API Routes

Expo API routes are files ending with +api.ts (or .tsx, .js, .jsx) under /app/. They export HTTP method handlers (GET, POST, etc.) and are accessed via standard fetch() calls. In browser-metro, these run entirely in-browser - no server or service worker needed.

How it works

Client Bundle                    API Bundle (separate)
+---------------------------+   +---------------------------+
| /__expo_ctx.js            |   | /__api_routes.js          |
|   (page routes only,      |   |   (only +api files,       |
|    +api files excluded)   |   |    keyed by URL path)     |
| /index.tsx                |   |                           |
|   ExpoRoot + App          |   | Sets window.__API_ROUTES__|
+---------------------------+   +---------------------------+

The API bundle is loaded in the iframe before the client bundle. A fetch interceptor patches window.fetch to check if the request pathname matches an API route. If matched, it calls the handler directly in-browser and returns the Response. Otherwise, it falls through to the real fetch.

File path to URL mapping

Dynamic segments ([id]) are supported with parameter extraction.

Writing an API route

// /app/api/hello+api.ts
export function GET(request: Request) {
  return Response.json({
    message: "Hello from the API!",
    timestamp: Date.now(),
  });
}
 
export function POST(request: Request) {
  const body = await request.json();
  return Response.json({
    received: body,
    echo: true,
  });
}

Key components

• isApiRouteFile(path) - detects +api files by filename pattern
• filePathToApiRoute(path) - converts file paths to URL paths
• buildApiRoutesEntry(vfs) - generates /__api_routes.js with route map and match() function
• buildApiBundle(vfs, url) - creates a separate Bundler instance for API routes

Watch mode

When +api files change, the worker rebuilds the API bundle separately and sends it alongside the client bundle. Client-only changes still use granular HMR; API changes trigger a full API bundle rebuild (API bundles are small, so this is fast).