findyourpilot/.agent/skills/tanstack-start-best-practices/rules/env-functions.md

# env-functions: Use Environment Functions for Configuration

## Priority: MEDIUM

## Explanation

Environment functions provide type-safe access to environment variables on the server. They ensure secrets stay server-side, provide validation, and enable different configurations per environment (development, staging, production).

## Bad Example

```tsx
// Accessing env vars directly - no validation, potential leaks
export const getApiData = createServerFn()
  .handler(async () => {
    // No validation - may be undefined
    const apiKey = process.env.API_KEY

    // Accidentally exposed in error messages
    if (!apiKey) {
      throw new Error(`Missing API_KEY: ${process.env}`)
    }

    return fetch(url, { headers: { Authorization: apiKey } })
  })

// Or importing env in shared files
// lib/config.ts
export const config = {
  apiKey: process.env.API_KEY,  // Bundled into client!
  dbUrl: process.env.DATABASE_URL,
}
```

## Good Example: Validated Environment Configuration

```tsx
// lib/env.server.ts
import { z } from 'zod'

const envSchema = z.object({
  // Required
  DATABASE_URL: z.string().url(),
  SESSION_SECRET: z.string().min(32),

  // API Keys
  STRIPE_SECRET_KEY: z.string().startsWith('sk_'),
  STRIPE_WEBHOOK_SECRET: z.string().startsWith('whsec_'),

  // Optional with defaults
  NODE_ENV: z.enum(['development', 'staging', 'production']).default('development'),
  LOG_LEVEL: z.enum(['debug', 'info', 'warn', 'error']).default('info'),

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

export type Env = z.infer<typeof envSchema>

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

  if (!parsed.success) {
    console.error('Invalid environment variables:')
    console.error(parsed.error.flatten().fieldErrors)
    throw new Error('Invalid environment configuration')
  }

  return parsed.data
}

// Validate once at startup
export const env = validateEnv()

// Usage in server functions
export const getPaymentIntent = createServerFn({ method: 'POST' })
  .handler(async () => {
    const stripe = new Stripe(env.STRIPE_SECRET_KEY)
    // Type-safe, validated access
  })
```

## Good Example: Public vs Private Config

```tsx
// lib/env.server.ts - Server only (secrets)
export const serverEnv = {
  databaseUrl: process.env.DATABASE_URL!,
  sessionSecret: process.env.SESSION_SECRET!,
  stripeSecretKey: process.env.STRIPE_SECRET_KEY!,
}

// lib/env.ts - Public config (safe for client)
export const publicEnv = {
  appUrl: process.env.VITE_APP_URL ?? 'http://localhost:3000',
  stripePublicKey: process.env.VITE_STRIPE_PUBLIC_KEY!,
  sentryDsn: process.env.VITE_SENTRY_DSN,
}

// Vite exposes VITE_ prefixed vars to client
// Non-prefixed vars are server-only
```

## Good Example: Environment-Specific Behavior

```tsx
// lib/env.server.ts
export const env = validateEnv()

export const isDevelopment = env.NODE_ENV === 'development'
export const isProduction = env.NODE_ENV === 'production'
export const isStaging = env.NODE_ENV === 'staging'

// lib/logger.server.ts
import { env, isDevelopment } from './env.server'

export function log(level: string, message: string, data?: unknown) {
  if (isDevelopment) {
    console.log(`[${level}]`, message, data)
    return
  }

  // Production: send to logging service
  if (env.SENTRY_DSN) {
    // Send to Sentry
  }
}

// Server function with environment checks
export const debugInfo = createServerFn()
  .handler(async () => {
    if (isProduction) {
      throw new Error('Debug endpoint not available in production')
    }

    return {
      nodeVersion: process.version,
      env: env.NODE_ENV,
    }
  })
```

## Good Example: Feature Flags via Environment

```tsx
// lib/features.server.ts
import { env } from './env.server'

export const features = {
  newCheckout: env.FEATURE_NEW_CHECKOUT === 'true',
  betaDashboard: env.FEATURE_BETA_DASHBOARD === 'true',
  aiAssistant: env.FEATURE_AI_ASSISTANT === 'true',
}

// Usage in server functions
export const getCheckoutUrl = createServerFn()
  .handler(async () => {
    if (features.newCheckout) {
      return '/checkout/v2'
    }
    return '/checkout'
  })

// Usage in loaders
export const Route = createFileRoute('/dashboard')({
  loader: async () => {
    return {
      showBetaFeatures: features.betaDashboard,
    }
  },
})
```

## Good Example: Type-Safe env.d.ts

```tsx
// env.d.ts - TypeScript declarations for env vars
declare namespace NodeJS {
  interface ProcessEnv {
    // Required
    DATABASE_URL: string
    SESSION_SECRET: string

    // Optional
    NODE_ENV?: 'development' | 'staging' | 'production'
    SENTRY_DSN?: string

    // Vite public vars
    VITE_APP_URL?: string
    VITE_STRIPE_PUBLIC_KEY: string
  }
}
```

## Environment Variable Checklist

| Variable | Prefix | Accessible On |
|----------|--------|---------------|
| `DATABASE_URL` | None | Server only |
| `SESSION_SECRET` | None | Server only |
| `STRIPE_SECRET_KEY` | None | Server only |
| `VITE_APP_URL` | `VITE_` | Server + Client |
| `VITE_STRIPE_PUBLIC_KEY` | `VITE_` | Server + Client |

## Context

- Never import `.server.ts` files in client code
- Use `VITE_` prefix for client-accessible variables
- Validate at startup to fail fast on misconfiguration
- Use Zod or similar for runtime validation
- Keep secrets out of error messages and logs
- Consider using `.env.local` for local overrides (gitignored)