Expo Environment Configuration Skill

Expo Environment Configuration

Overview

This skill enforces type-safe, validated environment variable management for Expo/React Native using Zod schemas. Environment variables are validated at build time and provide full TypeScript inference, regardless of their source (.env files, EAS Build secrets, CI/CD pipelines, or command-line exports).

Why This Pattern?

Unlike NestJS's @nestjs/config, Expo has no official type-safe env solution. The Zod validation pattern provides:

Type safety - Full TypeScript inference via z.infer<typeof schema>
Build-time validation - Fails fast with clear error messages before deployment
Source agnostic - Works with .env, EAS secrets, CI variables
Testing support - Easy mocking via module aliases

Core Pattern

Environment Schema (`src/lib/env.ts`)

import { z } from "zod";

/**
 * Environment variable schema with Zod validation.
 * Variables are validated at module load time.
 */
const envSchema = z.object({
  // Required variables
  EXPO_PUBLIC_API_URL: z.string().url(),
  EXPO_PUBLIC_APP_ENV: z.enum(["development", "staging", "production"]),

  // Optional variables with defaults
  EXPO_PUBLIC_SENTRY_DSN: z.string().optional(),
  EXPO_PUBLIC_FEATURE_FLAG: z
    .string()
    .transform(v => v === "true")
    .default("false"),
});

/**
 * Validated environment configuration.
 * Throws at module load if validation fails.
 */
export const env = envSchema.parse(process.env);

/**
 * Type-safe environment configuration.
 */
export type Env = z.infer<typeof envSchema>;

Usage in Components/Hooks

import { env } from "@/lib/env";

// Full autocomplete and type safety
const apiUrl = env.EXPO_PUBLIC_API_URL; // string
const isDev = env.EXPO_PUBLIC_APP_ENV === "development"; // boolean

Build-Time Validation (`app.config.ts`)

For variables needed during the build process, validate in app.config.ts:

// app.config.ts
const { z } = require("zod");

const buildEnvSchema = z.object({
  EXPO_PUBLIC_API_URL: z.string().url(),
  EXPO_PUBLIC_APP_ENV: z.enum(["development", "staging", "production"]),
  // Build-only secrets (not exposed to client)
  SENTRY_AUTH_TOKEN: z.string().optional(),
});

// Throws during `eas build` if invalid
const env = buildEnvSchema.parse(process.env);

module.exports = {
  name: "MyApp",
  slug: "my-app",
  extra: {
    apiUrl: env.EXPO_PUBLIC_API_URL,
    appEnv: env.EXPO_PUBLIC_APP_ENV,
  },
};

Variable Sources

Environment variables arrive in process.env from multiple sources:

| Source | When Available | How Set | |--------|----------------|---------| | .env.local | Local dev | Expo CLI auto-loads | | .env.development | Local dev | Copied to .env.local via npm script | | eas.json env | EAS Build | build.production.env section | | EAS Secrets | EAS Build | eas secret:create | | CI Variables | CI builds | GitHub Actions / GitLab CI settings |

The Zod pattern validates process.env directly - it doesn't care how variables got there.

Testing Pattern

Jest Setup (`jest.setup.local.ts`)

// Mock the env module for all tests
jest.mock("@/lib/env", () => ({
  env: {
    EXPO_PUBLIC_API_URL: "https://test.example.com",
    EXPO_PUBLIC_APP_ENV: "development",
    EXPO_PUBLIC_SENTRY_DSN: undefined,
    EXPO_PUBLIC_FEATURE_FLAG: false,
  },
}));

Override in Specific Tests

import { env } from "@/lib/env";

jest.mock("@/lib/env");

describe("ProductionFeature", () => {
  beforeEach(() => {
    (env as jest.Mocked<typeof env>).EXPO_PUBLIC_APP_ENV = "production";
  });

  it("should behave differently in production", () => {
    // Test production-specific behavior
  });
});

ESLint Enforcement

This pattern is enforced by ESLint's no-restricted-syntax rule in eslint.config.mjs:

"no-restricted-syntax": [
  "error",
  {
    selector: "MemberExpression[object.name='process'][property.name='env']",
    message: "Direct process.env access is forbidden. Import { env } from '@/lib/env' instead.",
  },
],

Exceptions (files allowed to use process.env):

lib/env.ts - The env validation module itself
app.config.ts - Expo build config
codegen.ts - GraphQL codegen config
playwright.config.ts - E2E test config
lighthouserc.js - Lighthouse CI config

Core Rules

1. Always Prefix with EXPO_PUBLIC_

Variables without this prefix are not available in client code:

// CORRECT - available in client
EXPO_PUBLIC_API_URL=https://api.example.com

// INCORRECT - only available at build time
API_URL=https://api.example.com

2. Never Access process.env Directly

Always use the validated env object:

// CORRECT - type-safe, validated
import { env } from "@/lib/env";
const url = env.EXPO_PUBLIC_API_URL;

// INCORRECT - untyped, unvalidated
const url = process.env.EXPO_PUBLIC_API_URL;

3. Validate Early, Fail Fast

Validation happens at module load. If a required variable is missing, the app fails immediately with a clear error rather than at runtime.

4. Use Transforms for Non-String Types

Environment variables are always strings. Use Zod transforms:

const envSchema = z.object({
  // Boolean from string
  EXPO_PUBLIC_DEBUG: z
    .string()
    .transform(v => v === "true")
    .default("false"),

  // Number from string
  EXPO_PUBLIC_TIMEOUT_MS: z
    .string()
    .transform(v => parseInt(v, 10))
    .default("5000"),

  // Array from comma-separated string
  EXPO_PUBLIC_ALLOWED_HOSTS: z
    .string()
    .transform(v => v.split(",").map(s => s.trim()))
    .default(""),
});

5. Separate Client vs Build-Only Variables

Keep sensitive build-time variables out of the client schema:

// Client variables (embedded in JS bundle)
const clientSchema = z.object({
  EXPO_PUBLIC_API_URL: z.string().url(),
});

// Build-only variables (NOT in bundle)
const buildSchema = z.object({
  SENTRY_AUTH_TOKEN: z.string(),
  EAS_PROJECT_ID: z.string(),
});

File Organization

src/
  lib/
    env.ts              # Main env schema and exports
app.config.ts           # Build-time validation (if needed)
.env.localhost          # Local development (git-ignored)
.env.development        # Development environment
.env.staging            # Staging environment
.env.production         # Production environment

Detailed Reference

For comprehensive patterns, transforms, and testing examples:

references/validation-patterns.md - Advanced Zod schemas, transforms, and refinements

Anti-Patterns to Avoid

Never use process.env directly in components

// WRONG - untyped, could be undefined
const Component = () => {
  const url = process.env.EXPO_PUBLIC_API_URL;
  // url is string | undefined, no validation
};

// CORRECT - validated and typed
import { env } from "@/lib/env";
const Component = () => {
  const url = env.EXPO_PUBLIC_API_URL;
  // url is string, guaranteed to be valid URL
};

Never skip validation for "simple" variables

// WRONG - skipping validation
export const API_URL = process.env.EXPO_PUBLIC_API_URL ?? "http://localhost:3000";

// CORRECT - always validate
const envSchema = z.object({
  EXPO_PUBLIC_API_URL: z.string().url().default("http://localhost:3000"),
});
export const { EXPO_PUBLIC_API_URL: API_URL } = envSchema.parse(process.env);

Never store secrets in EXPO_PUBLIC_ variables

// WRONG - secrets exposed in client bundle
EXPO_PUBLIC_API_SECRET=super-secret-key

// CORRECT - secrets only at build time, passed securely
SENTRY_AUTH_TOKEN=secret  # Build-only, not in bundle

Validation Checklist

When adding or modifying environment variables:

[ ] Variable is prefixed with EXPO_PUBLIC_ (if needed in client code)
[ ] Variable is added to the Zod schema in src/lib/env.ts
[ ] Appropriate Zod type/transform is used (url, enum, boolean transform, etc.)
[ ] Default value provided for optional variables
[ ] Jest mock updated in jest.setup.local.ts
[ ] Variable documented in .env.example or .env.development
[ ] Sensitive values use EAS Secrets, not .env files

Agent Skills: Expo Environment Configuration

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