envSchema.parse(process.env)

// Type-safe access
export type Env = z.infer<typeof envSchema>
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// src/lib/stripe.ts -- Use env instead of process.env
import { env } from "@/lib/env"
import Stripe from "stripe"

export const stripe = new Stripe(env.STRIPE_SECRET_KEY)
// env.STRIPE_SECRET_KEY is string (not string | undefined)
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If STRIPE_SECRET_KEY is missing, you get a clear error at server start instead of a cryptic runtime failure.

Secrets in Client Code: A Common Mistake

This is the most dangerous env var mistake:

// WRONG: exposes secret key to every browser that loads this page
"use client"

export function PayButton() {
  // This gets bundled into your client JavaScript
  const secret = process.env.STRIPE_SECRET_KEY  // undefined in browser, but...
}
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Even if process.env.STRIPE_SECRET_KEY returns undefined in the browser, Next.js sometimes inlines env vars at build time in certain configurations. Never reference secret keys in Client Components.

The correct pattern:

// Client Component
"use client"

export function PayButton() {
  const publishableKey = process.env.NEXT_PUBLIC_STRIPE_PUBLISHABLE_KEY  // OK
  // Secret operations go through API routes, not client code
}

// Server-side (API route)
import { env } from "@/lib/env"

export async function POST(req: Request) {
  // env.STRIPE_SECRET_KEY is fine here -- runs on server
  const session = await stripe.checkout.sessions.create(...)
}
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Different Values Per Environment

Use .env.development and .env.production for environment-specific values:

# .env.development
DATABASE_URL=postgresql://localhost/myapp_dev
STRIPE_SECRET_KEY=sk_test_...
NEXT_PUBLIC_STRIPE_PUBLISHABLE_KEY=pk_test_...
NEXT_PUBLIC_APP_URL=http://localhost:3000

# .env.production (committed -- no secrets)
NEXT_PUBLIC_APP_URL=https://myapp.com

# Vercel dashboard (production secrets -- never committed)
DATABASE_URL=postgresql://neon.tech/...
STRIPE_SECRET_KEY=sk_live_...
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The .env.example Pattern

Always maintain a .env.example file with all required variables but no values:

# .env.example (committed to git)
DATABASE_URL=
NEXTAUTH_URL=http://localhost:3000
NEXTAUTH_SECRET=
ANTHROPIC_API_KEY=
STRIPE_SECRET_KEY=
STRIPE_WEBHOOK_SECRET=
NEXT_PUBLIC_STRIPE_PUBLISHABLE_KEY=
NEXT_PUBLIC_APP_URL=http://localhost:3000
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New developer onboarding:

cp .env.example .env.local
# Fill in the values
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Add to your validation schema: any variable in .env.example should be validated.

Runtime vs Build Time

Some env vars are needed at build time (static generation). Others are needed at runtime.

// Available at BUILD TIME -- statically inlined
const appName = process.env.NEXT_PUBLIC_APP_NAME

// Available at RUNTIME only -- not safe to use in static paths
const dbUrl = process.env.DATABASE_URL
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For edge runtime (middleware, edge functions), note that Node.js process.env access patterns differ slightly -- test your middleware specifically.

Debugging Missing Variables

When you get undefined from process.env:

// Quick debug -- remove after fixing
console.log({
  hasDb: !!process.env.DATABASE_URL,
  hasStripe: !!process.env.STRIPE_SECRET_KEY,
  nodeEnv: process.env.NODE_ENV,
})
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Common causes:

  1. Variable not in .env.local (typo in the key name)
  2. Client Component accessing server-only variable
  3. Vercel dashboard variable not set for the current environment (Preview vs Production)
  4. Variable added to .env.example but forgot to add to .env.local

Environment variable setup is pre-configured in the AI SaaS Starter Kit, including Zod validation, .env.example, and a lib/env.ts singleton.

AI SaaS Starter Kit ($99) ->

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