Better Auth Fastify Integration Guide
This guide provides step-by-step instructions for configuring both essential handlers and CORS settings.
A configured Better Auth instance is required before proceeding. If you haven't set this up yet, please consult our Installation Guide.
Prerequisites
Verify the following requirements before integration:
- Node.js Environment: v16 or later installed
- ES Module Support: Enable ES modules in either:
package.json:{ "type": "module" }- TypeScript
tsconfig.json:{ "module": "ESNext" }
- Fastify Dependencies:
npm install fastify @fastify/cors
For TypeScript: Ensure your
tsconfig.json includes "esModuleInterop": true for optimal compatibility. Authentication Handler Setup
Configure Better Auth to process authentication requests by creating a catch-all route:
import Fastify from "fastify";
import { auth } from "./auth"; // Your configured Better Auth instance
const fastify = Fastify({ logger: true });
// Register authentication endpoint
fastify.route({
method: ["GET", "POST"],
url: "/api/auth/*",
async handler(request, reply) {
try {
// Construct request URL
const url = new URL(request.url, `http://${request.headers.host}`);
// Convert Fastify headers to standard Headers object
const headers = new Headers();
Object.entries(request.headers).forEach(([key, value]) => {
if (value) headers.append(key, value.toString());
});
// Create Fetch API-compatible request
const req = new Request(url.toString(), {
method: request.method,
headers,
body: request.body ? JSON.stringify(request.body) : undefined,
});
// Process authentication request
const response = await auth.handler(req);
// Forward response to client
reply.status(response.status);
response.headers.forEach((value, key) => reply.header(key, value));
reply.send(response.body ? await response.text() : null);
} catch (error) {
fastify.log.error("Authentication Error:", error);
reply.status(500).send({
error: "Internal authentication error",
code: "AUTH_FAILURE"
});
}
}
});
// Initialize server
fastify.listen({ port: 4000 }, (err) => {
if (err) {
fastify.log.error(err);
process.exit(1);
}
console.log("Server running on port 4000");
});Trusted origins
When a request is made from a different origin, the request will be blocked by default. You can add trusted origins to the auth instance.
export const auth = betterAuth({
trustedOrigins: ["http://localhost:3000", "https://example.com"],
});Configuring CORS
Secure your API endpoints with proper CORS configuration:
import fastifyCors from "@fastify/cors";
// Configure CORS policies
fastify.register(fastifyCors, {
origin: process.env.CLIENT_ORIGIN || "http://localhost:3000",
methods: ["GET", "POST", "PUT", "DELETE", "OPTIONS"],
allowedHeaders: [
"Content-Type",
"Authorization",
"X-Requested-With"
],
credentials: true,
maxAge: 86400
});
// Mount authentication handler after CORS registration
// (Use previous handler configuration here) Always restrict CORS origins in production environments. Use environment variables for dynamic configuration.