How to Convert Your Application from Express.js to Fastify

Transitioning from Express to Fastify: A Developer's Guide

In the ever-evolving landscape of web development, the choice of frameworks and languages can significantly impact the performance, maintainability, and scalability of web applications. Express.js has long been the go-to framework for Node.js developers due to its simplicity and flexibility. However, as applications grow and performance demands increase, developers may seek alternatives that offer better throughput and lower latency. This is where Fastify comes into the picture, a web framework highly focused on providing excellent performance and developer experience.

The decision to transition from Express to Fastify can be driven by various factors, such as the need for higher performance, the desire for a more modern and feature-rich framework, or simply the curiosity to explore new technologies. Fastify is designed to be as expressive as Express while being significantly faster, thanks to its efficient request and response handling and its use of modern JavaScript features.

In this article, we will explore the key differences between Express and Fastify and provide a comprehensive guide on how to convert an existing Express application to Fastify. We will also include a comparison table that highlights the differences in syntax between the two frameworks.

Understanding the Differences

Before diving into the conversion process, it's essential to understand the fundamental differences between Express and Fastify. Both frameworks are designed for building web applications on top of Node.js, but they differ in architecture, performance, and the way they handle certain tasks.

Aspect	Express	Fastify
Performance	Good	Excellent
Architecture	Middleware-based	Plugin-based
Schema Validation	External libraries	Built-in
Serialization	Manual	Automatic
Async/Await Support	Limited	Native

Syntax Differences

When converting from Express to Fastify, one of the first things you'll notice is the difference in syntax. While both frameworks share a similar approach to defining routes and handling requests, there are some notable distinctions.

Operation	Express Syntax	Fastify Syntax
Server Initialization	`const express = require('express'); const app = express();`	`const fastify = require('fastify')({ logger: true });`
Route Definition	`app.get('/', (req, res) => { res.send('Hello World!'); });`	`fastify.get('/', async (request, reply) => { return 'Hello World!'; });`
Middleware	`app.use((req, res, next) => { console.log('Middleware'); next(); });`	`fastify.addHook('onRequest', (request, reply, done) => { console.log('Hook'); done(); });`
Server Start	`app.listen(3000, () => { console.log('Server running on port 3000'); });`	fastify.listen(3000, (err, address) => { if (err) throw err; console.log(`Server listening on ${address}`); });

Converting an Express Application to Fastify

The conversion process involves several steps, from setting up the Fastify instance to migrating routes, middleware, and error handling. Let's walk through the process step by step.

1. Setting Up Fastify

const fastify = require('fastify')({ logger: true });

2. Migrating Routes

Convert your Express routes to Fastify routes. Note that Fastify supports async functions out of the box.

fastify.get('/', async (request, reply) => {
        return 'Hello World!';
    });

3. Migrating Middleware

In Fastify, middleware is replaced by hooks. Here's how you can convert an Express middleware to a Fastify hook.

fastify.addHook('onRequest', (request, reply, done) => {
        console.log('Hook');
        done();
    });

4. Error Handling

Fastify has a different approach to error handling. Here's an example of a custom error handler in Fastify.

fastify.setErrorHandler((error, request, reply) => {
        reply.status(error.statusCode || 500).send(error.message);
    });

5. Starting the Server

Finally, start the Fastify server similarly to how you would start an Express server, but with a slightly different syntax.

fastify.listen(3000, (err, address) => {
        if (err) throw err;
        console.log(`Server listening on ${address}`);
    });

Conclusion

Transitioning from Express to Fastify can be a smooth process if approached methodically. By understanding the differences in syntax and architecture, you can leverage Fastify's performance benefits and features to enhance your web applications. Remember to test thoroughly during the migration process to ensure that your application continues to function as expected.

Express to Fastify Conversion Checklist

Initial Setup

Install Fastify in your project using npm or yarn.
Remove Express-specific middleware that has Fastify alternatives.
Set up a basic Fastify server instance to replace the Express app.

Routes and Handlers

Convert Express routes to Fastify routes.
Update route handler function signatures to match Fastify's request and reply parameters.
Replace Express middleware with Fastify hooks or plugins where necessary.

Middleware Conversion

Identify custom middleware and convert them into Fastify plugins.
Replace body-parser with Fastify's built-in body parsing.
Update error handling to use Fastify's error handling mechanisms.

Plugins and Decorators

Search for Fastify equivalents of any Express plugins in use.
Register plugins with the Fastify instance.
Use Fastify decorators to add custom properties or methods to the request/reply objects.

Testing and Debugging

Update integration tests to work with Fastify's API.
Debug any issues that arise during the conversion process.
Ensure all routes and middleware function as expected.

Performance Optimization

Review Fastify's best practices for performance optimization.
Implement Fastify's built-in performance features like schema-based validation.

Code Examples

Below are some code snippets to help with the conversion:

Basic Express to Fastify server setup:

const fastify = require('fastify')({ logger: true });

fastify.get('/', async (request, reply) => {
  return { hello: 'world' }
});

fastify.listen(3000, function (err, address) {
  if (err) {
    fastify.log.error(err);
    process.exit(1);
  }
  fastify.log.info(`server listening on ${address}`);
});

Converting Express middleware to Fastify:

fastify.register(async (instance, opts, done) => {
  instance.addHook('preHandler', async (request, reply) => {
    // Custom middleware logic here
  });
  done();
});

Options

Toggles