How to Use the Fastify Adapter in NestJS: A Complete Guide

To use the Fastify adapter in NestJS, install @nestjs/platform-fastify and pass new FastifyAdapter() to NestFactory.create() instead of the default Express adapter.

The NestJS framework abstracts the underlying HTTP server through platform adapters, with Express being the default. Switching to Fastify requires only minimal bootstrap changes while unlocking significant performance benefits, as implemented in the nestjs/nest repository.

Installing the Fastify Platform Package

Before modifying your bootstrap code, add the Fastify platform package to your dependencies:

npm install @nestjs/platform-fastify

This package contains the FastifyAdapter class located at packages/platform-fastify/adapters/fastify-adapter.ts, which implements the Nest AbstractHttpAdapter contract.

Bootstrapping with FastifyAdapter

Replace the default Express bootstrap with Fastify by importing FastifyAdapter and NestFastifyApplication from the platform package:

import { NestFactory } from '@nestjs/core';
import {
  FastifyAdapter,
  NestFastifyApplication,
} from '@nestjs/platform-fastify';
import { AppModule } from './app.module';

async function bootstrap() {
  const app = await NestFactory.create<NestFastifyApplication>(
    AppModule,
    new FastifyAdapter(),
  );
  await app.listen(3000);
}
bootstrap();

The generic type NestFastifyApplication (defined in packages/platform-fastify/interfaces/nest-fastify-application.interface.ts) provides TypeScript support for Fastify-specific methods.

FastifyAdapter Architecture and Implementation Details

Understanding the internal structure helps when debugging or extending the adapter functionality.

Adapter Class Structure

The FastifyAdapter class extends AbstractHttpAdapter and translates Nest's generic HTTP handling into Fastify's API. The constructor (lines 46-60 in packages/platform-fastify/adapters/fastify-adapter.ts) creates a new Fastify instance or accepts an existing one, then configures the router with version-constraint handling.

Middleware Support with Middie

Fastify requires the middie plugin for Express-style middleware compatibility. When you call use() before the adapter initializes, the adapter queues middleware internally (lines 333-341). During app.init(), Nest automatically registers middie and flushes the queued middlewares.

Type Identification

The getType() method returns the string 'fastify' (lines 292-294), allowing Nest to identify the active platform and load the correct internal modules.

Configuring Fastify-Specific Features

The adapter exposes helper methods that wrap native Fastify plugins for common web server features.

Server Options and Logging

Pass standard FastifyServerOptions to the constructor for core behavior configuration:

const fastifyAdapter = new FastifyAdapter({
  logger: true,
  ignoreTrailingSlash: true,
});

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

Static Assets and View Engines

Serve static files using the useStaticAssets method (implemented at lines 558-580), which internally delegates to @fastify/static:

import { join } from 'path';

fastifyAdapter.useStaticAssets({
  root: join(__dirname, '..', 'public'),
});

Configure template engines via setViewEngine (lines 674-679), which loads @fastify/view:

fastifyAdapter.setViewEngine({
  engine: {
    handlebars: require('handlebars'),
  },
  templates: 'views',
});

CORS Configuration

Enable cross-origin resource sharing through the adapter's enableCors method, which utilizes @fastify/cors:

fastifyAdapter.enableCors({ origin: true });

Using Express-Style Middleware

Despite running on Fastify, you can register middleware using the familiar Express pattern. The adapter maintains compatibility through its internal queuing system:

fastifyAdapter.use((req, res, next) => {
  console.log('Request received:', req.raw.url);
  next();
});

According to the source at lines 67-81, the adapter registers onRequest and onResponse hooks to integrate Nest's global guards, pipes, and interceptors into Fastify's lifecycle.

Summary

• Install the @nestjs/platform-fastify package to access the Fastify adapter.
• Import FastifyAdapter and NestFastifyApplication from the platform package for type-safe bootstrapping.
• Instantiate new FastifyAdapter() with optional FastifyServerOptions and pass it to NestFactory.create().
• Configure Fastify-specific features using enableCors, useStaticAssets, and setViewEngine helper methods.
• Reference the implementation at packages/platform-fastify/adapters/fastify-adapter.ts for advanced customization or debugging.

Frequently Asked Questions

What is the difference between FastifyAdapter and the default Express adapter in NestJS?

The FastifyAdapter translates NestJS abstractions into Fastify's high-performance HTTP engine, while the default adapter uses Express. Both implement the same AbstractHttpAdapter contract, so your controllers, services, and decorators remain identical. Fastify offers superior throughput and lower overhead, though the Express adapter has broader middleware ecosystem compatibility by default.

Can I use Express middleware with FastifyAdapter?

Yes. The FastifyAdapter queues Express-style middleware calls and automatically registers the middie plugin during initialization (source lines 333-341). This allows you to use app.use() or fastifyAdapter.use() with traditional (req, res, next) signature functions, though native Fastify plugins often provide better performance.

How do I configure Fastify plugins like static file serving?

Use the adapter's convenience methods rather than manual plugin registration. Call fastifyAdapter.useStaticAssets() for static files (wrapping @fastify/static) and fastifyAdapter.setViewEngine() for templates (wrapping @fastify/view). These methods handle plugin initialization and TypeScript type safety automatically.

Where is the FastifyAdapter implementation located in the NestJS repository?

The core implementation resides at packages/platform-fastify/adapters/fastify-adapter.ts in the nestjs/nest repository. Key supporting files include the interface definitions at packages/platform-fastify/interfaces/nest-fastify-application.interface.ts and working examples in sample/10-fastify/src/main.ts and sample/17-mvc-fastify/src/main.ts.

{
  "mcpServers": {
    "instagit": {
      "command": "npx",
      "args": ["-y", "instagit@latest"]
    }
  }
}