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Atlas Whoff
Atlas Whoff

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Type-Safe Environment Variables in Node.js with Zod

#node #typescript #tutorial #javascript

The Problem: Runtime Surprises 

const apiKey = process.env.STRIPE_SECRET_KEY;
apiKey.startsWith('sk_'); // TypeError: Cannot read properties of undefined
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You deployed to production. A required env var wasn't set. Your app crashes at the worst possible moment.

The Solution: Validate at Startup

Parse and validate environment variables when the process starts. Fail fast with clear errors instead of mysterious crashes later.

import { z } from 'zod';

const envSchema = z.object({
  // Required strings
  DATABASE_URL: z.string().url(),
  STRIPE_SECRET_KEY: z.string().startsWith('sk_'),
  NEXTAUTH_SECRET: z.string().min(32),

  // Optional with defaults
  NODE_ENV: z.enum(['development', 'test', 'production']).default('development'),
  PORT: z.coerce.number().default(3000),
  LOG_LEVEL: z.enum(['debug', 'info', 'warn', 'error']).default('info'),

  // Optional fields
  SENTRY_DSN: z.string().url().optional(),
  REDIS_URL: z.string().url().optional(),

  // Booleans from strings
  ENABLE_FEATURE_FLAGS: z.string().transform(v => v === 'true').default('false'),
});

type Env = z.infer<typeof envSchema>;

function validateEnv(): Env {
  const result = envSchema.safeParse(process.env);

  if (!result.success) {
    console.error('Invalid environment variables:');
    result.error.issues.forEach(issue => {
      console.error(`  ${issue.path.join('.')}: ${issue.message}`);
    });
    process.exit(1);
  }

  return result.data;
}

export const env = validateEnv();
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Now instead of process.env.PORT, you use env.PORT—fully typed, validated, guaranteed non-null.

Full Error Output

When vars are missing or malformed:

Invalid environment variables:
  DATABASE_URL: Invalid url
  STRIPE_SECRET_KEY: Required
  NEXTAUTH_SECRET: String must contain at least 32 character(s)
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You know exactly what's wrong before your app ever starts handling requests.

Next.js Integration

Next.js has a split environment: server vs client. Handle both:

// src/env.mjs
import { z } from 'zod';

// Server-only env vars
const serverSchema = z.object({
  DATABASE_URL: z.string().url(),
  STRIPE_SECRET_KEY: z.string(),
  NEXTAUTH_SECRET: z.string(),
  NODE_ENV: z.enum(['development', 'test', 'production']),
});

// Client-safe env vars (must be prefixed NEXT_PUBLIC_)
const clientSchema = z.object({
  NEXT_PUBLIC_APP_URL: z.string().url(),
  NEXT_PUBLIC_STRIPE_PUBLISHABLE_KEY: z.string().startsWith('pk_'),
});

// Only validate server schema on server
const serverEnv = typeof window === 'undefined'
  ? serverSchema.parse(process.env)
  : serverSchema.parse({});

const clientEnv = clientSchema.parse({
  NEXT_PUBLIC_APP_URL: process.env.NEXT_PUBLIC_APP_URL,
  NEXT_PUBLIC_STRIPE_PUBLISHABLE_KEY: process.env.NEXT_PUBLIC_STRIPE_PUBLISHABLE_KEY,
});

export const env = { ...serverEnv, ...clientEnv };
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Advanced: Transformations 

const envSchema = z.object({
  // Comma-separated list → array
  ALLOWED_ORIGINS: z.string()
    .transform(s => s.split(',').map(s => s.trim()))
    .default('http://localhost:3000'),

  // JSON string → object  
  FEATURE_FLAGS: z.string()
    .transform(s => JSON.parse(s) as Record<string, boolean>)
    .optional(),

  // URL → parsed URL object
  DATABASE_URL: z.string().url()
    .transform(s => new URL(s)),

  // Number ranges
  MAX_CONNECTIONS: z.coerce.number().min(1).max(100).default(10),

  // Secret length validation
  JWT_SECRET: z.string().min(32, 'JWT secret must be at least 32 characters'),
});
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t3-env: The Production-Ready Version

For Next.js + TypeScript projects, @t3-oss/env-nextjs handles the boilerplate:

npm install @t3-oss/env-nextjs zod
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import { createEnv } from '@t3-oss/env-nextjs';
import { z } from 'zod';

export const env = createEnv({
  server: {
    DATABASE_URL: z.string().url(),
    STRIPE_SECRET_KEY: z.string(),
  },
  client: {
    NEXT_PUBLIC_APP_URL: z.string().url(),
  },
  // Map to process.env
  runtimeEnv: {
    DATABASE_URL: process.env.DATABASE_URL,
    STRIPE_SECRET_KEY: process.env.STRIPE_SECRET_KEY,
    NEXT_PUBLIC_APP_URL: process.env.NEXT_PUBLIC_APP_URL,
  },
});
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Built-in: tree-shaking (client vars don't leak to server bundle), runtime validation, TypeScript autocomplete.

The .env.example Pattern

Maintain a .env.example with all required variables (no real values):

# .env.example — commit this
DATABASE_URL=postgresql://user:password@localhost:5432/mydb
STRIPE_SECRET_KEY=sk_test_...
NEXTAUTH_SECRET=generate-with-openssl-rand-base64-32
PORT=3000
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Combined with startup validation, new developers know exactly what to set and get immediate feedback if anything is missing.

Stop shipping apps that silently fail because someone forgot to set SENDGRID_API_KEY in production.

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Top comments (1)

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mickyarun profile image
arun rajkumar •

The .env.example pattern combined with startup validation is the move. We run Zod validation across all our NestJS services at Atoa and the single best change we made was environment-aware strictness: in dev mode, a missing optional var logs a warning. In prod, it calls process.exit(1). That distinction sounds small but it changes developer behaviour — nobody ignores a process exit.

The multi-service angle is where it gets interesting. We have a shared env schema that every service validates against on startup. Any service that boots successfully has implicitly passed the contract. The one thing I'd add to this article: the .env.example isn't just for new developers, it's your schema documentation. We generate ours directly from the Zod schema so they can never drift out of sync.