Build Serverless API Routes with Expo

When to Use API Routes

Use API routes when you need:

• Server-side secrets - API keys, database credentials, or tokens that must never reach the client
• Database operations - Direct database queries that shouldn't be exposed
• Third-party API proxies - Hide API keys when calling external services (OpenAI, Stripe, etc.)
• Server-side validation - Validate data before database writes
• Webhook endpoints - Receive callbacks from services like Stripe or GitHub
• Rate limiting - Control access at the server level
• Heavy computation - Offload processing that would be slow on mobile

When NOT to Use API Routes

Avoid API routes when:

• Data is already public - Use direct fetch to public APIs instead
• No secrets required - Static data or client-safe operations
• Real-time updates needed - Use WebSockets or services like Supabase Realtime
• Simple CRUD - Consider Firebase, Supabase, or Convex for managed backends
• File uploads - Use direct-to-storage uploads (S3 presigned URLs, Cloudflare R2)
• Authentication only - Use Clerk, Auth0, or Firebase Auth instead

File Structure

API routes live in the app directory with +api.ts suffix:

app/
  api/
    hello+api.ts          → GET /api/hello
    users+api.ts          → /api/users
    users/[id]+api.ts     → /api/users/:id
  (tabs)/
    index.tsx

Basic API Route

// app/api/hello+api.ts
export function GET(request: Request) {
  return Response.json({ message: "Hello from Expo!" });
}

HTTP Methods

Export named functions for each HTTP method:

// app/api/items+api.ts
export function GET(request: Request) {
  return Response.json({ items: [] });
}

export async function POST(request: Request) {
  const body = await request.json();
  return Response.json({ created: body }, { status: 201 });
}

export async function PUT(request: Request) {
  const body = await request.json();
  return Response.json({ updated: body });
}

export async function DELETE(request: Request) {
  return new Response(null, { status: 204 });
}

Dynamic Routes

// app/api/users/[id]+api.ts
export function GET(request: Request, { id }: { id: string }) {
  return Response.json({ userId: id });
}

Request Handling

Query Parameters

export function GET(request: Request) {
  const url = new URL(request.url);
  const page = url.searchParams.get("page") ?? "1";
  const limit = url.searchParams.get("limit") ?? "10";

  return Response.json({ page, limit });
}

Headers

export function GET(request: Request) {
  const auth = request.headers.get("Authorization");

  if (!auth) {
    return Response.json({ error: "Unauthorized" }, { status: 401 });
  }

  return Response.json({ authenticated: true });
}

JSON Body

export async function POST(request: Request) {
  const { email, password } = await request.json();

  if (!email || !password) {
    return Response.json({ error: "Missing fields" }, { status: 400 });
  }

  return Response.json({ success: true });
}

Environment Variables

Use process.env for server-side secrets:

// app/api/ai+api.ts
export async function POST(request: Request) {
  const { prompt } = await request.json();

  const response = await fetch("https://api.openai.com/v1/chat/completions", {
    method: "POST",
    headers: {
      "Content-Type": "application/json",
      Authorization: `Bearer ${process.env.OPENAI_API_KEY}`,
    },
    body: JSON.stringify({
      model: "gpt-4",
      messages: [{ role: "user", content: prompt }],
    }),
  });

  const data = await response.json();
  return Response.json(data);
}

Set environment variables:

• Local: Create .env file (never commit)
• EAS Hosting: Use eas env:create or Expo dashboard

CORS Headers

Add CORS for web clients:

const corsHeaders = {
  "Access-Control-Allow-Origin": "*",
  "Access-Control-Allow-Methods": "GET, POST, PUT, DELETE, OPTIONS",
  "Access-Control-Allow-Headers": "Content-Type, Authorization",
};

export function OPTIONS() {
  return new Response(null, { headers: corsHeaders });
}

export function GET() {
  return Response.json({ data: "value" }, { headers: corsHeaders });
}

Error Handling

export async function POST(request: Request) {
  try {
    const body = await request.json();
    // Process...
    return Response.json({ success: true });
  } catch (error) {
    console.error("API error:", error);
    return Response.json({ error: "Internal server error" }, { status: 500 });
  }
}

Testing Locally

Start the development server with API routes:

npx expo serve

This starts a local server at http://localhost:8081 with full API route support.

Test with curl:

curl http://localhost:8081/api/hello
curl -X POST http://localhost:8081/api/users -H "Content-Type: application/json" -d '{"name":"Test"}'

Deployment to EAS Hosting

Prerequisites

npm install -g eas-cli
eas login

Deploy

eas deploy

This builds and deploys your API routes to EAS Hosting (Cloudflare Workers).

Environment Variables for Production

# Create a secret
eas env:create --name OPENAI_API_KEY --value sk-xxx --environment production

# Or use the Expo dashboard

Custom Domain

Configure in eas.json or Expo dashboard.

EAS Hosting Runtime (Cloudflare Workers)

API routes run on Cloudflare Workers. Key limitations:

Missing/Limited APIs

• No Node.js filesystem - fs module unavailable
• No native Node modules - Use Web APIs or polyfills
• Limited execution time - 30 second timeout for CPU-intensive tasks
• No persistent connections - WebSockets require Durable Objects
• fetch is available - Use standard fetch for HTTP requests

Use Web APIs Instead

// Use Web Crypto instead of Node crypto
const hash = await crypto.subtle.digest(
  "SHA-256",
  new TextEncoder().encode("data")
);

// Use fetch instead of node-fetch
const response = await fetch("https://api.example.com");

// Use Response/Request (already available)
return new Response(JSON.stringify(data), {
  headers: { "Content-Type": "application/json" },
});

Database Options

Since filesystem is unavailable, use cloud databases:

• Cloudflare D1 - SQLite at the edge
• Turso - Distributed SQLite
• PlanetScale - Serverless MySQL
• Supabase - Postgres with REST API
• Neon - Serverless Postgres

Example with Turso:

// app/api/users+api.ts
import { createClient } from "@libsql/client/web";

const db = createClient({
  url: process.env.TURSO_URL!,
  authToken: process.env.TURSO_AUTH_TOKEN!,
});

export async function GET() {
  const result = await db.execute("SELECT * FROM users");
  return Response.json(result.rows);
}

Calling API Routes from Client

// From React Native components
const response = await fetch("/api/hello");
const data = await response.json();

// With body
const response = await fetch("/api/users", {
  method: "POST",
  headers: { "Content-Type": "application/json" },
  body: JSON.stringify({ name: "John" }),
});

Common Patterns

Authentication Middleware

// utils/auth.ts
export async function requireAuth(request: Request) {
  const token = request.headers.get("Authorization")?.replace("Bearer ", "");

  if (!token) {
    throw new Response(JSON.stringify({ error: "Unauthorized" }), {
      status: 401,
      headers: { "Content-Type": "application/json" },
    });
  }

  // Verify token...
  return { userId: "123" };
}

// app/api/protected+api.ts
import { requireAuth } from "../../utils/auth";

export async function GET(request: Request) {
  const { userId } = await requireAuth(request);
  return Response.json({ userId });
}

Proxy External API

// app/api/weather+api.ts
export async function GET(request: Request) {
  const url = new URL(request.url);
  const city = url.searchParams.get("city");

  const response = await fetch(
    `https://api.weather.com/v1/current?city=${city}&key=${process.env.WEATHER_API_KEY}`
  );

  return Response.json(await response.json());
}

Rules

• NEVER expose API keys or secrets in client code
• ALWAYS validate and sanitize user input
• Use proper HTTP status codes (200, 201, 400, 401, 404, 500)
• Handle errors gracefully with try/catch
• Keep API routes focused - one responsibility per endpoint
• Use TypeScript for type safety
• Log errors server-side for debugging

Limitations

• Use this skill only when the task clearly matches the scope described above.
• Do not treat the output as a substitute for environment-specific validation, testing, or expert review.
• Stop and ask for clarification if required inputs, permissions, safety boundaries, or success criteria are missing.