Understanding CORS and Setting it up with NestJS + Fastify 🚀

CORS, or Cross-Origin Resource Sharing, is a mechanism that allows a server to indicate any origins
(domain, scheme, or port) other than its own from which a browser should permit loading resources.
Without CORS, modern browsers would block cross-origin requests for security reasons, preventing
malicious behavior.

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Why is CORS Necessary?

When building modern web applications, the frontend and backend often run on different domains or
ports, especially during development. For example:

• Frontend: http://localhost:3000
• Backend: http://localhost:5000

By default, browsers block requests from the frontend to the backend due to the Same-Origin
Policy. This is where CORS comes in—it acts as a controlled way to bypass the same-origin
restriction, enabling secure communication.

A typical CORS scenario involves the frontend making an API call to the backend, and the backend
explicitly allowing such requests by including certain headers in its response.

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Configuring CORS in NestJS with Fastify

While enabling CORS in a NestJS application using Express is straightforward, using Fastify requires
a slightly different approach. When I was working on a project, I decided to switch from Vite’s
proxy server to enabling CORS on my Fastify server, both as a learning opportunity and for better
alignment with production setups. However, I found the documentation and available articles on
enabling CORS for Fastify in NestJS to be sparse.

Here’s how I tackled it.

Step 1: Define CORS Options

To keep things clean and reusable, I created a dedicated cors.options.ts file for my CORS
configuration. Here's what it looks like:

import { type CorsOptions } from "@nestjs/common/interfaces/external/cors-options.interface";

const CorsOptions: CorsOptions = {
    origin: [
        // ? Development environment
        "http://localhost:3000",
        // * Add Production and Staging URLs here
    ],
    methods: "GET,HEAD,PUT,PATCH,POST,DELETE",
    allowedHeaders: "Content-Type, Authorization",
    credentials: true,
    maxAge: 86400,
};

export { CorsOptions };

Step 2: Use CORS Options in the Main Application

In the main.ts file, I configured the Fastify server to use these options:

import { NestFactory } from "@nestjs/core";
import { FastifyAdapter, NestFastifyApplication } from "@nestjs/platform-fastify";
import helmet from "@fastify/helmet";

import { CorsOptions } from "./options/cors.options";
import { AppModule } from "./app.module";

const bootstrap = async (): Promise<void> => {
    const FastifyModule = new FastifyAdapter();

    // Enabling things for Fastify server
    FastifyModule.register(helmet); // Secure headers
    FastifyModule.enableCors(CorsOptions); // Enable CORS with custom options

    const app = await NestFactory.create<NestFastifyApplication>(AppModule, FastifyModule);

    // Start the application
    const PORT = 5000;
    await app.listen(PORT, () => {
        console.log(`Server is running on http://localhost:${PORT}`);
    });
};

bootstrap();

Key Points to Note

• Origins: I specified the allowed origins explicitly, such as http://localhost:3000 for development. Add production or staging URLs as needed.
• Methods: Only specific HTTP methods like GET, POST, and DELETE are allowed.
• Credentials: By setting credentials: true, the server allows cookies or authentication tokens to be sent with requests.
• Modular Approach: Placing the CORS options in a separate file helps maintain modularity and makes it easier to modify configurations in the future.

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Axios Configuration for the Frontend

Here is my axiosConfig.ts file, showing how I have configured Axios for the frontend:

import axios, { AxiosRequestConfig } from "axios";

import { ContentType, ResponseType, ResponseEncoding } from "../config/requestConfig";

const baseURL = (import.meta.env.VITE_API_BASE_URL as string) ?? "http://localhost:5000";

const axiosRequestConfig: AxiosRequestConfig = {
    baseURL,
    headers: {
        "Content-Type": ContentType.JSON,
    },
    timeout: 330000,
    withCredentials: true,
    responseType: ResponseType.JSON,
    responseEncoding: ResponseEncoding.UTF8,
    maxContentLength: 50000,
    validateStatus(status) {
        return status >= 200 && status < 300;
    },
    maxRedirects: 5,
};

const AxiosInstance = axios.create(axiosRequestConfig);

export { axiosRequestConfig, AxiosInstance };

Learnings and Final Thoughts

While configuring CORS for Fastify in NestJS, I initially used Express-specific solutions, which
felt incorrect and hacky. After thorough research, I found the proper way to enable it using the
enableCors method of the Fastify adapter.

If you’re using my Verdant turborepo template, you can
find an example of this CORS configuration there. The repo also demonstrates how to structure a
modern monorepo using tools like TurboRepo and Vite.

If you want to learn more about CORS, check out this
MDN article, which does a fantastic job of
explaining the concept in detail.

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CORS can seem tricky initially, but with the right approach, it’s straightforward to configure and
ensures secure communication between your frontend and backend.